Core Java Interview Questions – Part 2

In java interview questions series: part 1, we discussed some important questions which interviewer ask most of the time. Now is the time to take that discussion forward. In this post, I will talk about below given list of questions.

Why finalize() method should be avoided?
Why HashMap should not be used in multithreaded environment? Can it cause infinite loop as well?
Explain abstraction and encapsulation? How are they related?
Difference between interfaces and abstract classes?
How StringBuffer save the memory?
Why wait and notify is declared in Object class instead of Thread ?
Write Java program to create deadlock in Java and fix it ?
What happens if your Serializable class contains a member which is not  serializable? How do you fix it?
Explain transient and volatile keywords in java?
Difference between Iterator and ListIterator?

Why finalize() method should be avoided?

We all know the basic statement that finalize() method is called by garbage collector thread before reclaiming the memory allocated to the object. See this program which prove that finalize() invocation is not guaranteed at all. Other reasons can be:

  1. 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.
  2. 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 your log files. So bad, isn’t it?
  3. Also, There is some performance penalty when finalize() in included in your class. In Effective java (2nd edition ) Joshua bloch says,

    “Oh, and one more thing: there is a severe performance penalty for using finalizers. On my machine, the time to create and destroy a simple object is about 5.6 ns. Adding a finalizer increases the time to 2,400 ns. In other words, it is about 430 times slower to create and destroy objects with finalizers.”

Why HashMap should not be used in multithreaded environment? Can it cause infinite loop as well?

We know that 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 reasons e.g. to avoid dirty reads and to maintain data consistency. In worst case, this mutithreaded environment can result in infinite loop as well.

Yes, it is true. HashMap.get() can cause an infinite loop. Lets see how??

If you look at the source code HashMap.get(Object key) method, it looks like this:

public Object get(Object key) {
    Object k = maskNull(key);
    int hash = hash(k);
    int i = indexFor(hash, table.length);
    Entry e = table[i];
    while (true) {
        if (e == null)
            return e;
        if (e.hash == hash && eq(k, e.key))
            return e.value;
        e =;

while(true){...} can always be a victim of infinite loop at runtime in multithreaded environment, IF, somehow can point to itself. This will result in infinite loop. But, how will point to itself (i.e.).

This can happen in void transfer(Entry[] newTable) method, which is invoked at time the HashMap resizing is done.

do {
    Entry next =;
    int i = indexFor(e.hash, newCapacity); = newTable[i];
    newTable[i] = e;
    e = next;
} while (e != null);

This piece of code is prone to produce above condition, if resizing happen and at the same time other threads tried to modify the map instance.

Only way to avoid this scenario is to use synchronization in code, or better, use synchronized collection.

Explain abstraction and encapsulation? How are they related?


Abstraction captures only those details about an object that are relevant to the current perspective.

In object-oriented programming theory, abstraction involves the facility to define objects that represent abstract “actors” that can perform work, report on and change their state, and “communicate” with other objects in the system.

Abstraction in any programming language works in many ways. It can be seen from creating subroutines to defining interfaces for making low level language calls. Some abstractions try to limit the breadth of concepts a programmer needs, by completely hiding the abstractions they in turn are built on, e.g. design patterns.

Typically abstraction can be seen in two ways:

Data abstraction is the way to create complex data types and exposing only meaningful operations to interact with data type, where as hiding all the implementation details from outside works.

Control abstraction is the process of identifying all such statements and expose them as a unit of work. We normally use this feature when we create a function to perform any work.


Wrapping data and methods within classes in combination with implementation hiding (through access control) is often called encapsulation. The result is a data type with characteristics and behaviors. Encapsulation essentially has both i.e. information hiding and implementation hiding.

Whatever changes, encapsulate it“. It has been quoted as a famous design principle. For that matter in any class, changes can happen in data in runtime and changes in implementation can happen in future releases. So, encapsulation applies to both i.e. data as well as implementation.

