8.1 Classes & Inheritance

8.1.1 Inheritance • Objects are created to model ‘things’• Sometimes, ‘things’ may be different, but still

have many attributes in common• We can create an object that models their

common attributes

8.1.2 Abstraction • A superclass can be used

to model common attributes or behavior

• Sometimes, the superclass cannot be an object itself

• The superclass may define the behaviors that subclasses must implement

8.1.3 The problem of multiple inheritance

• Some languages allow one class to have multiple super-classes

• Inheritance provides an “is a” relationship• Multiple inheritance creates the potential for

conflicting attributes and behaviors

8.2.1 Java language keywords in inheritance

• extends: Declares that the current class is a subclass of another

• super: A keyword used by the subclass to access the attributes of the superclass

• protected: An access modifier that restricts access to subclasses

• abstract: Declares that a class is an interface• implements: Allows a class to take on the

properties of an interface

8.3.1 Role of access modifiers in inheritance

• The subclass inherits all the attributes and behaviors of the superclass except those declared as private

8.4.1 Method overriding

• Subclasses may need to customize the behavior of the superclass

• The subclass defines a method that already exists in the superclass

• When this method is called, it is the one defined in the subclass that is used

• The superclass method is still accessible from the subclass using the super keyword

8.4.3 Overloading versus overriding

• Overriding is when a method in the subclass is designed to replace a method of the same name in the superclass

• Overloading is having several methods with the same name in the same class

• Overriding allows you to customize behavior

• Overloading creates several ways to do the same thing!

8.6.1 Handling constructors in inheritance

• Constructors can be overloaded• Whenever an subclass is ‘constructed’ the

superclass(es) constructors are also called• The subclass constructor can explicitly call

the superclass constructor using the super keyword

• If there is no corresponding constructor in the superclass, the compilation will fail

8.7.1 Abstract classes

• Superclasses are supposed to model more generalized attributes and behavior

• High up the tree, the classes are more a guideline that a model

• Abstract classes define methods that have no body and must be overridden

• The abstract class does not know how to perform the behavior, but does force subclasses to create a behavior with this name

8.7.2 Final classes

• Use the final keyword to declare an attribute immutable

• Also use the final keyword to prevent a method from being overridden in a subclass

8.8.1 What and why of interfaces

• Java prevents multiple inheritance

• However, a class can inherit attributes from multiple sources using interfaces

• Interfaces are a blueprint for behavior The interface does not define the behavior, but forces the subclass to do it