What is String in Java? String is a data type?

String is a Class in java and defined in java.lang package. It’s not a primitive data type like int and long. String class represents character Strings. String is used in almost all the Java applications and there are some interesting facts we should know about String. String in immutable and final in Java and JVM uses String Pool to store all the String objects.

Some other interesting things about String is the way we can instantiate a String object using double quotes and overloading of “+” operator for concatenation.

To create immutable class in java, you have to do following steps.

· Declare the class as final so it can’t be extended.

· Make all fields private so that direct access is not allowed.

· Don’t provide setter methods for variables

· Make all mutable fields final so that it’s value can be assigned only once.

· Initialize all the fields via a constructor performing deep copy.

· Perform cloning of objects in the getter methods to return a copy rather than returning the actual object reference.

Write a method that will remove given character from the String?

We can use replaceAll method to replace all the occurance of a String with another String. The important point to note is that it accepts String as argument, so we will use Character class to create String and use it to replace all the characters with empty String.

private static String removeChar(String str, char c)

{

if(str==null){

return null;

}

return str.replaceAll(Character.toString(c), "");

}

How to convert String to char and vice versa?

String is a sequence of characters, so we can’t convert it to a single character. We can use use charAt method to get the character at given index or we can

use toCharArray()method to convert String to character array.

How to convert String to byte array and vice versa?

We can use String getBytes() method to convert String to byte array and we can use String constructor new String(byte[] arr) to convert byte array to String.

Difference between String, StringBuffer and StringBuilder?

String is immutable and final in java, so whenever we do String manipulation, it creates a new String. String manipulations are resource consuming, so java provides two utility classes for String manipulations – StringBuffer and StringBuilder.

StringBuffer and StringBuilder are mutable classes. StringBuffer operations are thread- safe and synchronized where StringBuilder operations are not thread-safe. So when multiple threads are working on same String, we should use StringBuffer but in single threaded environment we should use StringBuilder.

StringBuilder performance is fast than StringBuffer because of no overhead of synchronization.

Why String is immutable or final in Java

There are several benefits of String because it’s immutable and final.

· String Pool is possible because String is immutable in java.

· It increases security because any hacker can’t change its value and it’s used for storing sensitive information such as database username, password etc.

· Since String is immutable, it’s safe to use in multi-threading and we don’t need any synchronization.

· Strings are used in java classloader and immutability provides security that correct class is getting loaded by Classloader.

How to Split String in java?

We can use split(String regex) to split the String into String array based on the provided regular expression.

Why Char array is preferred over String for storing password?

String is immutable in java and stored in String pool. Once it’s created it stays in the pool until unless garbage collected, so even though we are done with password it’s available in memory for longer duration and there is no way to avoid it. It’s a security risk because anyone having access to memory dump can find the password as clear text.

If we use char array to store password, we can set it to blank once we are done with it. So we can control for how long it’s available in memory that avoids the security threat with String.

How do you check if two Strings are equal in Java?

There are two ways to check if two Strings are equal or not – using “==” operator or using equals method. When we use “==” operator, it checks for value of String as well as reference but in our programming, most of the time we are checking equality of String for value only. So we should use equals method to check if two Strings are equal or not.

There is another function equalsIgnoreCase that we can use to ignore case.

What is String Pool?

As the name suggests, String Pool is a pool of Strings stored in Java heap memory. We know that String is special class in java and we can create String object using new operator as well as providing values in double quotes.

What does String intern() method do?

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

This method always return a String that has the same contents as this string, but is guaranteed to be from a pool of unique strings.

Does String is thread-safe in Java?

Strings are immutable, so we can’t change it’s value in program. Hence it’s thread-safe and can be safely used in multi-threaded environment.

Why String is popular HashMap key in Java?

Since String is immutable, its hashcode is cached at the time of creation and it doesn’t need to be calculated again. This makes it a great candidate for key in a Map and it’s processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

Java Multithreading Interview Questions and Answers

1. What is the difference between Process and Thread?

A process is a self contained execution environment and it can be seen as a program or application whereas Thread is a single task of execution within the process. Java runtime environment runs as a single process which contains different classes and programs as processes. Thread can be called lightweight process. Thread requires less resources to create and exists in the process, thread shares the process resources.

2. What are the benefits of multi-threaded programming?

In Multi-Threaded programming, multiple threads are executing concurrently that improves the performance because CPU is not idle incase some thread is waiting to get some resources. Multiple threads share the heap memory, so it’s good to create multiple threads to execute some task rather than creating multiple processes. For example, Servlets are better in performance than CGI because Servlet support multi-threading but CGI doesn’t.

3. What is difference between user Thread and daemon Thread?

When we create a Thread in java program, it’s known as user thread. A daemon thread runs in background and doesn’t prevent JVM from terminating. When there are no user threads running, JVM shutdown the program and quits. A child thread created from daemon thread is also a daemon thread.

4. How can we create a Thread in Java?

There are two ways to create Thread in Java – first by implementing Runnable interface and then creating a Thread object from it and second is to extend the Thread Class. Read this post to learn more about creating threads in java.

5. What are different states in lifecycle of Thread?

When we create a Thread in java program, its state is New. Then we start the thread that change it’s state to Runnable. Thread Scheduler is responsible to allocate CPU to threads in Runnable thread pool and change their state to Running. Other Thread states are Waiting, Blocked and Dead. Read this post to learn more about life cycle of thread.

6. Can we call run() method of a Thread class?

Yes, we can call run() method of a Thread class but then it will behave like a normal method. To actually execute it in a Thread, we need to start it

using Thread.start() method.

7. How can we pause the execution of a Thread for specific time?

We can use Thread class sleep() method to pause the execution of Thread for certain time. Note that this will not stop the processing of thread for specific time, once the thread awake from sleep, it’s state gets changed to runnable and based on thread scheduling, it gets executed.

8. What do you understand about Thread Priority?

Every thread has a priority, usually higher priority thread gets precedence in execution but it depends on Thread Scheduler implementation that is OS dependent. We can specify the priority of thread but it doesn’t guarantee that higher priority thread will get executed before lower priority thread. Thread priority is an int whose value varies from 1 to 10 where 1 is the lowest priority thread and 10 is the highest priority thread.

