Inheritance - Introduduction to Jave Programming - Lecture Slides, Slides of Java Programming

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Inheritance

Highlights

• Extending classes (inheritance)
• super keyword

Story time

Story time

Extending classes

(See: DuneCat.java and ArrakianSandworm.java) Extended (child) classes get objects and methods from their parent class For example, DuneCat (child) got the roar() method from ArrakianSandworm (parent) (not possible to extend from 2 classes)

What do child classes get?

Child classes get all objects and (public) methods from the parent class Remember: private means they can only be used inside the class where it is defined To access and modify private data, public methods are needed (get___() and set___()) (See: Parent.java and Child.java)

AD&D example

Using child and parent classes

You can think of a child class as a more specific instance of a parent class This means you can think of a child object as type parent (but not parent as type child) (See: UsingInheritance.java)

Oddities of super() constructor

If you do not code super() on the first line of a child class's constructor, then... ● The parent's default constructor will run ● If the parent does not have a default constructor, there will be an error This is a reason why it is normally useful to have a default constructor defined, even if it does nothing

this

We introduced this a while back, but you can also use this to run constructors (like super) If you use this as a constructor (like super), then it must happen on the first line of code (Note: you can only use this in this manner inside a constructor) (See: ThisConstructor.java)

Override

Not in book: You should always add @Override before every method you override (very important!) When overriding, you can change modifiers and return types under these restrictions:

1. The modifier must be more “open” (e.g. private -> public)
2. The return type is a descendant (child) of the original return type (e.g. Parent -> Child) (See: OverrideChild.java and OverrideParent.java)

Override vs Overload

When you override, you redefine an existing method (name) (same name, number of arguments and types) When you overload, you add a new way of using a method (name) (same name, but different arguments)