SO, they can relate like following :

– Abstraction is more about ‘What‘ a class can do. [Idea]
– Encapsulation is more about ‘How‘ to achieve that functionality. [Implementation]

Difference between interfaces and abstract classes?

Basic differences between interfaces and abstract class can be counted as follows:

  • An interface cannot have any methods, whereas an abstract class can[Not true after java 8 default methods]
  • A class can implement many interfaces but can have only one superclass (abstract or not)
  • An interface is not part of the class hierarchy. Unrelated classes can implement the same interface

You should remember that : “When you can fully describe the concept in terms of “what it does” without needing to specify any of “how it does“, then you should use an interface. If you need to include some implementation details, then you will need to represent your concept in an abstract class.”

Also, if i talk differently : Are there many classes that can be “grouped together” and described by one noun? If so, have an abstract class by the name of this noun, and inherit the classes from it. For example Cat and Dog can both inherit from abstract class Animal, and this abstract base class will implement a method void breathe() which all animals will thus do in exactly the same fashion.

What kinds of verbs can be applied to my class, that might in general also be applied to others? Create an interface for each of these verbs. For example, All animals can be fed, so I will create an interface called IFeedable and have Animal implement that. Only Dog and Horse are nice enough though to implement ILikeable, but some are not.

As said by someone: the main difference is where you want your implementation. By creating an interface, you can move your implementation to any class that implements your interface. By creating an abstract class, you can share implementation for all derived classes in one central place, and avoid lots of bad things like code duplication.

How StringBuffer save the memory?

A String is implemented as an immutable object; that is, when you initially decide to put something into a String object, the JVM allocates a fixed-width array of exactly the size of your initial value. This is then treated as a constant inside the JVM, which allows for very significant performance savings in the case where the String’s value is not changed. However, if you decide to change the String’s contents in any way, what the JVM then essentially does is copy the contents of the original String into a temporary space, make your changes, then save those changes into a whole new memory array. Thus, making changes to a String’s value after initialization is a fairly expensive operation.

StringBuffer, on the other hand, is implemented as a dynamically – growable array inside the JVM, which means that any change operation can occur on the existing memory location, with new memory allocated only as-needed. However, there is no opportunity for the JVM to make optimizations around the StringBuffer, since its contents are assumed to be changeable at any instance.

Why wait and notify is declared in Object class instead of Thread?

Wait, notify, notifyAll methods are only required when you want your threads to access a shared resource and a shared resource could be any java object which is on the heap. So, these methods are defined on the core Object class so that each object has control of allowing threads to wait on it’s monitor. Java doesn’t have any special object which is used for sharing a common resource. No such data structure is defined.So, onus is given on the Object class to be able to become shared resource providing it will helper methods like wait(),notify() and notifyAll().

Java is based on Hoare‘s monitors idea. In Java all object has a monitor. Threads waits on monitors so, to perform a wait, we need 2 parameters:

– a Thread
– a monitor (any object)

In the Java design, the thread can not be specified, it is always the current thread running the code. However, we can specify the monitor (which is the object we call wait on). This is a good design, because if we could make any other thread to wait on a desired monitor, this would lead to an “intrusion”, posing difficulties on designing /programming concurrent programs. Remember that in Java all operations that are intrusive in another thread’s execution are deprecated (e.g. stop()).

Write Java program to create deadlock in Java and fix it ?

In java, a deadlock is a situation where minimum two threads are holding lock on some different resource, and both are waiting for other resource to complete its task. And, none is able to leave the lock on resource it is holding.

Read More: Writing a deadlock and resolving in java

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

In this case, NotSerializableException will be thrown at runtime. To fix this issue, a very simple solution is to mark such fields transient. It means these fields will not be serialized. If you want to save the state of these fields as well then you should consider reference variables which already implements Serializable interface.

You also might need to use readResolve() and writeResolve() methods. Lets summarize this:

  • First, make your non-serialisable field transient.
  • In writeObject(), first call defaultWriteObject() on the stream to store all the non-transient fields, then call other methods to serialize the individual properties of your non-serializable object.
  • In readObject(), first call defaultReadObject() on the stream to read back all the non-transient fields, then call other methods (corresponding to the ones you added to writeObject) to deserialise your non-serializable object.