9. What is Thread Scheduler and Time Slicing?

Thread Scheduler is the Operating System service that allocates the CPU time to the available runnable threads. Once we create and start a thread, it’s execution depends on the implementation of Thread Scheduler. Time Slicing is the process to divide the available CPU time to the available runnable threads. Allocation of CPU time to threads can be based on thread priority or the thread waiting for longer time will get more priority in getting CPU time. Thread scheduling can’t be controlled by java, so it’s always better to control it from application itself.

10. What is context-switching in multi-threading?

Context Switching is the process of storing and restoring of CPU state so that Thread execution can be resumed from the same point at a later point of time. Context Switching is the essential feature for multitasking operating system and support for multi-threaded environment.

11. How can we make sure main() is the last thread to finish in Java Program?

We can use Thread join() method to make sure all the threads created by the program is dead before finishing the main function. Here is an article about Thread join method.

12. How does thread communicate with each other?

When threads share resources, communication between Threads is important to coordinate their efforts. Object class wait(), notify() and notifyAll() methods allows threads to communicate about the lock status of a resource. Check this post to learn more about thread wait, notify and notifyAll.

13. Why thread communication methods wait(), notify() and notifyAll() are in Object class?

In Java every Object has a monitor and wait, notify methods are used to wait for the Object monitor or to notify other threads that Object monitor is free now. There is no monitor on threads in java and synchronization can be used with any Object, that’s why it’s part of Object class so that every class in java has these essential methods for inter thread communication.

14. Why wait(), notify() and notifyAll() methods have to be called from synchronized method or block?

When a Thread calls wait() on any Object, it must have the monitor on the Object that it will leave and goes in wait state until any other thread call notify() on this Object. Similarly when a thread calls notify() on any Object, it leaves the monitor on the Object and other waiting threads can get the monitor on the Object. Since all these methods require Thread to have the Object monitor, that can be achieved only by synchronization, they need to be called from synchronized method or block.

15. Why Thread sleep() and yield() methods are static?

Thread sleep() and yield() methods work on the currently executing thread. So there is no point in invoking these methods on some other threads that are in wait state. That’s why these methods are made static so that when this method is called statically, it works on the current executing thread and avoid confusion to the programmers who might think that they can invoke these methods on some non-running threads.

16. How can we achieve thread safety in Java?

There are several ways to achieve thread safety in java – synchronization, atomic concurrent classes, implementing concurrent Lock interface, using volatile keyword, using immutable classes and Thread safe classes. Learn more at thread safety tutorial.

17. What is volatile keyword in Java

When we use volatile keyword with a variable, all the threads read it’s value directly from the memory and don’t cache it. This makes sure that the value read is the same as in the memory.

18. Which is more preferred – Synchronized method or Synchronized block?

Synchronized block is more preferred way because it doesn’t lock the Object, synchronized methods lock the Object and if there are multiple synchronization blocks in the class, even though they are not related, it will stop them from execution and put them in wait state to get the lock on Object.

19. How to create daemon thread in Java?

Thread class setDaemon(true) can be used to create daemon thread in java. We need to call this method before calling start() method else it will throw IllegalThreadStateException.

20. What is ThreadLocal?

Java ThreadLocal is used to create thread-local variables. We know that all threads of an Object share it’s variables, so if the variable is not thread safe, we

can use synchronization but if we want to avoid synchronization, we can use ThreadLocal variables.

Every thread has it’s own ThreadLocal variable and they can use it’s get() and set() methods to get the default value or change it’s value local to Thread.

ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread. Check this post for small example program showing ThreadLocal Example.

21. What is Thread Group? Why it’s advised not to use it?

ThreadGroup is a class which was intended to provide information about a thread group. ThreadGroup API is weak and it doesn’t have any functionality that is not provided by Thread. Two of the major feature it had are to get the list of active threads in a thread group and to set the uncaught exception handler for the thread. But Java 1.5 has

added setUncaughtExceptionHandler(UncaughtExceptionHandler eh) method using which we can add uncaught exception handler to the thread. So ThreadGroup is obsolete and hence not advised to use anymore.

t1.setUncaughtExceptionHandler(new UncaughtExceptionHandler(){

@Override

public void uncaughtException(Thread t, Throwable e) {

System.out.println("exception occured:"+e.getMessage());

}

});

22. What is Java Thread Dump, How can we get Java Thread dump of a Program?

Thread dump is list of all the threads active in the JVM, thread dumps are very helpful in analyzing bottlenecks in the application and analyzing deadlock situations. There are many ways using which we can generate Thread dump – Using Profiler, Kill -3 command, jstack tool etc. I prefer jstack tool to generate thread dump of a program because it’s easy to use and comes with JDK installation. Since it’s a terminal based tool, we can create script to generate thread dump at regular intervals to analyze it later on. Read this post to know more about generating thread dump in java.

23. What is Deadlock? How to analyze and avoid deadlock situation?

Deadlock is a programming situation where two or more threads are blocked forever, this situation arises with at least two threads and two or more resources.

To analyze a deadlock, we need to look at the java thread dump of the application, we need to look out for the threads with state as BLOCKED and then the resources it’s waiting to lock, every resource has a unique ID using which we can find which thread is already holding the lock on the object.

Avoid Nested Locks, Lock Only What is Required and Avoid waiting indefinitely are common ways to avoid deadlock situation, read this post to learn how

to analyze deadlock in java with sample program.

24. What is Java Timer Class? How to schedule a task to run after specific interval?

java.util.Timer is a utility class that can be used to schedule a thread to be executed at certain time in future. Java Timer class can be used to schedule a task to be run one-time or to be run at regular intervals.

java.util.TimerTask is an abstract class that implements Runnable interface and we need to extend this class to create our own TimerTask that can be scheduled using java Timer class.

Check this post for java Timer example.

25. What is Thread Pool? How can we create Thread Pool in Java?

A thread pool manages the pool of worker threads, it contains a queue that keeps tasks waiting to get executed.

