Chapter 6 Inheritance Pg 151-163

Chapter 6 Inheritance Pg 151-163

1. makes code extensible in any OOP language

2. An Overview of Inheritance

   • Extending a class is done by creating a new class. This creates a parent child relationship.
     • This can be repeated ad infititum unless you declare the class to be final
       • Inheritance allows you to add functionality that doesn't exist in the original class

3. The extends Keyword

   • Extend a class using the extends keyword in a class declaration
     • public class Child extends Parent {}
     • All Java classes by default extend the Java.lang.Object class. B/c Object is the ultimate superclass in Java
     • In Java a class can only extend one class

4. The is-a Relationship

   • When extending a class an 'is-a' relationship is formed
     • If Animal is a superclass the a subclass can be a bird
       • A bird 'is-a' animal
       • subclasses inherits the methods within the superclass

5. Accessibility

   • subclasses can access superclasses public and protected methods and fields, but not it's private methods

6. Method Overriding

   • When extending a class the behavior of a method in a parent class can be modified in the subclass
     • Done when a subclasses method has the same signature as a parent class's method
       • Simply write the new methods in the sublcass
       • You cannot override a final method

7. Calling the Superclass's Constructors

   • When you create a child object, all its parent classes are also instantiated
     • Super represents an instance of the direct superclass of the current object
       • You can explicitly call the parents constructor from a subclass's constructor using the super keyword, but super must be the first statement in the constructor
         • public Sub(String s){super(s)}
         • It makes sense for a child class to call its parent's constructor from its own constructor b/c an instance of a subclass must always be accompanied by an instance of each its parents

8. Calling the Superclass's Hidden Members

   • The super keyword can call a hidden member or an overridden method in a superclass
     • super.memberName returns the specified member in the parent class

9. Type Casting

   • You can only cast an instance of a subclass to its parent class
     • Child child = new Child(); Parent parent = child;
     • downcasting is where you cast an object down the inheritance hierarcy.
       • Child child = new Child; Parent parent = child; //parent point to an instance of a Child Child child2 = (Child) parent;//downcasting
         • Downcasting to a subclass is only allowed if the parent class reference is already pointing to an instance of the subclass

10. Final Classes

    • Prevents the extension of your classes
      • Final may appear after or before the access modifier
        • public final class pencil OR final public class pen

11. The instanceof Keyword

    • Instanceof keyword can be used to test if an object is of a specified type
      • usually part of if statement
        • String s = "Hello"; if(s instanceof java.lang.String) THIS RETURNS TRUE