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1) How to create a immutable object in Java and what are its benefits?

Ans: Setter methods are meant to change the state of object, don’t provide “setter” methods or methods that modify fields or objects referred to by fields.

• Make all fields final and private so that they will not be accessible outside the class. Making them final will ensure the even accidentally you can not change them.
• Declare the class as final. Final classes in java can not be overridden.
• Instance variables will be either mutable or immutable. Return new objects with copied content for all mutable objects.

Benefits of immutable class are given below;

• They are simple to construct, test, and use
• They are automatically thread-safe and have no synchronization issues
• They do not need a copy constructor
• They do not need an implementation of clone
• They allow hashCode to use lazy initialization, and to cache its return value
• They do not need to be copied defensively when used as a field
• They make good Map keys and Set elements

2) Explain Java 8 new features.

Ans: The most important Java 8 features are;

• Lambda expressions gives the ability to pass a functionality as a method argument. Lambda expression help us reduce the code clutter in using a single method class. For example, when we have to associate an action button click to a functionality, then lambda expressions will make the code look neat and compact.
• Method references provide easy-to-read lambda expressions for methods that already have a name.
• Default methods enable new functionality to be added to the interfaces of libraries and ensure binary compatibility with code written for older versions of those interfaces.
• Repeating annotations provide the ability to apply the same annotation type more than once to the same declaration or type use.
• Type annotations can be applied wherever a type is used like in new instance creates, exception throws clause etc. This will help to enforce stronger type checks and using this feature we can come up with a type checking framework itself.

3) Why should we avoid finalize() method ?

Ans: The various reasons are

• The finalize() method is called by garbage collector thread before reclaiming the memory allocated to the object. So finalize() invocation is not guaranteed at all.
• finalize() methods do not work in chaining like constructors. It means like when you call a constructor then constructors of all super classes will be invokes implicitly. But, in case of finalize methods, this is not followed. Super class’s finalize() should be called explicitly.
• Any Exception thrown by finalize method is ignored by GC thread and it will not be propagated further, in fact it will not be logged in the log files.

4) What is the difference between the Boolean & operator and the && operator?

Ans: If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.

5)What happens if your Serializable class contains a member which is not serializable? How can you fix it?

Ans: when serializable class contains a member which is not serializable, NotSerializableException will be thrown at runtime. To fix this issue, we can do the following;

• We have to mark such field as transient. It means this field will not be serialized. If you want to save the state of this field then a reference variables which already implements Serializable interface should be considered.
• In writeObject(), call defaultWriteObject() on the stream to store all the non-transient fields, then call other methods to serialize the individual properties of non-serializable object.
• In readObject(), call defaultReadObject() on the stream to read back all the non-transient fields, then call other methods corresponding to the ones added to writeObject to deserialise non-serializable object.

6) What is the difference between Iterator and ListIterator?

Ans: We use Iterator to traverse a Set or a List or a Map. But ListIterator can only be used to traverse a List only. With ListIterator we can –

• iterate backwards.
• obtain the index at any point.
• add a new value at any point.
• set a new value at that point.

7) What is synchronization? Explain Class level locking and object level locking?

Ans: Synchronization has reference to multi-threading. With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object’s value. This often leads to significant errors.A synchronized block of code can only be executed by one thread at a time. Java supports multiple threads to be executed. This may cause two or more threads to access the same fields or objects. Synchronization is a process which keeps all concurrent threads in execution to be in sync. Synchronization avoids memory consistence errors caused due to inconsistent view of shared memory. When a method is declared as synchronized; the thread holds the monitor for that method’s object If another thread is executing the synchronized method, thread is blocked until that thread releases the monitor.

Synchronization in java is achieved using synchronized keyword. You can use synchronized keyword in your class on defined methods or blocks. Keyword can not be used with variables or attributes in class definition.

• Object level locking is mechanism when you want to synchronize a non-static method or non-static code block such that only one thread will be able to execute the code block on given instance of the class. This should always be done to make instance level data thread safe.
• Class level locking prevents multiple threads to enter in synchronized block in any of all available instances on runtime. This means if at runtime there are 100 instances of SampleClass, then only one thread will be able to execute sampleMethod() in any one of instance at a time, and all other instances will be locked for other threads. This should always be done to make static data thread safe.

8) What is garbage collection? Can we enforce it?

Ans: Garbage collection is an automatic memory management feature in Java and is run within the JVM. A Garbage collection has two purposes;

• any unused memory should be freed.
• no memory should be freed unless the program will not use it anymore.

We can not force garbage collection. If we call System.gc() it is simply a hint to the garbage collector that you want it to do a collection. There is no way to force as the garbage collector is non-deterministic. Also, under the documentation for OutOfMemoryError it declares that it will not be thrown unless the VM has failed to reclaim memory following a full garbage collection. So if you keep allocating memory until you get the error, you will have already forced a full garbage collection.

9) Why HashMap should not be used in multithreaded environment?

Ans: HashMap is non-synchronized collection where as its synchronized counter-part is HashTable. So, when you are accessing the collection in multithreaded environment and all threads are accessing a single instance of collection, then its safer to use HashTable for various obvious reason to maintain data consistency. In worst case, this mutithreaded environment can result in infinite loop as well.

10) What is the use of the finally block? Is it guaranteed to be called? When it will be NOT called?

Ans: The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated. It will not allow having cleanup code accidentally bypassed by a return, continue, or break.

In following situations finally block is not called;

• If the JVM exits while the try or catch code is being executed.
• If the thread executing the try or catch code is interrupted or killed.