A thread pool manages the collection of Runnable threads and worker threads execute Runnable from the queue.

java.util.concurrent.Executors provide implementation of java.util.concurrent.Executor interface to create the thread pool in java. Thread Pool Example program shows how to create and use Thread Pool in java. Or

read ScheduledThreadPoolExecutor Example to know how to schedule tasks after certain delay.

26. What will happen if we don’t override Thread class run() method?

Thread class run() method code is as shown below.

public void run() {

if (target != null) {

}

Above target set in the init() method of Thread class and if we create an instance of Thread class as new TestThread(), it’s set to null. So nothing will happen if we don’t override the run() method. Below is a simple example demonstrating this.

public class TestThread extends Thread {

//not overriding Thread.run() method

//main method, can be in other class too

public static void main(String args[]){ Thread t = new TestThread();

System.out.println("Before starting thread"); t.start();

System.out.println("After starting thread");

}

It will print only below output and terminate.

Java Concurrency Interview Questions and

Answers

1. What is atomic operation? What are atomic classes in Java Concurrency API?

Atomic operations are performed in a single unit of task without interference from other operations. Atomic operations are necessity in multi-threaded environment to avoid data inconsistency.

int++ is not an atomic operation. So by the time one threads read it’s value and increment it by one, other thread has read the older value leading to wrong result.

To solve this issue, we will have to make sure that increment operation on count is atomic, we can do that using Synchronization but Java 5 java.util.concurrent.atomic provides wrapper classes for int and long that can be used to achieve this atomically without usage of Synchronization. Go to this article to learn more about atomic concurrent classes.

2. What is Lock interface in Java Concurrency API? What are it’s benefits over synchronization?

Lock interface provide more extensive locking operations than can be obtained using synchronized methods and statements. They allow more flexible structuring, may have quite different properties, and may support multiple associated Condition objects.

The advantages of a lock are

· it’s possible to make them fair

· it’s possible to make a thread responsive to interruption while waiting on a Lock object.

· it’s possible to try to acquire the lock, but return immediately or after a timeout if the lock can’t be acquired

· it’s possible to acquire and release locks in different scopes, and in different orders

3. What is Executors Framework?

In Java 5, Executor framework was introduced with the java.util.concurrent.Executor interface.

The Executor framework is a framework for standardizing invocation, scheduling, execution, and control of asynchronous tasks according to a set of execution policies.

Creating a lot many threads with no bounds to the maximum threshold can cause application to run out of heap memory. So, creating a ThreadPool is a better solution as a finite number of threads can be pooled and reused. Executors

framework facilitate process of creating Thread pools in java. Check out this post to learn with example code to create thread pool using Executors framework.

4. What is BlockingQueue? How can we implement Producer-Consumer problem using Blocking Queue?

java.util.concurrent.BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

BlockingQueue doesn’t accept null values and throw NullPointerException if you try to store null value in the queue.

BlockingQueue implementations are thread-safe. All queuing methods are atomic in nature and use internal locks or other forms of concurrency control.

BlockingQueue interface is part of java collections framework and it’s primarily used for implementing producer consumer problem.

Check this post for producer-consumer problem implementation using BlockingQueue.

5. What is Callable and Future?

Java 5 introduced java.util.concurrent.Callable interface in concurrency package that is similar to Runnable interface but it can return any Object and able to throw Exception.

Callable interface use Generic to define the return type of Object. Executors class provide useful methods to execute Callable in a thread pool. Since callable tasks run in parallel, we have to wait for the returned Object. Callable tasks return java.util.concurrent.Future object. Using Future we can find out the status of the Callable task and get the returned Object. It provides get() method that can wait for the Callable to finish and then return the result.

Check this post for Callable Future Example.

6. What is FutureTask Class?

FutureTask is the base implementation class of Future interface and we can use it with Executors for asynchronous processing. Most of the time we don’t need to use FutureTask class but it comes real handy if we want to override some of the

methods of Future interface and want to keep most of the base implementation. We can just extend this class and override the methods according to our requirements. Check out Java FutureTask Example post to learn how to use it and what are different methods it has.

7. What are Concurrent Collection Classes?

Java Collection classes are fail-fast which means that if the Collection will be changed while some thread is traversing over it using iterator, the iterator.next() will throw ConcurrentModificationException.

Concurrent Collection classes support full concurrency of retrievals and adjustable expected concurrency for updates.

Major classes are ConcurrentHashMap, CopyOnWriteArrayList and CopyOnWriteArraySet, check this post to learn how to avoid ConcurrentModificationException when using iterator.

8. What is Executors Class?

Executors class provide utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes.

Executors class can be used to easily create Thread Pool in java, also this is the only class supporting execution of Callable implementations.

9. What are some of the improvements in Concurrency API in Java 8?

Some important concurrent API enhancements are:

· ConcurrentHashMap compute(), forEach(), forEachEntry(), forEachKey(), forEachValue(), merge(), reduce() and search() methods.

· CompletableFuture that may be explicitly completed (setting its value and status).

· Executors newWorkStealingPool() method to create a work-stealing thread pool using all available processors as its target parallelism level.

1. What are Collection related features in Java 8?

Java 8 has brought major changes in the Collection API. Some of the changes are:

1. Java Stream API for collection classes for supporting sequential as well as parallel processing

2. Iterable interface is extended with forEach() default method that we can use to iterate over a collection. It is very helpful when used with lambda expressions because it’s argument Consumer is a function interface.

3. Miscellaneous Collection API improvements such

as forEachRemaining(Consumer action)method in Iterator interface, Map replaceAll(), compute(), merge() methods.

2. What is Java Collections Framework? List out some benefits of Collections framework?

Collections are used in every programming language and initial java release contained few classes for collections: Vector, Stack, Hashtable, Array. But looking at the larger scope and usage, Java 1.2 came up with Collections Framework that group all the collections interfaces, implementations and algorithms.

Java Collections have come through a long way with usage of Generics and Concurrent Collection classes for thread-safe operations. It also includes blocking interfaces and their implementations in java concurrent package.

Some of the benefits of collections framework are;

· Reduced development effort by using core collection classes rather than implementing our own collection classes.

· Code quality is enhanced with the use of well tested collections framework classes.

