Constructors in case of inheritance

Constructors in case of inheritance

• When a program creates a subclass object, the subclass constructor immediately calls the superclass constructor (explicitly, via super, or implicitly).
• The superclass constructor’s body executes to initialize the superclass’s instance variables that are part of the subclass object, then the subclass constructor’s body executes to initialize the subclass-only instance variables.(cont…)
• Java ensures that even if a constructor does not assign a value to an instance variable, the variable is still initialized to its default value (e.g., 0 for primitive numeric types, false for booleans, null for references).   
• Java can implicitly call only the no argument constructor of the super class.

Example:
        The following example illustrates the calling order of the constructors in case of inheritance when an object for subclass is created.

1. /* Constructor calling */
2. class one //Super class
3. {
4. one()
5. {
6. System.out.println("Inside the constructor of class one");
7. }
8. }
9.  
10. class two extends one //Sub class
11. {
12. two()
13. {
14. System.out.println("Inside the constructor of class two");
15. }
16. }
17.  
18. /* Main class */
19. class callconst
20. {
21. public static void main(String args[])
22. {
23. System.out.println("Creating instance of class two\n");
24. two t=new two(); //creating instance of class two
25. }
26. }

Output:
D:\Java>javac callconst.java
D:\Java>java callconst Creating instance of class two
Inside the constructor of class one Inside the constructor of class two