Java Inheritance - instance variables overriding

You can hide a field, but not override it.

Hiding means that a field will have a different value depending from which class it's accessed. The field in the subclass will "hide" the field in the super-class, but both exists.

That's an extremely bad practice to hide field, but works:

public class HideField {

    public static class A
    {   
        String name = "a";

        public void doIt1() { System.out.println( name ); };
        public void doIt2() { System.out.println( name ); };   
    }

    public static class B extends A
    {
        String name = "b";

        public void doIt2() { System.out.println( name ); };
    }

    public static void main(String[] args)
    {
        A a = new A();
        B b = new B();

        a.doIt1(); // print a
        b.doIt1(); // print a
        a.doIt2(); // print a
        b.doIt2(); // print b <-- B.name hides A.name
    }
}

Depending on whether the method was overriden, the field in A or B is accessed.

Never do that! That's never the solution to your problem and creates very subtle bugs related to inheritance.

Because:

• It may break the parent class code. For example consider following code(What would be behavior of line obB.getInt(); in following code if, instance variables overriding is allowed):

  class A
  {
      int aInt;
  
      public int getInt()
      {
          return aInt;
      }
  }
  
  class B extends A
  {
      int aInt;
  
      public int getInt2()
      {
          return aInt;
      }
  
      public static void main(String[] args)
      {
           B obB = new B();
  
           //What would be behavior in following line if, 
           //instance variables overriding is allowed
           obB.getInt(); 
      }
  }
  

• It is not logical because child class should have/reflect all behaviours of parent class.

So, you can only hide the inherited methods/variables in child class but can't override.

Following is an extract from Java doc from Oracle specifying what operation you can perform/expect from child class:

You can use the inherited members as is, replace them, hide them, or supplement them with new members:

• The inherited fields can be used directly, just like any other fields.
• You can declare a field in the subclass with the same name as the one in the superclass, thus hiding it (not recommended).