Abstract Classes & Interfaces. Lecture 6

Transcription

1 Abstract Classes & Interfaces Lecture 6

2 Abstraction refers to the act of representing essential features without including the background details or explanations.

3 Abstract Classes Java allows abstract classes use the modifier abstract on a class header to declare an abstract class abstract class Vehicle { An abstract class is a placeholder in a class hierarchy that represents a generic concept Vehicle Car Boat Plane

4 Abstract Classes Abstract classes are only used as super classes Abstract classes can not be instantiated - Classes are declared as abstract classes only if they will never be instantiated Abstract classes contain usually one or more abstract methods Example: public abstract class Mouse implements Direction { abstract void makemove( ); In fact, any class with an abstract method is automatically an abstract class

5 Abstract Methods Abstract methods have no body at all and just have their headers declared The only way to use an abstract class is to create a subclass that implements each abstract method Concrete classes are classes that implement each abstract method in their superclasses Example: abstract void makemove( );

6 Example abstract class public abstract class Shape { public abstract double area(); // Abstract methods: note public abstract double circumference(); // semicolon instead of body. class Circle extends Shape { public static final double PI = ; protected double r; // Instance data public Circle(double r) { this.r = r; // Constructor public double getradius() { return r; // Accessor //Implementations of abstract methods. public double area() { return PI*r*r; public double circumference() { return 2*PI*r;

7 Example abstract class (cont d) class Rectangle extends Shape { protected double w, h; // Instance data public Rectangle(double w, double h) { // Constructor this.w = w; this.h = h; public double getwidth() { return w; // Accessor method public double getheight() { return h; // Another accessor // Implementations of abstract methods. public double area() { return w*h; public double circumference() { return 2*(w + h);

8 Abstract Class: Example An abstract class often contains abstract methods, though it doesn t have to Abstract methods consist of only methods declarations, without any method body public abstract class Vehicle { String name; public String getname() { return name; \\ method body abstract public void move(); \\no body!

9 Abstract Classes The non-abstract child of an abstract class must override the abstract methods of the parent An abstract class cannot be instantiated (why?) The use of abstract classes is a design decision; it helps us establish common elements in a class that is too general to instantiate

10 Interfaces Lecture 7

11 Single vs. Multiple Inheritance Some object-oriented languages allow multiple inheritance, which allows a class to be derived from two or more classes, inheriting the members of all parents The price: collisions, such as the same variable name, same method name in two parents, have to be resolved Java decision: single inheritance, meaning that a derived class can have only one parent class

12 Java Interface A Java interface is a collection of constants and abstract methods abstract method: a method header without a method body; we declare an abstract method using the modifier abstract since all methods in an interface are abstract, the abstract modifier is usually left off Methods in an interface have public visibility by default

13 Interface: Syntax interface is a reserved word public interface Doable { public static final String NAME; public void dothis(); public int dothat(); public void dothis2 (float value, char ch); public boolean dotheother (int num); A semicolon immediately follows each method header No method in an interface has a definition (body)

14 Implementing an Interface A class formally implements an interface by stating so in the class header in the implements clause a class can implement multiple interfaces: the interfaces are listed in the implements clause, separated by commas If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will produce errors

15 Implementing Interfaces public class Something implements Doable { public void dothis () { implements is a // whatever reserved word public void dothat () { // whatever Each method listed in Doable is given a definition // etc. public class ManyThings implements Doable, AnotherDoable

16 Interfaces: An Example A class that implements an interface can implement other methods as well

17 Interfaces: Examples from Java Standard Class Library The Java Standard Class library defines many interfaces: the Iterator interface contains methods that allow the user to move through a collection of objects easily hasnext(), next(), remove() the Comparable interface contains an abstract method called compareto, which is used to compare two objects if (obj1.compareto(obj2) < 0) System.out.println( obj1 is less than obj2 );

18 Polymorphism via Interfaces Define a polymorphism reference through interface declare a reference variable of an interface type Doable obj; the obj reference can be used to point to any object of any class that implements the Doable interface the version of dothis depends on the type of object that obj is referring to: obj.dothis();

19 Interface Hierarchies Inheritance can be applied to interfaces as well as classes One interface can be used as the parent of another The child interface inherits all abstract methods of the parent A class implementing the child interface must define all methods from both the parent and child interfaces Note that class hierarchies and interface hierarchies are distinct (they do not overlap)

20 Interfaces (cont d) An interface in Java is like an abstract class, but it does not have any fields or constructors, and all its methods are abstract. public interface Dance { DanceStep getstep (int i); int gettempo (); int getbeat (int i); public abstract is not written because all the methods are public abstract.

21 Interfaces (cont d) We must officially state that a class implements an interface. A concrete class that implements an interface must supply all the methods of that interface. public class Waltz implements Dance {... // Methods: public DanceStep getstep (int i) {... public int gettempo () { return 750; public int getbeat (int i) {......

22 Interfaces (cont d) A class can implement several interfaces. Like an abstract class, an interface supplies a secondary data type to objects of a class that implements that interface. You can declare variables and parameters of an interface type. Dance d = new Waltz( ); Polymorphism fully applies to objects disguised as interface types.

23 Interfaces (cont d) public interface Edible { String getfoodgroup(); int getcaloriesperserving(); public class Pancake implements Edible {... public class Breakfast Polymorphism: { the correct private int mytotalcalories = 0; method is called... for any specific public void eat (Edible obj, int servings) type of Edible, { e.g., a Pancake mytotalcalories += obj.getcaloriesperserving ()* servings;...

24 Classes Interfaces Similarities A superclass provides a secondary data type to objects of its subclasses. An abstract class cannot be instantiated. An interface provides a secondary data type to objects of classes that implement that interface. An interface cannot be instantiated.

25 Classes Interfaces Similarities A concrete subclass of an abstract class must define all the inherited abstract methods. A class can extend another class. A subclass can add methods and override some of its superclass s methods. A concrete class that implements an interface must define all the methods specified by the interface. An interface can extend another interface (called its superinterface) by adding declarations of abstract methods.

26 Classes Interfaces Differences A class can extend only one class. A class can have fields. A class defines its own constructors (or gets a default constructor). A class can implement any number of interfaces. An interface cannot have fields (except, possibly, some public static final constants). An interface has no constructors.

27 Classes Interfaces Differences A concrete class has all its methods defined. An abstract class usually has one or more abstract methods. All methods declared in an interface are abstract. Every class is a part of a hierarchy of classes with Object at the top. An interface may belong to a small hierarchy of interfaces, but this is not as common.