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.

 1 class Complex {
 2  
 3     private double re, im;
 4      
 5     // A normal parametrized constructor 
 6     public Complex(double re, double im) {
 7         this.re = re;
 8         this.im = im;
 9     }
10      
11     // copy constructor
12     Complex(Complex c) {
13         System.out.println("Copy constructor called");
14         re = c.re;
15         im = c.im;
16     }
17       
18     // Overriding the toString of Object class
19     @Override
20     public String toString() {
21         return "(" + re + " + " + im + "i)";
22     }
23 }
24  
25 public class Main {
26  
27     public static void main(String[] args) {
28         Complex c1 = new Complex(10, 15);
29          
30         // Following involves a copy constructor call
31         Complex c2 = new Complex(c1);   
32  
33         // Note that following doesn't involve a copy constructor call as 
34         // non-primitive variables are just references.
35         Complex c3 = c2;   
36  
37         System.out.println(c2); // toString() of c2 is called here
38     }
39 }

Output:

Copy constructor

called (10.0 + 15.0i)

 1 class Complex {
 2  
 3     private double re, im;
 4  
 5     public Complex(double re, double im) {
 6         this.re = re;
 7         this.im = im;
 8     }
 9 }
10  
11 public class Main {
12      
13     public static void main(String[] args) {
14         Complex c1 = new Complex(10, 15);  
15         Complex c2 = new Complex(c1);  // compiler error here
16     }
17 }

Compile error