Java Tutorial: Extending a Class that has Explicit Constructors

Xah Lee, 2005-02-28, 2011-12-26

In the following code, why it doesn't compile, but does when B() is defined?

class B {
  int x;
  //B () {x=300;}
  B (int n) {x=n;}
  int returnMe () {return x;}
}

class C extends B {
}

public class Inh3 {
  public static void main(String[] args) {
  }
}

(answer below.)

Solution

the answer to the constructor mystery is that, if one provides any constructor, one must define all constructors.

Peter Molettiere on Apple's Java forum gave excellent answers:

Because there is no default constructor available in B, as the compiler error message indicates. Once you define a constructor in a class, the default constructor is not included. If you define *any* constructor, then you must define *all* constructors.

When you try to instantiate C, it has to call super() in order to initialize its super class. You don't have a super(), you only have a super(int n), so C can not be defined with the default constructor C() { super(); }. Either define a no-arg constructor in B, or call super(n) as the first statement in your constructors in C.

So, the following would work:

   class B {
       int x;
       B() { } // a constructor
       B( int n ) { x = n; } // a constructor
       int returnMe() { return x; }
   }

   class C extends B {
   }

or this:

   class B {
       int x;
       B( int n ) { x = n; } // a constructor
       int returnMe() { return x; }
   }

   class C extends B {
       C () { super(0); } // a constructor
       C (int n) { super(n); } // a constructor
   }

If you want to make sure that x is set in the constructor, then the second solution is preferable.

Also, see java lang spec http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#41652. Quote:

8.8.7 Default Constructor

If a class contains no constructor declarations, then a default
constructor that takes no parameters is automatically provided:

   * If the class being declared is the primordial class Object, then
   the default constructor has an empty body.

   * Otherwise, the default constructor takes no parameters and simply
   invokes the superclass constructor with no arguments.

A compile-time error occurs if a default constructor is provided by
the compiler but the superclass does not have an accessible
constructor that takes no arguments.