Also, I will highly recommend to read full guide on serialization in java.

Explain transient and volatile keywords in java?


The transient keyword in Java is used to indicate that a field should not be serialized.” According to language specification: Variables may be marked transient to indicate that they are not part of the persistent state of an object. For example, you may have fields that are derived from other fields, and should only be done so programmatically, rather than having the state be persisted via serialization.

For example, in class fields like principal and rate can be serialized while interest can be calculated any time even after de-serialization.

If we recall, each thread in java has its own local memory space as well and it does all read/write operations in its local memory. Once all operations are done, it write back the modified state of variable in main memory from where all threads access this variable. Normally, this is the default flow inside JVM. But, the volatile modifier tells the JVM that a thread accessing the variable must always reconcile its own private copy of the variable with the master copy in memory. It means every time thread want to read the state of variable, it must flush its local memory state and update the variable from main memory.


volatile is most useful in lock-free algorithms. You mark the variable holding shared data as volatile when you are not using locking to access that variable and you want changes made by one thread to be visible in another, or you want to create a “happens-after” relation to ensure that computation is not re-ordered, again, to ensure changes become visible at the appropriate time.

The volatile should be used to safely publish immutable objects in a multi-threaded Environment. Declaring a field like public volatile ImmutableObject foo secures that all threads always see the currently available instance reference.

Difference between Iterator and ListIterator?

We can use Iterator to traverse a Set or a List or a Map. But ListIterator can only be used to traverse a List only. Other differences can be listed as below.

You can –

  1. iterate backwards.
  2. obtain the index at any point.
  3. add a new value at any point.
  4. set a new value at that point.


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

ListIterator<String> listIterator = names.listIterator();

//Add a value at any place using ListIterator

listIterator = names.listIterator();

//Set a value at any place using ListIterator


[Alex, Bob, Charles]
[Alex, Lokesh, Bob, Lokesh, Charles, Lokesh]
[John, John, John, John, John, John]

Clearly, I am able to add elements at random places in list while iterating over it – and similarily I can change any element as well. Using Iterator, it is not possible.

Happy Learning !!

Was this post helpful?

Join 7000+ Fellow Programmers

Subscribe to get new post notifications, industry updates, best practices, and much more. Directly into your inbox, for free.

16 thoughts on “Core Java Interview Questions – Part 2”

  1. I want to solve one scenario. For ex: assume 50 classes(C1 to C50) implements interface I1 and provide imlementations for method m1. Now if in future m2 method is needed but at one condition that only 25 classes (C1 to C25) need to provide implementation for m2 and other remaining 25 must not.
    how can we achieve.

  2. Hey Lokesh!

    Very helpful blog! Thank you! One suggestion is that maybe volatile and java memory model should be covered in a lot more details – there are many questions and concepts around volatile and JMM

  3. Thanks for your wonderful blog!

    I’ve one doubt regarding HashMap.get() can cause an infinite loop? Currently, I believe this issue is fixed in recent J2SE version 8. Below code snippet from the HashMap Class

        public V get(Object key) {
            Node<K,V> e;
            return (e = getNode(hash(key), key)) == null ? null : e.value;

    Please correct me I’m wrong

  4. really help to understand concept better.. Thanks a lot.
    Could you please share knowledge on Multithreading,Executor framework .

  5. For listIterator, the below 2 points were shared:
    add a new value at any point.
    set a new value at that point.

    Could you explain this please?

  6. Hi Lokesh,

    Under “Difference between Iterator and ListIterator?” you mentioned 4 points, but it’s not clear whether they are applicable for Iterator or ListIterator.


  7. I am unable to understand the Control and Data Abstraction that you have mentioned in the discussion about Abstraction and Encapsulation. Can you please elaborate?


Leave a Comment


A blog about Java and its related technologies, the best practices, algorithms, interview questions, scripting languages, and Python.