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JVM / Java / OOP / Constructors

1. Multiple constructors

 
package oop.constructors;

public class MultipleConstructors {
    public static void main(String[] args) {

        new A();        // Default constructor used
        new A("aaa");   // Constructor 1 used - aaa
        new A(100);     // Constructor 2 used - 100
    }
}

class A {

    // Default contructor (written explicitly for clarity)
    public A() {
        System.out.println("Default constructor used");
    }

    // Constructor 1
    public A(String s) {
        System.out.println("Constructor 1 used: " + s);
    }

    // Constructor 2
    public A(int n) {
        System.out.println("Constructor 2 used: " + n);
    }
}

2. This method

To call another constructor from the same class use this() method.

 
package oop.constructors;

public class ThisMethod {
    public static void main(String[] args) {
        
        Rectangle a = new Rectangle();
        Rectangle b = new Rectangle(100, 200);
        Rectangle c = new Rectangle(300, 400, 11, 22); // Look Here

        System.out.println(a);  // W: 0 H: 0 x: 0 y: 0
        System.out.println(b);  // W: 100 H: 200 x: 0 y: 0
        System.out.println(c);  // W: 300 H: 400 x: 11 y: 22
    }
    
}

class Rectangle {

    private int x, y;
    private int width, height;
    
    public Rectangle() {
        this(0, 0, 0, 0); // Look Here
    }
    
    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }
    
    public Rectangle(int width, int height, int x, int y) {
        this(width, height);
        this.x = x;
        this.y = y;
    }

    public String toString() {
        return String.format(
            "W: %s H: %s x: %s y: %s", width, height, x, y
        );
    }
}

3. Super method

The super() method is used to call parent constructor. Constructor call must be the first statement in a controller.

 
package oop.constructors;

public class SuperMethod {
    public static void main(String[] args) {
        new MainClass();  // Parent constructor called.
    }
}

class BaseClass {
    public BaseClass() {
        System.out.println("Parent constructor called.");
    }
}

class MainClass extends BaseClass {
    public MainClass() {  // Subclass controller
        super();  // Parent constructor called
    }

    /*
        // This will not (void is the first statement)
        public void MainClass(String s) {
           super(s);
        }
    */
}