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.

Here’s a basic algorithm for implementing a copy constructor in Java:

• Define a class: Create a class that represents the object you want to manage.
• Define instance variables: Within the class, define instance variables that represent the data you want to manage.
• Define a constructor: Define a constructor for the class that takes an instance of the same class as its argument. This constructor will be used to create a copy of the object.
• Initialize the instance variables: Within the constructor, initialize the instance variables with the values from the argument object.
• Use the this keyword to refer to the instance variables: To refer to the instance variables of the class within the constructor, use the this keyword.
• Check for null values: If the argument object is null, return a new instance of the class with default values for the instance variables.
• Implement deep copying: If the instance variables are objects, create new instances of those objects within the constructor and initialize them with the values from the argument object. This is called deep copying and ensures that changes to the copied object do not affect the original object.

example implementation of a copy constructor for a simple class called Person:

Java

class Person {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public Person(Person another) {
        this(another.name, another.age);
    }

    // Getters and setters for the instance variables
}

Example 1

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 2

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.

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