Copy Constructor in Java

Like C++, Java also supports a copy constructor. But, unlike C++, Java doesn’t create a default copy constructor if you don’t write your own. A prerequisite prior to learning copy constructors is to learn about constructors in java to deeper roots. Below is an example Java program that shows a simple use of a copy constructor.

Example 1A

Java

// Java Program to Illustrate Copy Constructor

// Class 1
class Complex {

    // Class data members
    private double re, im;

    // Constructor 1
    // Parameterized constructor
    public Complex(double re, double im)
    {

        // this keyword refers to current instance itself
        this.re = re;
        this.im = im;
    }

    // Constructor 2
    // 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)";
    }
}

// Class 2
// Main class
public class Main {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating object of above class
        Complex c1 = new Complex(10, 15);

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

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

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

Output

Copy constructor called
(10.0 + 15.0i)

Example 1B

Java

// Java Program to Illustrate Copy Constructor

// Class 1
class Complex {

    // Class data members
    private double re, im;

    // Constructor
    public Complex(double re, double im)
    {
        // this keyword refers to current instance itself
        this.re = re;
        this.im = im;
    }
}

// Class 2
// Main class
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Creating object of above class
        // inside main() method
        Complex c1 = new Complex(10, 15);

        // Note: compiler error here
        Complex c2 = new Complex(c1);
    }
}

Output:

Now, in the above code, the line calling the function with the object c1 as the parameter will give the error as the type of the parameter in the constructors is of ‘double’ type while the passed content is of ‘object’ type.

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