Abstract and final classes [Horstmann, pp ] An abstract class is kind of a cross between a class and an interface.

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1 Abstract and final classes [Horstmann, pp ] An abstract class is kind of a cross between a class and an interface. In a class, all methods are defined. In an interface, methods are declared rather than defined. In an abstract class, typically, some methods are defined while others are just declared. The point of declaring a method is to force subclasses to define it. A good example would be a paint()method in a graphics package. It says that an element has to be displayable on the screen in some way, but it doesn t say how. public abstract class GraphicsComponent { public abstract void paint(); A class that contains one or more abstract methods cannot be instantiated. (What does that mean?) The opposite of an abstract class is a final class. Unlike an abstract class, which must be subclassed, a final class or a final method can t be subclassed. This is good when the programmer wants to make sure that methods are not overridden to do something dangerous. public final class String { public final boolean checkpassword(string password) { Access specifiers [Horstmann 13.7] In Java, there are four different ways of specifiying who can access an instance variable, method, or class. Lecture 10 Programming Concepts Java 1

2 public private protected default (package) Package access is a good default for But it is bad for Why? Here is a table telling what can access a member with each kind of access specifier. Modifier Class Package Subclass World private Y N N N no modifier Y Y N N protected Y Y Y N public Y Y Y Y What s dangerous about using protected access? It allows subclasses to depend on not only the interface, but also the of a superclass. Besides subclasses, read or modify the variable. can also Here is an example of the danger of having a subclass depend on the implementation. Here is a specialized class of Rectangle that keeps track of how many times its width is changed (from Skrien, An Introduction to Object-Oriented Design and Design Patterns Using Java). The superclass contains these methods: CSC 216 Lecture Notes Spring 2008

3 public void setwidth(int newwidth) { width = newwidth; public void setheight(int newheight) { height = newheight; public void setsize(int w, int h) { setwidth(w); setheight(h); The subclass overrides setwidth: public void setwidth(int newwidth) { if( newwidth!= width ) { widthchangecounter ++; width = newwidth; If the implementation of setsize is changed, the subclass s setwidth may not work anymore. Why not? Consider this example of a program containing two packages. class A class Asub class B class C Fill in the following table, showing which members of class A are visible to each other class, for each possible access specifier. Modifier A Asub B C private Y no modifier protected public Y Y Y Lecture 10 Programming Concepts Java 3

4 Now repeat, for members of class Asub. Modifier A Asub B C private no modifier protected public And for members of class B. Modifier A Asub B C private no modifier protected public Class Object [Horstmann 13.8] Class Object is an ancestor ( superclass ) of all classes. Therefore, all methods defined in Object are accessible to any class. Class Object has nine methods, one of which has three overloaded varieties. All of them are quite useful, but in this course, we ll only be concerned with three of them: Method String tostring() Effect Returns a string representation of object boolean equals(object o) Tests whether two objects are equal Object clone() Suppose we had a class defined as Makes a copy of an existing object. CSC 216 Lecture Notes Spring 2008

5 public class Student { private String name; private int number; private String major; private String hometown; What would be a good tostring() method for this class? What about equals()? What does it do if not overridden? What s the difference between x == y and x.equals(y)? Suppose we had a class Fraction: public class Fraction { private int numerator; private int denominator; What would be a good equals( ) method? Lecture 10 Programming Concepts Java 5

6 Cloning objects [Horstmann ] Copying an object reference just gives two references to the same object. Student s, t; Student s = new Student( ); t = s; If clone() is implemented for Student, then s.clone()creates a new copy of the student object. Most objects are made up of other objects. There are two different ways of copying them. Create new references to each of the subobjects (shallow copy). Orig. x y z Copy Copy each of the subobjects (deep copy). Orig. x Copy x y y z z The clone() method does a shallow copy. Why? CSC 216 Lecture Notes Spring 2008