Abstract Classes Brookwood High Scool抽象类布鲁克伍德学校.pptVIP

Abstract Classes Mrs. C. Furman November 10, 2008 Abstract Classes Used when we have methods that will be overridden in every subclass. A superclass which represents an abstract concept and should NOT be instantiated. Abstract classes can have instance variables and concrete methods. public abstract class Shape { private String myName; public Shape (String name) { myName = name;} public String getName () { return myName;} //abstract methods public abstract double area(); public abstract double perimeter(); public double semiPerimeter () { return perimeter() / 2; } } Abstract Methods Methods which will be overridden in every subclass. They act as placeholders for Dynamic Binding. public class Circle extends Shape {private double myRadius; public Circle (double radius, String name) { super (name); myRadius = radius; } public double perimeter () { return 2 * Math.PI * myRadius;} public double area() { return Math.PI * Math.pow (myRadius,2);} } Shape a; a = new Shape (“blob”); //causes an error // we can not create abstract objects. a = new Circle (3, “circle”); Interface a collection of related methods whose headers are provided without implementation. All the methods are both public and abstract. A class that implements an interface must provide implementations for ALL methods declared in the interface. Additional methods can be implemented as well. If we do not override all the methods in the interface, we must be abstract. Interface vs. Abstract Interface can NOT provide implementation for any of its methods, abstract can. Interface can NOT contain instance variables, abstract can. Both interfaces and abstracts can declare constants. Interfaces can NOT be constructed. No instances of Abstract can be created. public interface PersonalInfo { String getName(); String getAddress(); int getSocialSecurityNo(); } //NO access specifiers. //They are all public abstract!! public class Student implements PersonalInfo { … } An