· Reduced effort for code maintenance by using collection classes shipped with JDK.

· Reusability and Interoperability

3. What is the benefit of Generics in Collections Framework?

Java 1.5 came with Generics and all collection interfaces and implementations use it heavily. Generics allow us to provide the type of Object that a collection can contain, so if you try to add any element of other type it throws compile time error. This avoids ClassCastException at Runtime because you will get the error at compilation. Also Generics make code clean since we don’t need to use casting and instanceof operator. I would highly recommend to go through Java Generic Tutorial to understand generics in a better way.

4. What are the basic interfaces of Java Collections Framework?

Collection is the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Java platform doesn’t provide any direct implementations of this interface.

Set is a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the deck of cards.

List is an ordered collection and can contain duplicate elements. You can access any element from it’s index. List is more like array with dynamic length.

A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value.

Some other interfaces

are Queue, Dequeue, Iterator, SortedSet,

and ListIterator.

5. Why Collection doesn’t extend Cloneable and Serializable interfaces?

Collection interface specifies group of Objects known as elements. How the elements are maintained is left up to the concrete implementations of Collection. For example, some Collection implementations like List allow duplicate elements whereas other implementations like Set don’t.

A lot of the Collection implementations have a public clone method. However, it does’t really make sense to include it in all implementations of Collection. This is because Collection is an abstract representation. What matters is the implementation.

The semantics and the implications of either cloning or serializing come into play when dealing with the actual implementation; so concrete implementation should decide how it should be cloned or serialized, or even if it can be cloned or serialized.

So mandating cloning and serialization in all implementations is actually less flexible and more restrictive. The specific implementation should make the decision as to whether it can be cloned or serialized.

6. Why Map interface doesn’t extend Collection interface?

Although Map interface and it’s implementations are part of Collections Framework, Map are not collections and collections are not Map. Hence it doesn’t make sense for Map to extend Collection or vice versa.

If Map extends Collection interface, then where are the elements? Map contains key-value pairs and it provides methods to retrieve list of Keys or values as Collection but it doesn’t fit into the “group of elements” paradigm.

7. What is an Iterator?

Iterator interface provides methods to iterate over any Collection. We can get iterator instance from a Collection using iterator() method. Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection during the iteration. Java Collection iterator provides a generic way for traversal through the elements of a collection and implements Iterator Design Pattern.

8. What is difference between Enumeration and Iterator interface?

Enumeration is twice as fast as Iterator and uses very less memory. Enumeration is very basic and fits to basic needs. But Iterator is much safer as compared to Enumeration because it always denies other threads to modify the collection object which is being iterated by it.

Iterator takes the place of Enumeration in the Java Collections Framework. Iterators allow the caller to remove elements from the underlying collection that is not possible with Enumeration. Iterator method names have been improved to make it’s functionality clear.

9. Why there is not method like Iterator.add() to add elements to the collection?

The semantics are unclear, given that the contract for Iterator makes no guarantees about the order of iteration. Note, however, that ListIterator does provide an add operation, as it does guarantee the order of the iteration.

10. Why Iterator don’t have a method to get next element directly without moving the cursor?

It can be implemented on top of current Iterator interface but since it’s use will be rare, it doesn’t make sense to include it in the interface that everyone has to implement.

11. What is different between Iterator and ListIterator?

· We can use Iterator to traverse Set and List collections whereas ListIterator can be used with Lists only.

· Iterator can traverse in forward direction only whereas ListIterator can be used to traverse in both the directions.

· ListIterator inherits from Iterator interface and comes with extra functionalities like adding an element, replacing an element, getting index position for previous and next elements.

12. What are different ways to iterate over a list?

We can iterate over a list in two different ways – using iterator and using for-each loop.

List<String> strList = new ArrayList<>();

//using for-each loop for(String obj : strList){

System.out.println(obj);

}

//using iterator

Iterator<String> it = strList.iterator(); while(it.hasNext()){

String obj = it.next();

System.out.println(obj);

}

Using iterator is more thread-safe because it makes sure that if underlying list elements are modified, it will throw ConcurrentModificationException.

13. What do you understand by iterator fail-fast property?

Iterator fail-fast property checks for any modification in the structure of the underlying collection everytime we try to get the next element. If there are any modifications found, it throws ConcurrentModificationException. All the implementations of Iterator in Collection classes are fail-fast by design except the concurrent collection classes like ConcurrentHashMap and CopyOnWriteArrayList.

14. What is difference between fail-fast and fail- safe?

Iterator fail-safe property work with the clone of underlying collection, hence it’s not affected by any modification in the collection. By design, all the collection classes in java.util package are fail-fast whereas collection classes

in java.util.concurrent are fail-safe.

Fail-fast iterators throw ConcurrentModificationException whereas fail-safe iterator never throws ConcurrentModificationException.

Check this post for CopyOnWriteArrayList Example.

15. How to avoid ConcurrentModificationException while iterating a collection?

We can use concurrent collection classes to

avoid ConcurrentModificationException while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList.

Check this post for ConcurrentHashMap Example.

16. Why there are no concrete implementations of Iterator interface?

Iterator interface declare methods for iterating a collection but it’s implementation is responsibility of the Collection implementation classes. Every collection class that returns an iterator for traversing has it’s own Iterator implementation nested class.

This allows collection classes to chose whether iterator is fail-fast or fail-safe. For example ArrayList iterator is fail-fast whereas CopyOnWriteArrayList iterator is fail- safe.

17. What is UnsupportedOperationException?

UnsupportedOperationException is the exception used to indicate that the operation is not supported. It’s used extensively in JDK classes, in collections framework java.util.Collections.UnmodifiableCollection throws this exception for all add and removeoperations.

18. How HashMap works in Java?

HashMap stores key-value pair in Map.Entry static nested class implementation. HashMap works on hashing algorithm and uses hashCode() and equals() method in put and get methods.

When we call put method by passing key-value pair, HashMap uses Key hashCode() with hashing to find out the index to store the key-value pair. The Entry is stored in the LinkedList, so if there are already existing entry, it uses equals() method to check if the passed key already exists, if yes it overwrites the value else it creates a new entry and store this key-value Entry.

