Constructor Demo in Java.

ConstructorDemo.java

/*
 * Constructor Demo.
 *
 * - Constructor is syntactically similar to a method, but it lacks of return type.
 * - Constructor will have the same name as class name in which is declared.
 * - Contructors are invoked immediately after creation of the objects.
 *
 * There are two types of constructors:
 * - default constructor;
 * - parameterised constructor;
 *
 * Note:
 *   If a class don't have explicit constructor,
 *   compiler will add a default constructor
 *   so that class can be instantiated.
 *
 * Note:
 *   If default constructor is missing, but parameterised constructor is defined,
 *   Java compiler will generate an error about mismatch.
 *   However if you don't write any constructor,
 *   compiler will add superclass constructor by it-self.
 *   So if you're adding parameterised constructor, always define default constructor.
 */

class A {
   A() { // Default constructor.
     System.out.println("From default.");
   }
   A(int a, int b) { // Parameterised constructor.
     System.out.println("From param constructor.");
     System.out.println("a = " + a + "; b = " + b);
   }
 }

class ConstructorDemo {
    public static void main(String[] args) {
        new A();            // Invokes the default constructor.
        new A(100, 200);    // Invokes param constructor.
    }
 }