Java Interview Question

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Can we override the static methods?

No, we can't override static methods.

44) What is the static block?

Static block is used to initialize the static data member. It is executed before the main method, at the time of class loading.

45) Can we execute a program without main() method?

Ans) No, It was possible before JDK 1.7 using the static block. Since JDK 1.7, it is not possible. More Details.

46) What if the static modifier is removed from the signature of the main method?

Program compiles. However, at runtime, It throws an error "NoSuchMethodError."

47) What is the difference between static (class) method and instance method?

static or class methodinstance method
1)A method that is declared as static is known as the static method.A method that is not declared as static is known as the instance method.
2)We don't need to create the objects to call the static methods.The object is required to call the instance methods.
3)Non-static (instance) members cannot be accessed in the static context (static method, static block, and static nested class) directly.Static and non-static variables both can be accessed in instance methods.
4)For example: public static int cube(int n){ return n*n*n;}For example: public void msg(){...}.

48) Can we make constructors static?

As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.

49) Can we make the abstract methods static in Java?

In Java, if we make the abstract methods static, It will become the part of the class, and we can directly call it which is unnecessary. Calling an undefined method is completely useless therefore it is not allowed.

50) Can we declare the static variables and methods in an abstract class?

Yes, we can declare static variables and methods in an abstract method. As we know that there is no requirement to make the object to access the static context, therefore, we can access the static context declared inside the abstract class by using the name of the abstract class. Consider the following example.


51) What is this keyword in java?

The this keyword is a reference variable that refers to the current object. There are the various uses of this keyword in Java. It can be used to refer to current class properties such as instance methods, variable, constructors, etc. It can also be passed as an argument into the methods or constructors. It can also be returned from the method as the current class instance.



52) What are the main uses of this keyword?

There are the following uses of this keyword.

  • this can be used to refer to the current class instance variable.
  • this can be used to invoke current class method (implicitly)
  • this() can be used to invoke the current class constructor.
  • this can be passed as an argument in the method call.
  • this can be passed as an argument in the constructor call.
  • this can be used to return the current class instance from the method.

53) Can we assign the reference to this variable?

No, this cannot be assigned to any value because it always points to the current class object and this is the final reference in Java. However, if we try to do so, the compiler error will be shown. Consider the following example.

54) Can this keyword be used to refer static members?

Yes, It is possible to use this keyword to refer static members because this is just a reference variable which refers to the current class object. However, as we know that, it is unnecessary to access static variables through objects, therefore, it is not the best practice to use this to refer static members. Consider the following example.

55) How can constructor chaining be done using this keyword?

Constructor chaining enables us to call one constructor from another constructor of the class with respect to the current class object. We can use this keyword to perform constructor chaining within the same class. Consider the following example which illustrates how can we use this keyword to achieve constructor chaining.

56) What are the advantages of passing this into a method instead of the current class object itself?

As we know, that this refers to the current class object, therefore, it must be similar to the current class object. However, there can be two main advantages of passing this into a method instead of the current class object.

  • this is a final variable. Therefore, this cannot be assigned to any new value whereas the current class object might not be final and can be changed.
  • this can be used in the synchronized block.

57) What is the Inheritance?

Inheritance is a mechanism by which one object acquires all the properties and behavior of another object of another class. It is used for Code Reusability and Method Overriding. The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also. Inheritance represents the IS-A relationship which is also known as a parent-child relationship.

There are five types of inheritance in Java.

  • Single-level inheritance
  • Multi-level inheritance
  • Multiple Inheritance
  • Hierarchical Inheritance
  • Hybrid Inheritance

Multiple inheritance is not supported in Java through class.

More Details.

58) Why is Inheritance used in Java?

There are various advantages of using inheritance in Java that is given below.

  • Inheritance provides code reusability. The derived class does not need to redefine the method of base class unless it needs to provide the specific implementation of the method.
  • Runtime polymorphism cannot be achieved without using inheritance.
  • We can simulate the inheritance of classes with the real-time objects which makes OOPs more realistic.
  • Inheritance provides data hiding. The base class can hide some data from the derived class by making it private.
  • Method overriding cannot be achieved without inheritance. By method overriding, we can give a specific implementation of some basic method contained by the base class.

59) Which class is the superclass for all the classes?

The object class is the superclass of all other classes in Java.

60) Why is multiple inheritance not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.

Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile time error.

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