When we call get method by passing Key, again it uses the hashCode() to find the index in the array and then use equals() method to find the correct Entry and return it’s value. Below image will explain these detail clearly.

The other important things to know about HashMap are capacity, load factor, threshold resizing. HashMap initial default capacity is 16 and load factor is 0.75. Threshold is capacity multiplied by load factor and whenever we try to add an entry, if map size is greater than threshold, HashMap rehashes the contents of map into a new array with a larger capacity. The capacity is always power of 2, so if you know that you need to store a large number of key-value pairs, for example in caching data from database, it’s good idea to initialize the HashMap with correct capacity and load factor.

19. What is the importance of hashCode() and equals() methods?

HashMap uses Key object hashCode() and equals() method to determine the index to put the key-value pair. These methods are also used when we try to get value from HashMap. If these methods are not implemented correctly, two

different Key’s might produce same hashCode() and equals() output and in that case rather than storing it at different location, HashMap will consider them same and overwrite them.

Similarly all the collection classes that doesn’t store duplicate data use hashCode() and equals() to find duplicates, so it’s very important to implement them correctly. The implementation of equals() and hashCode() should follow these rules.

· If o1.equals(o2), then o1.hashCode() == o2.hashCode()should always be true.

· If o1.hashCode() == o2.hashCode is true, it doesn’t mean that o1.equals(o2) will be true.

20. Can we use any class as Map key?

We can use any class as Map Key, however following points should be considered before using them.

· If the class overrides equals() method, it should also override hashCode() method.

· The class should follow the rules associated with equals() and hashCode() for all instances. Please refer earlier question for these rules.

· If a class field is not used in equals(), you should not use it in hashCode() method.

· Best practice for user defined key class is to make it immutable, so that hashCode() value can be cached for fast performance. Also immutable classes make sure that hashCode() and equals() will not change in future that will solve any issue with mutability.

For example, let’s say I have a class MyKey that I am using for HashMap key.

· //MyKey name argument passed is used for equals() and hashCode()

· MyKey key = new MyKey("Pankaj"); //assume hashCode=1234

· myHashMap.put(key, "Value");

· // Below code will change the key hashCode() and equals()

· // but it's location is not changed.

· key.setName("Amit"); //assume new hashCode=7890

· //below will return null, because HashMap will try to look for key

· //in the same index as it was stored but since key is mutated,

· //there will be no match and it will return null.

· myHashMap.get(new MyKey("Pankaj"));

This is the reason why String and Integer are mostly used as HashMap keys.

21. What are different Collection views provided by Map interface?

Map interface provides three collection views:

0. Set<K> keySet(): Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

1. Collection<V> values(): Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator’s own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

2. Set<Map.Entry<K, V>> entrySet(): Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator’s own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

22. What is difference between HashMap and Hashtable?

HashMap and Hashtable both implements Map interface and looks similar, however there are following difference between HashMap and Hashtable.

0. HashMap allows null key and values whereas Hashtable doesn’t allow null key and values.

1. Hashtable is synchronized but HashMap is not synchronized. So HashMap is better for single threaded environment, Hashtable is suitable for multi- threaded environment.

2. LinkedHashMap was introduced in Java 1.4 as a subclass of HashMap, so incase you want iteration order, you can easily switch from HashMap to LinkedHashMap but that is not the case with Hashtable whose iteration order is unpredictable.

3. HashMap provides Set of keys to iterate and hence it’s fail-fast but Hashtable provides Enumeration of keys that doesn’t support this feature.

4. Hashtable is considered to be legacy class and if you are looking for modifications of Map while iterating, you should use ConcurrentHashMap.

23. How to decide between HashMap and TreeMap?

For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

24. What are similarities and difference between ArrayList and Vector?

ArrayList and Vector are similar classes in many ways.

0. Both are index based and backed up by an array internally.

1. Both maintains the order of insertion and we can get the elements in the order of insertion.

2. The iterator implementations of ArrayList and Vector both are fail-fast by design.

3. ArrayList and Vector both allows null values and random access to element using index number.

These are the differences between ArrayList and Vector.

4. Vector is synchronized whereas ArrayList is not synchronized. However if you are looking for modification of list while iterating, you should use CopyOnWriteArrayList.

5. ArrayList is faster than Vector because it doesn’t have any overhead because of synchronization.

6. ArrayList is more versatile because we can get synchronized list or read-only list from it easily using Collections utility class.

25. What is difference between Array and ArrayList? When will you use Array over ArrayList?

Arrays can contain primitive or Objects whereas ArrayList can contain only Objects.

Arrays are fixed size whereas ArrayList size is dynamic.

Arrays doesn’t provide a lot of features like ArrayList, such as addAll, removeAll, iterator etc.

Although ArrayList is the obvious choice when we work on list, there are few times when array are good to use.

· If the size of list is fixed and mostly used to store and traverse them.

· For list of primitive data types, although Collections use autoboxing to reduce the coding effort but still it makes them slow when working on fixed size primitive data types.

· If you are working on fixed multi-dimensional situation, using [][] is far more easier than List<List<>>

26. What is difference between ArrayList and LinkedList?

ArrayList and LinkedList both implement List interface but there are some differences between them.

0. ArrayList is an index based data structure backed by Array, so it provides random access to it’s elements with performance as O(1) but LinkedList stores data as list of nodes where every node is linked to it’s previous and next node. So even though there is a method to get the element using index, internally it traverse from start to reach at the index node and then return the element, so performance is O(n) that is slower than ArrayList.

1. Insertion, addition or removal of an element is faster in LinkedList compared to ArrayList because there is no concept of resizing array or updating index when element is added in middle.

2. LinkedList consumes more memory than ArrayList because every node in LinkedList stores reference of previous and next elements.

27. Which collection classes provide random access of it’s elements?

ArrayList, HashMap, TreeMap, Hashtable classes provide random access to it’s elements. Download java collections pdf for more information.

28. What is EnumSet?

java.util.EnumSet is Set implementation to use with enum types. All of the elements in an enum set must come from a single enum type that is specified, explicitly or implicitly, when the set is created. EnumSet is not synchronized and null elements are not allowed. It also provides some useful methods like copyOf(Collection c), of(E first, E… rest) and complementOf(EnumSet s).

