Instance Variable in Java

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Instance Variable
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Instance Method
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16 Inheritance
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There are two fundamental mechanisms for building new classes from existing ones: inheritance and aggregation It makes sense to inherit from an existing class Vehicle to define a class Car, since a car is a vehicle The class Vehicle has several parts; therefore, it makes sense to define a composite object of the class Vehicle that has constituent objects of such classes as Motor, Axle, and GearBox, which make up a vehicle Inheritance is illustrated by an example that implements a stack of characters that can print its elements on the terminal This new stack has all the properties and behaviors of the CharStack class, but it also has the additional capability of printing its elements Given that this printable stack is a stack of characters, it can be derived from the CharStack class This relationship is shown in Figure 16 The class PrintableCharStack is called the subclass, and the class CharStack is called the superclass The CharStack class is a generalization for all stacks of characters, whereas the
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Class Diagram Depicting Inheritance Relationship
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16: INHERITANCE
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class PrintableCharStack is a specialization of stacks of characters that can also print their elements In Java, deriving a new class from an existing class requires the use of the extends clause in the subclass declaration A subclass can extend only one superclass The subclass can inherit members of the superclass The following code fragment implements the PrintableCharStack class:
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class PrintableCharStack extends CharStack { // Instance method public void printStackElements() { // implementation of the method } // (1) // (2)
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// The constructor calls the constructor of the superclass explicitly public PrintableCharStack(int capacity) { super(capacity); } // (3) }
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The PrintableCharStack class extends the CharStack class at (1) Implementing the printStackElements() method in the PrintableCharStack class requires access to the field stackArray from the superclass CharStack However, this field is private and therefore not accessible in the subclass The subclass can access these fields if the accessibility of the fields is changed to protected in the CharStack class Example 13 uses a version of the class CharStack, which has been modified accordingly Implementation of the printStackElements() method is shown at (2) The constructor of the PrintableCharStack class at (3) calls the constructor of the superclass CharStack in order to initialize the stack properly
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Example 13
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Defining a Subclass
// Source Filename: CharStackjava public class CharStack { // Instance variables protected char[] stackArray; // The array that implements the stack protected int topOfStack; // The top of the stack // The rest of the definition is the same as in Example 12 }
//Filename: PrintableCharStackjava public class PrintableCharStack extends CharStack {
// (1)
// Instance method public void printStackElements() { // (2) for (int i = 0; i <= topOfStack; i++) Systemoutprint(stackArray[i]); // print each char on terminal Systemoutprintln(); } // Constructor calls the constructor of the superclass explicitly PrintableCharStack(int capacity) { super(capacity); } // (3) }
CHAPTER 1: BASICS OF JAVA PROGRAMMING
Objects of the PrintableCharStack class will respond just like the objects of the CharStack class, but they will also have the additional functionality defined in the subclass:
PrintableCharStack pcStack = new PrintableCharStack(3); pcStackpush('H'); pcStackpush('i'); pcStackpush('!'); pcStackprintStackElements(); // Prints "Hi!" on the terminal
17 Aggregation
When building new classes from existing classes using aggregation, a composite object is built from the constituent objects that are its parts Java supports aggregation of objects by reference, since objects cannot contain other objects explicitly The fields can only contain values of primitive data types or reference values to other objects Each object of the CharStack class has a field to store the reference value of an array object that holds the characters Each stack object also has a field of primitive data type int to store the index value that denotes the top of stack This is reflected in the definition of the CharStack class, which contains an instance variable for each of these parts In contrast to the constituent objects whose reference values are stored in fields, the values of primitive data types are themselves stored in the fields of the composite object The aggregation relationship is depicted by the UML diagram in Figure 17, showing that each object of the CharStack class will have one array object of type char associated with it