AP Computer Science A Unit 9 Inheritance

Unit 9: Inheritance Creating Superclasses and Subclasses: class B extends A { } Inheritance essentially copies all of the methods and instance variables from class A and pastes those into class B at run time. The code from A is run from within class B. Note: Java does not allow one class to extend more than one other class. B is said to be a subclass of A. Equivalently, A is a superclass of B. A subclass can only have a single superclass, but a superclass can have many subclasses. Writing Constructors for Subclasses: Constructors are not inherited from a superclass. In general, subclass constructors must be explicitly written in the subclass. Within a subclass constructor, the super keyword can be used to call a superclass constructor, as in the following example: public class Pet { private String name; public Pet() { name = ""; } public Pet(String petName) { name = petName; } } public class Dog extends Pet { public Dog(dogName, dogBreed) { private String breed; super(dogName); breed = dogBreed; } } In this example, the Dog constructor calls the Pet constructor and passes dogName to it so that it can be stored in the name instance variable. The call to a superclass constructor must be the first line in a subclass constructor. If the superclass has a default (no parameter) constructor, the call to super is optional; the default constructor will be called automatically if it is omitted. It is very important to remember that although a subclass inherits all the attributes and behaviors of its superclass, it cannot access private variables or methods from the superclass. Overriding Methods: Overriding an inherited method means providing a public method in a subclass with the same method as a public method in the superclass. The method in the subclass will be called instead of the method in the superclass. When you extend a class, you inherit all methods and instance variables. You can override the original methods by implementing one with the same signature. Super Keyword: super – refers to the parent class super.toString( ); //legal statement super.super.toString(); //illegal statement Super is typically used to call parent class constructors and to call parent methods that have been overridden in the sub class. Super is necessary to distinguish between methods when the subclass and superclass have a method with the same signature. This example and explanation below is from Westlake Notes: class Skeleton extends Monster { private double speed; public Skeleton( ) { speed=100; } public Skeleton( String name, double speed ) { super(name); this.speed=speed; } public String toString( ) { return super.toString() + " " + speed; } } In this example above, if a call is not made to a super() constructor, Java will automatically call the default super() constructor on the first line of every subclass constructor. In the toString() method of Skeleton, the super().toString() is called to retrieve the super class data. If the super. was not placed in front of the toString() call, a recursive process would begin. Polymorphism: This is the ability of one general thing to behave like other specific things. Ex: public class Mammal{ … } public class Dog extends Mammal{ … } This creates the following relationship: A Dog is a Mammal. Because of this relationship, the following assignments are all valid: Mammal test= new Mammal(); Dog tea= new Dog(); Mammal rose= new Dog(); Remember: a child class can never refer to a parent class but a parent can always refer to a child class. Object Superclass: Class Object is the one true super class. Object does not extend any other class. All classes extend Object. Class Object is the parent of every other class in Java. All classes in Java start out with all of the same data / instance variables and methods as class Object. The most common Object methods used are equals(), toString(), clone(), and hashCode(). There are many more as well, but these are the more common ones. Because String extends Object, String will contain all methods and instance variables / data from Object as well as all methods and instance variables / data written in class String. String is an Object.