Check this post for java enum tutorial.

29. Which collection classes are thread-safe?

Vector, Hashtable, Properties and Stack are synchronized classes, so they are thread-safe and can be used in multi-threaded environment. Java 1.5 Concurrent API included some collection classes that allows modification of collection while iteration because they work on the clone of the collection, so they are safe to use in multi-threaded environment.

30. What are concurrent Collection Classes?

Java 1.5 Concurrent package (java.util.concurrent) contains thread-safe collection classes that allow collections to be modified while iterating. By design Iterator implementation in java.utilpackages are fail-fast and throws ConcurrentModificationException. But Iterator implementation

in java.util.concurrent packages are fail-safe and we can modify the collection while iterating. Some of these classes

are CopyOnWriteArrayList, ConcurrentHashMap, CopyOnWriteArraySet. Read these posts to learn about them in more detail.

· Avoid ConcurrentModificationException

· CopyOnWriteArrayList Example

· HashMap vs ConcurrentHashMap

31. What is BlockingQueue?

BlockingQueue interface is part of java collections framework and it’s primarily used for implementing producer consumer problem. We don’t need to worry about

waiting for the space to be available for producer or object to be available for consumer in BlockingQueue as it’s handled by implementation classes of BlockingQueue.

Java provides several BlockingQueue implementations such as ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue etc.

Check this post for use of BlockingQueue for producer-consumer problem.

32. What is Queue and Stack, list their differences?

Both Queue and Stack are used to store data before processing

them. java.util.Queue is an interface whose implementation classes are present in java concurrent package. Queue allows retrieval of element in First-In-First-Out (FIFO) order but it’s not always the case. There is also Deque interface that allows elements to be retrieved from both end of the queue.

Stack is similar to queue except that it allows elements to be retrieved in Last-In- First-Out (LIFO) order.

Stack is a class that extends Vector whereas Queue is an interface.

33. What is Collections Class?

java.util.Collections is a utility class consists exclusively of static methods that operate on or return collections. It contains polymorphic algorithms that operate on collections, “wrappers”, which return a new collection backed by a specified collection, and a few other odds and ends.

This class contains methods for collection framework algorithms, such as binary search, sorting, shuffling, reverse etc.

34. What is Comparable and Comparator interface?

Java provides Comparable interface which should be implemented by any custom class if we want to use Arrays or Collections sorting methods. Comparable interface has compareTo(T obj) method which is used by sorting methods. We should override this method in such a way that it returns a negative integer, zero, or a positive integer if “this” object is less than, equal to, or greater than the object passed as argument.

But, in most real life scenarios, we want sorting based on different parameters. For example, as a CEO, I would like to sort the employees based on Salary, an HR would like to sort them based on the age. This is the situation where we need to use Comparator interface because Comparable.compareTo(Object o) method implementation can sort based on one field only and we can’t chose the field on which we want to sort the Object.

Comparator interface compare(Object o1, Object o2) method need to be implemented that takes two Object argument, it should be implemented in such a way that it returns negative int if first argument is less than the second one and returns zero if they are equal and positive int if first argument is greater than second one.

Check this post for use of Comparable and Comparator interface to sort objects.

35. What is difference between Comparable and Comparator interface?

Comparable and Comparator interfaces are used to sort collection or array of objects.

Comparable interface is used to provide the natural sorting of objects and we can use it to provide sorting based on single logic.

Comparator interface is used to provide different algorithms for sorting and we can chose the comparator we want to use to sort the given collection of objects.

36. How can we sort a list of Objects?

If we need to sort an array of Objects, we can use Arrays.sort(). If we need to sort a list of objects, we can use Collections.sort(). Both these classes have overloaded sort() methods for natural sorting (using Comparable) or sorting based on criteria (using Comparator).

Collections internally uses Arrays sorting method, so both of them have same performance except that Collections take sometime to convert list to array.

37. While passing a Collection as argument to a function, how can we make sure the function will not be able to modify it?

We can create a read-only collection

using Collections.unmodifiableCollection(Collection c)method before

passing it as argument, this will make sure that any operation to change the collection will throw UnsupportedOperationException.

38. How can we create a synchronized collection from given collection?

We can use Collections.synchronizedCollection(Collection c) to get a synchronized (thread-safe) collection backed by the specified collection.

39. What are common algorithms implemented in Collections Framework?

Java Collections Framework provides algorithm implementations that are commonly used such as sorting and searching. Collections class contain these method implementations. Most of these algorithms work on List but some of them are applicable for all kinds of collections.

Some of them are sorting, searching, shuffling, min-max values.

40. What is Big-O notation? Give some examples?

The Big-O notation describes the performance of an algorithm in terms of number of elements in a data structure. Since Collection classes are actually data structures, we usually tend to use Big-O notation to chose the collection implementation to use based on time, memory and performance.

Example 1: ArrayList get(index i) is a constant-time operation and doesn’t depend on the number of elements in the list. So it’s performance in Big-O notation is O(1).

Example 2: A linear search on array or list performance is O(n) because we need to search through entire list of elements to find the element.

41. What are best practices related to Java Collections Framework?

· Chosing the right type of collection based on the need, for example if size is fixed, we might want to use Array over ArrayList. If we have to iterate over the Map in order of insertion, we need to use TreeMap. If we don’t want duplicates, we should use Set.

· Some collection classes allows to specify the initial capacity, so if we have an estimate of number of elements we will store, we can use it to avoid rehashing or resizing.

· Write program in terms of interfaces not implementations, it allows us to change the implementation easily at later point of time.

· Always use Generics for type-safety and avoid ClassCastException at runtime.

· Use immutable classes provided by JDK as key in Map to avoid implementation of hashCode() and equals() for our custom class.

· Use Collections utility class as much as possible for algorithms or to get read-only, synchronized or empty collections rather than writing own implementation. It will enhance code-reuse with greater stability and low maintainability.

