Copy Constructor in Java

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

Following is an example Java program that shows a simple use of copy constructor.

// filename: Main.java 
  
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)

Now try the following Java program:

// filename: Main.java 
  
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 
    } 
} 

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

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