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