42. What is Java Priority Queue?

PriorityQueue is an unbounded queue based on a priority heap and the elements are ordered in their natural order or we can provide Comparator for ordering at the time of creation. PriorityQueue doesn’t allow null values and we can’t add any object that doesn’t provide natural ordering or we don’t have any comparator for them for ordering. Java PriorityQueue is not thread-safe and provided O(log(n)) time for enqueing and dequeing operations. Check this post for java priority queue example.

43. Why can’t we write code as List<Number> numbers = new ArrayList<Integer>();?

Generics doesn’t support sub-typing because it will cause issues in achieving type safety. That’s why List<T> is not considered as a subtype of List<S> where S is the super-type of T. To understanding why it’s not allowed, let’s see what could have happened if it has been supported.

List<Long> listLong = new ArrayList<Long>(); listLong.add(Long.valueOf(10));

List<Number> listNumbers = listLong; // compiler error listNumbers.add(Double.valueOf(1.23));

As you can see from above code that IF generics would have been supporting sub-typing, we could have easily add a Double to the list of Long that would have caused ClassCastException at runtime while traversing the list of Long.

44. Why can’t we create generic array? or write code as List<Integer>[] array = new ArrayList<Integer>[10];

We are not allowed to create generic arrays because array carry type information of it’s elements at runtime. This information is used at runtime to

throw ArrayStoreException if elements type doesn’t match to the defined type. Since generics type information gets erased at compile time by Type Erasure, the array store check would have been passed where it should have failed. Let’s understand this with a simple example code.

List<Integer>[] intList = new List<Integer>[5]; // compile error

Object[] objArray = intList;

List<Double> doubleList = new ArrayList<Double>(); doubleList.add(Double.valueOf(1.23));

objArray[0] = doubleList; // this should fail but it would pass because at runtime intList and doubleList both are just List

Arrays are covariant by nature i.e S[] is a subtype of T[] whenever S is a subtype of T but generics doesn’t support covariance or sub-typing as we saw in last question. So if we would have been allowed to create generic arrays, because of type erasure we would not get array store exception even though both types are not related.

1. What is Exception in Java?

Exception is an error event that can happen during the execution of a program and disrupts it’s normal flow. Exception can arise from different kind of situations such as wrong data entered by user, hardware failure, network connection failure etc.

Whenever any error occurs while executing a java statement, an exception object is created and then JRE tries to find exception handler to handle the exception. If suitable exception handler is found then the exception object is passed to the handler code to process the exception, known as catching the exception. If no handler is found then application throws the exception to runtime environment and JRE terminates the program.

Java Exception handling framework is used to handle runtime errors only, compile time errors are not handled by exception handling framework.

2. What are the Exception Handling Keywords in Java?

There are four keywords used in java exception handling.

1. throw: Sometimes we explicitly want to create exception object and then throw it to halt the normal processing of the program. throw keyword is used to throw exception to the runtime to handle it.

2. throws: When we are throwing any checked exception in a method and not handling it, then we need to use throws keyword in method signature to let caller program know the exceptions that might be thrown by the method. The caller method might handle these exceptions or propagate it to it’s caller method using throws keyword. We can provide multiple exceptions in the throws clause and it can be used with main() method also.

3. try-catch: We use try-catch block for exception handling in our code. try is the start of the block and catch is at the end of try block to handle the exceptions. We can have multiple catch blocks with a try and try-catch block can be nested also. catch block requires a parameter that should be of type Exception.

4. finally: finally block is optional and can be used only with try-catch block. Since exception halts the process of execution, we might have some resources open that will not get closed, so we can use finally block. finally block gets executed always, whether exception occurrs or not.

3. Explain Java Exception Hierarchy?

Java Exceptions are hierarchical and inheritance is used to categorize different types of exceptions. Throwable is the parent class of Java Exceptions Hierarchy and it has two child objects – Error and Exception. Exceptions are further divided into checked exceptions and runtime exception.

Errors are exceptional scenarios that are out of scope of application and it’s not possible to anticipate and recover from them, for example hardware failure, JVM crash or out of memory error.

Checked Exceptions are exceptional scenarios that we can anticipate in a program and try to recover from it, for example FileNotFoundException. We should catch this exception and provide useful message to user and log it properly for debugging purpose. Exception is the parent class of all Checked Exceptions.

Runtime Exceptions are caused by bad programming, for example trying to retrieve an element from the Array. We should check the length of array first before trying to retrieve the element otherwise it might

throw ArrayIndexOutOfBoundException at runtime. RuntimeException is the parent class of all runtime exceptions.

4. What are important methods of Java Exception Class?

Exception and all of it’s subclasses doesn’t provide any specific methods and all of the methods are defined in the base class Throwable.

1. String getMessage() – This method returns the message String of Throwable and the message can be provided while creating the exception through it’s constructor.

2. String getLocalizedMessage() – This method is provided so that subclasses can override it to provide locale specific message to the calling program. Throwable class implementation of this method simply

use getMessage() method to return the exception message.

3. synchronized Throwable getCause() – This method returns the cause of the exception or null id the cause is unknown.

4. String toString() – This method returns the information about Throwable in String format, the returned String contains the name of Throwable class and localized message.

5. void printStackTrace() – This method prints the stack trace information to the standard error stream, this method is overloaded and we can pass PrintStream or PrintWriter as argument to write the stack trace information to the file or stream.

5. Explain Java 7 ARM Feature and multi-catch block?

If you are catching a lot of exceptions in a single try block, you will notice that catch block code looks very ugly and mostly consists of redundant code to log the error, keeping this in mind Java 7 one of the feature was multi-catch block where we can catch multiple exceptions in a single catch block. The catch block with this feature looks like below:

Most of the time, we use finally block just to close the resources and sometimes we forget to close them and get runtime exceptions when the resources are exhausted. These exceptions are hard to debug and we might need to look into each place where we are using that type of resource to make sure we are closing it. So java 7 one of the improvement was try-with-resources where we can create a resource in the try statement itself and use it inside the try-catch block. When the execution comes out of try-catch block, runtime environment automatically close these resources. Sample of try-catch block with this improvement is:

6. What is difference between Checked and Unchecked Exception in Java?

1. Checked Exceptions should be handled in the code using try-catch block or else method should use throws keyword to let the caller know about the checked exceptions that might be thrown from the method. Unchecked Exceptions are not required to be handled in the program or to mention them in throws clause of the method.

