(1) A subclass inherits all variables and the normal methods from its superclass:

(2) A subclass do not inherit any constructor method from its superclass:

But instead:   a constructor in the subclass must invoke some constructor in the superclass -- discussed next

Notice that a subclass object always contains a superclass object:

Recall:   objects are initialized using a constructor    --    how to makes sure that a subclass object is initialized ?

(3) Rule:   a constructor of the subclass must invoke some constructor of its superclass as the first statement

The syntax super( parameters ) is used to invoke a constructor in its superclass

public class myProg
{
  public static void main()
  {
    NewClass a = new NewClass();
    NewClass b = new NewClass(44);
  }
}




  

public class NewClass
       extends SomeClass
{
  public NewClass()
  {
    super(); //Calls: SomeClass()
  }
    
  public NewClass(int a)
  {
    super(a); //Calls: SomeClass(a)
  }
}
  

public class SomeClass
{
  public int x;

  public SomeClass()
  {
    x = 99;
  }
    
  public SomeClass(int a)      
  {
    x = a;
  }
} 

DEMO: demo/13-inheritance/06-super

DEMO: trace the execution in BlueJ

(4) Compliance rule:   if a constructor in the subclass does NOT invoke any constructor in its superclass:

Then... (can you guess what will happen ??? Remember the rules about default constructors ?

(4) Then:   the Java compiler will automatically insert the call super( ) as the first statement:

I.e.:   if the first statement in a constructor is not super(...), the Java compiler will call the default constructor!

public class myProg
{
  public static void main()
  {
    NewClass a = new NewClass();
  }
}






  

public class NewClass
       extends SomeClass
{
  public NewClass()
  {
    // No super( ) call
    // Java will insert super()
  }
    
  public NewClass(int a)     
  {
    super(a);      
  }
}
  

public class SomeClass
{
  public int x;

  public SomeClass()
  {
    x = 99;
  }
    
  public SomeClass(int a)   
  {
    x = a;
  }
}
  

DEMO: demo/13-inheritance/07-super

DEMO: trace the execution in BlueJ

DEMO: demo/13-inheritance/09-chaining/

public class SomeClass
{
    public int x;

    public SomeClass()
    {
        x = 99;
    }

    public void method1( )
    {
        System.out.println("I am SomeClass.method1(). x = " + x);
    }
    
    public void method2( )
    {
        System.out.println("I am SomeClass.method2(). x = " + x);
    }
}

public class NewClass extends SomeClass
{
    NewClass()
    {
        
    }
}

public class NewerClass extends NewClass
{
    NewerClass()
    {
        
    }
}

public class myProg
{
    public static void main(String[] args)
    {
        NewerClass x = new NewerClass(); // Trace this 
    }
}