Commonly Asked Java Programming Interview Questions | Set 2

In this article, some of the most important Java Interview Questions and Answers are discussed, to give you the cutting edge in your interviews. Java is one of the most popular and widely used programming language and platform. Java is fast, reliable and secure. From desktop to web applications, scientific supercomputers to gaming consoles, cell phones to the Internet, Java is used in every nook and corner. This is why you should master these questions if you want to stand out.

Let us start by taking a look at some of the most frequently asked Java interview questions,

So let’s get started with the set of Java Interview Questions.

Q1. Can we Overload or Override static methods in java?



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Q2. Why the main method is static in java?
The method is static because otherwise there would be ambiguity: which constructor should be called? Especially if your class looks like this:

public class JavaClass
{
  protected JavaClass(int x)
  {   }
   public void main(String[] args) 
   {

    }
}

Should the JVM call new JavaClass(int)? What should it pass for x? If not, should the JVM instantiate JavaClass without running any constructor method? because that will special-case your entire class – sometimes you have an instance that hasn’t been initialized, and you have to check for it in every method that could be called. There are just too many edge-cases and ambiguities for it to make sense for the JVM to have to instantiate a class before the entry point is called. That’s why main is static.

 

Q3. What happens if you remove static modifier from the main method?
Program compiles successfully. But at runtime throws an error “NoSuchMethodError”.

 

Q4. What is the scope of variables in Java in following cases? 



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Q5. What is “this” keyword in java?
Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.
Usage of this keyword

 

Q6. What is an abstract class? How abstract classes are similar or different in Java from C++?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.

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Q7. Which class is the superclass for every class?
Object class

 

Q8. Can we overload main() method?
The main method in Java is no extra-terrestrial method. Apart from the fact that main() is just like any other method & can be overloaded in a similar manner, JVM always looks for the method signature to launch the program.



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Q9. What is object cloning?
Object cloning means to create an exact copy of the original object. If a class needs to support cloning, it must implement java.lang.Cloneable interface and override clone() method from Object class. Syntax of the clone() method is :

protected Object clone() throws CloneNotSupportedException

If the object’s class doesn’t implement Cloneable interface then it throws an exception ‘CloneNotSupportedException’ .

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Q10. How is inheritance in C++  different from Java?

  1. In Java, all classes inherit from the Object class directly or indirectly. Therefore, there is always a single inheritance tree of classes in Java, and Object class is the root of the tree.
  2. In Java, members of the grandparent class are not directly accessible. See this G-Fact for more details.
  3. The meaning of protected member access specifier is somewhat different in Java. In Java, protected members of a class “A” are accessible in other class “B” of the same package, even if B doesn’t inherit from A (they both have to be in the same package).
  4. Java uses extends keyword for inheritance. Unlike C++, Java doesn’t provide an inheritance specifier like public, protected or private. Therefore, we cannot change the protection level of members of the base class in Java, if some data member is public or protected in base class then it remains public or protected in the derived class. Like C++, private members of base class are not accessible in the derived class.
    Unlike C++, in Java, we don’t have to remember those rules of inheritance which are the combination of base class access specifier and inheritance specifier.
  5. In Java, methods are virtual by default. In C++, we explicitly use virtual keyword. See this G-Fact for more details.
  6. Java uses a separate keyword interface for interfaces, and abstract keyword for abstract classes and abstract functions.
  7. Unlike C++, Java doesn’t support multiple inheritance. A class cannot inherit from more than one class. A class can implement multiple interfaces though.
  8. In C++, default constructor of parent class is automatically called, but if we want to call parametrized constructor of a parent class, we must use Initializer list. Like C++, default constructor of the parent class is automatically called in Java, but if we want to call parameterized constructor then we must use super to call the parent constructor.

See examples here

 

Q11. Why method overloading is not possible by changing the return type in java?
In C++ and Java, functions can not be overloaded if they differ only in the return type. The return type of functions is not a part of the mangled name which is generated by the compiler for uniquely identifying each function. The No of arguments, Type of arguments & Sequence of arguments are the parameters which are used to generate the unique mangled name for each function. It is on the basis of these unique mangled names that compiler can understand which function to call even if the names are same(overloading).



 

Q12. Can we override private methods in Java?
No, a private method cannot be overridden since it is not visible from any other class. Read more

 

Q13. What is blank final variable?
A final variable in Java can be assigned a value only once, we can assign a value either in declaration or later.

    final int i = 10;
    i = 30; // Error because i is final.

A blank final variable in Java is a final variable that is not initialized during declaration. Below is a simple example of blank final.

    // A simple blank final example 
    final int i;
    i = 30;

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Q14. What is “super” keyword in java?
The super keyword in java is a reference variable that is used to refer parent class objects. The keyword “super” came into the picture with the concept of Inheritance. Whenever you create the instance of a subclass, an instance of parent class is created implicitly i.e. referred by super reference variable.
Various scenarios of using java super Keyword:

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Q15. What is static variable in Java?
The static keyword in java is used for memory management mainly. We can apply java static keyword with variables, methods, blocks and nested class. The static keyword belongs to the class than the instance of the class.

The static can be:

 

Q16. Differences between HashMap and HashTable in Java.
1. HashMap is non synchronized. It is not-thread safe and can’t be shared between many threads without proper synchronization code whereas Hashtable is synchronized. It is thread-safe and can be shared with many threads.
2. HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.
3. HashMap is generally preferred over HashTable if thread synchronization is not needed
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Q17. How are Java objects stored in memory?
In Java, all objects are dynamically allocated on Heap. This is different from C++ where objects can be allocated memory either on Stack or on Heap. In C++, when we allocate abject using new(), the object is allocated on Heap, otherwise on Stack if not global or static.
In Java, when we only declare a variable of a class type, only a reference is created (memory is not allocated for the object). To allocate memory to an object, we must use new(). So the object is always allocated memory on the heap.  Read more

 

Q18. What are C++ features missing in Java?
Try to answer this on your own before seeing the answer – here.

 

See also:

 

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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