2. Exception is the super class of all checked exceptions

whereas RuntimeException is the super class of all unchecked exceptions. Note that RuntimeException is the child class of Exception.

3. Checked exceptions are error scenarios that requires to be handled in the code, or else you will get compile time error. For example, if you use FileReader to read a file, it throws FileNotFoundException and we must catch it in the try-catch block or throw it again to the caller method. Unchecked exceptions are mostly caused by poor programming, for example NullPointerException when invoking a method on an object reference without making sure that it’s not null. For example, I can write a method to remove all the vowels from the string. It’s the caller responsibility to make sure not to pass null string. I might change the method to handle these scenarios but ideally the caller should take care of this.

7. What is difference between throw and throws keyword in Java?

throws keyword is used with method signature to declare the exceptions that the method might throw whereas throw keyword is used to disrupt the flow of program and handing over the exception object to runtime to handle it.

8. How to write custom exception in Java?

We can extend Exception class or any of it’s subclasses to create our custom exception class. The custom exception class can have it’s own variables and methods that we can use to pass error codes or other exception related information to the exception handler.

A simple example of custom exception is shown below.

9. What is OutOfMemoryError in Java?

OutOfMemoryError in Java is a subclass of java.lang.VirtualMachineError and it’s thrown by JVM when it ran out of heap memory. We can fix this error by providing more memory to run the java application through java options.

$>java MyProgram -Xms1024m -Xmx1024m -XX:PermSize=64M - XX:MaxPermSize=256m

10. What are different scenarios causing “Exception in thread main”?

Some of the common main thread exception scenarios are:

· Exception in thread main java.lang.UnsupportedClassVersionError: This exception comes when your java class is compiled from another JDK version and you are trying to run it from another java version.

· Exception in thread main java.lang.NoClassDefFoundError: There are two variants of this exception. The first one is where you provide the class full name with .class extension. The second scenario is when Class is not found.

· Exception in thread main java.lang.NoSuchMethodError: main: This exception comes when you are trying to run a class that doesn’t have main method.

· Exception in thread “main” java.lang.ArithmeticException: Whenever any exception is thrown from main method, it prints the exception is console. The first part explains that exception is thrown from main method, second part prints the exception class name and then after a colon, it prints the exception message.

Read more about these at Java Exception in Thread main.

11. What is difference between final, finally and finalize in Java?

final and finally are keywords in java whereas finalize is a method.

final keyword can be used with class variables so that they can’t be reassigned, with class to avoid extending by classes and with methods to avoid overriding by subclasses, finally keyword is used with try-catch block to provide statements that will always gets executed even if some exception arises, usually finally is used to close resources. finalize() method is executed by Garbage Collector before the object is destroyed, it’s great way to make sure all the global resources are closed.

Out of the three, only finally is related to java exception handling.

12. What happens when exception is thrown by main method?

When exception is thrown by main() method, Java Runtime terminates the program and print the exception message and stack trace in system console.

13. Can we have an empty catch block?

We can have an empty catch block but it’s the example of worst programming. We should never have empty catch block because if the exception is caught by that block, we will have no information about the exception and it wil be a nightmare to debug it. There should be at least a logging statement to log the exception details in console or log files.

14. Provide some Java Exception Handling Best Practices?

Some of the best practices related to Java Exception Handling are:

· Use Specific Exceptions for ease of debugging.

· Throw Exceptions Early (Fail-Fast) in the program.

· Catch Exceptions late in the program, let the caller handle the exception.

· Use Java 7 ARM feature to make sure resources are closed or use finally block to close them properly.

· Always log exception messages for debugging purposes.

· Use multi-catch block for cleaner close.

· Use custom exceptions to throw single type of exception from your application API.

· Follow naming convention, always end with Exception.

· Document the Exceptions Thrown by a method using @throws in javadoc.

· Exceptions are costly, so throw it only when it makes sense. Else you can catch them and provide null or empty response.

Read more about them in detail at Java Exception Handling Best Practices.

15. What is the problem with below programs and how do we fix it?

In this section, we will look into some programming questions related to java exceptions.

0. What is the problem with below program?

Above program won’t compile and you will get error message as “The exception FileNotFoundException is already caught by the alternative IOException”. This is because FileNotFoundException is subclass of IOException, there are two ways to solve this problem.

First way is to use single catch block for both the exceptions.

Another way is to remove the FileNotFoundException from multi-catch block. You can chose any of these approach based on your catch block code.

1. What is the problem with below program?

The program won’t compile because FileNotFoundException is subclass of IOException, so the catch block of FileNotFoundException is unreachable and you will get error message as “Unreachable catch block for FileNotFoundException. It is already handled by the catch block for IOException”.

You need to fix the catch block order to solve this issue. Notice that JAXBException is not related to IOException or

FileNotFoundException and can be put anywhere in above catch block

hierarchy.

2. What is the problem with below program?

The program won’t compile because JAXBException is a checked exception and foo() method should throw this exception to catch in the calling method. You will get error message as “Unreachable catch block for JAXBException. This exception is never thrown from the try statement body”.

To solve this issue, you will have to remove the catch block of JAXBException.

Notice that catching NullPointerException is valid because it’s an unchecked exception.

3. What is the problem with below program?

This is a trick question, there is no problem with the code and it will compile successfully. We can always catch Exception or any unchecked exception even if it’s not in the throws clause of the method.

Similarly if a method (foo) declares unchecked exception in throws clause, it is not mandatory to handle that in the program.

4. What is the problem with below program?

The above program won’t compile because start() method signature is not same in subclass. To fix this issue, we can either change the method singnature in subclass to be exact same as superclass or we can remove throws clause from subclass method as shown below.

5. What is the problem with below program?

The above program won’t compile because exception object in multi-catch block is final and we can’t change it’s value. You will get compile time error as “The parameter e of a multi-catch block cannot be assigned”.

We have to remove the assignment of “e” to new exception object to solve this error.

Java Intermediate Questions