Inheritance in Java

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So far we have outlined a number of powerful facilities contained in the Java programming language which are used to construct objects and define the methods which correspond to the messages that Java objects receive This section introduces what is certainly the most powerful facility within Java: that of inheritance In order to introduce the idea we will first describe three increasingly complex examples of where inheritance is useful The first example will introduce the idea of inheritance in an abstract way Later chapters of the book show how inheritance is used within more realistic applets and applications 331 The augmented set The first example concerns a set where we need methods which find out the size of the set and add integers to the set, together with an operation which finds the sum of the elements in the set One very efficient way of implementing such a set in such a way that summation is not too inefficient an operation is to have a class which has two instance variables: intSet, the set of integers, and an integer sum which contains the current sum of the integers This is an efficient implementation because any method which needs to find the sum only needs to look it up in the variable sum rather than iterating through intSet An example of an object described by this class is shown in Figure 32 Here the instance variable sum contains the current sum of the integers in the instance variable intSet Let us assume that we have already implemented a set of integers described by the class intSet shown below, where the code for each of the classes is not shown and where we have just listed the instance variables without defining their types The class does not require a summation method and hence does not require an instance variable to contain a sum Class IntSet { int intSetvar[]; public boolean includes(int no) {
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} public int size(int no) { } public void add (int no) { } public void remove(int no) { } } Let us assume that this set has been implemented for another project and is associated with the four methods shown above which check that a particular number is in the set, find the size of the set, add an element to the set and removes an element from the set Now let us assume that we need a set to which we wish to send messages that calculate its sum We are faced with one choice immediately: we can program a new class from scratch However, another choice is to use a facility known as inheritance which takes advantage of the set class we have just written Before seeing it in action it is worth providing some definitions Inheritance is a relationship between two classes: if a class A inherits from a class B, then class A, as well as being able to use all of its own instance variables and methods, can use all the methods that B can use and can also use any instance variables that B can use As an example of this consider the two classes X and Y below without the code for their methods
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intSet 9 13
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Figure 32 An implementation of a set of integers
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Class X { // Declarations for u v and w // Code for method A
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// Code for method B // Code for method C } Class Y { // Declarations for l and m // Code for method R // Code for method S // Code for method T } If class Y inherits from class X, then, firstly, all the methods in class Y can refer to not only the instance variables in class Y but also the instance variables in class X; and secondly an object described by class Y can have messages corresponding not only to the methods R, S and T but also the methods A, B and C Classes can inherit from classes which inherit from other classes As an example consider the three classes shown below: Class X { // Declarations for u v and w // Code for method A // Code for method B // Code for method C } Class Y { // Declarations of l and m // Code for method R // Code for method S // Code for method T } Class Z { // Declarations of n and o // Code for method G // Code for method H // Code for method I }
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