Copy Constructor in Java

Reference: TutorialPoints, GeekforGeeks

The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously. The copy constructor is used to:

• Initialize one object from another of the same type.

• Copy an object to pass it as an argument to a function.

• Copy an object to return it from a function.

In C++, if a copy constructor is not defined in a class, the compiler itself defines one.

Java also supports copy constructor. But, unlike C++, Java doesn’t create a default copy constructor if you don’t write your own.

 class Complex {

     private double re, im;

     // A normal parametrized constructor
     public Complex(double re, double im) {
         this.re = re;
         this.im = im;
     }

     // copy constructor
     Complex(Complex c) {
         System.out.println("Copy constructor called");
         re = c.re;
         im = c.im;
     }

     // Overriding the toString of Object class
     @Override
     public String toString() {
         return "(" + re + " + " + im + "i)";
     }
 }

 public class Main {

     public static void main(String[] args) {
         Complex c1 = new Complex(10, 15);

         // Following involves a copy constructor call
         Complex c2 = new Complex(c1);   

         // Note that following doesn't involve a copy constructor call as
         // non-primitive variables are just references.
         Complex c3 = c2;   

         System.out.println(c2); // toString() of c2 is called here
     }
 }

Output:

Copy constructor

called (10.0 + 15.0i)

 class Complex {

     private double re, im;

     public Complex(double re, double im) {
         this.re = re;
         this.im = im;
     }
 }

 public class Main {

     public static void main(String[] args) {
         Complex c1 = new Complex(10, 15);
         Complex c2 = new Complex(c1);  // compiler error here
     }
 }

Compile error