Single Inheritance for Classes in Java

The previous section mainly concentrated on the default constructor of the super class.  In this section, we shall concentrate on the default constructor of the sub class.

Modify your code in “second.java” as follows:

  public class Second extends First {

    /** Creates a new instance of Second */
    public Second() {
        x = 90;
        y = 80;
    }

    public int getProduct() {
        int p;
        p = x * y;
        return p;
    }
  }

You need not change anything apart from the above.

From the code fragments above, you can observe that I am assigning the values of members directly within the constructor of the sub class (“Second”). 

Once you execute your program, you will see a different output. That means the default constructor of the sub class also gets executed along with the default constructor of the parent class. But the important issue is that the execution starts from the super class and gradually moves to the sub class. 

From this concept, you can conclude that the default constructor of a super class gets automatically executed when you create an instance of a sub class, followed by the execution of the default constructor of the sub class. 

If you don’t include any default constructors, JVM automatically creates them for you at run-time and executes them (virtually).