Core Java - Interview Questions and Answers

31) What do you understand by copy constructor in Java?

There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++.

There are many ways to copy the values of one object into another in java. They are:

By constructor
By assigning the values of one object into another
By clone() method of Object class

In this example, we are going to copy the values of one object into another using java constructor.

 //Java program to initialize the values from one object to another class Student6{     int id;     String name;     //constructor to initialize integer and string     Student6(int i,String n){     id = i;     name = n;     }     //constructor to initialize another object     Student6(Student6 s){     id = s.id;     name =s.name;     }     void display(){System.out.println(id+" "+name);}       public static void main(String args[]){     Student6 s1 = new Student6(111,"Karan");     Student6 s2 = new Student6(s1);     s1.display();     s2.display();    } } 

Test it Now

Output:

 111 Karan 111 Karan

32) What are the differences between the constructors and methods?

There are many differences between constructors and methods. They are given below.

Java Constructor	Java Method
A constructor is used to initialize the state of an object.	A method is used to expose the behavior of an object.
A constructor must not have a return type.	A method must have a return type.
The constructor is invoked implicitly.	The method is invoked explicitly.
The Java compiler provides a default constructor if you don't have any constructor in a class.	The method is not provided by the compiler in any case.
The constructor name must be same as the class name.	The method name may or may not be same as class name.

33) What is the static variable?

The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.

 //Program of static variable  class Student8{    int rollno;    String name;    static String college ="ITS";        Student8(int r,String n){    rollno = r;    name = n;    }  void display (){System.out.println(rollno+" "+name+" "+college);}   public static void main(String args[]){  Student8 s1 = new Student8(111,"Karan");  Student8 s2 = new Student8(222,"Aryan");    s1.display();  s2.display();  } } 

Test it Now

 Output:111 Karan ITS        222 Aryan ITS

34) What is the static method?

A static method belongs to the class rather than the object.
There is no need to create the object to call the static methods.
A static method can access and change the value of the static variable.

35) What are the restrictions that are applied to the Java static methods?

Two main restrictions are applied to the static methods.

The static method can not use non-static data member or call the non-static method directly.
this and super cannot be used in static context as they are non-static.

36) Why is the main method static?

Because the object is not required to call the static method. If we make the main method non-static, JVM will have to create its object first and then call main() method which will lead to the extra memory allocation.

37)

Can we override the static methods?

No, we can't override static methods.

38) 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.

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

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

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

static or class method	instance 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 nnn;}	For example: public void msg(){...}.

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