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For example, in the code fragment Integer intobj = new Integer(7); if (intobj instanceof Integer) { System.out.println("intobj is an Integer"); } if (intobj instanceof Object) { System.out.println("intobj is an Object"); } since java.lang.Integer is a subclass of java.lang.Object, both if statements are true. 5.3 Abstract Classes and Methods In our bank account example, we can think of a savings account, checking (or current) account, and long-term deposit account all being physical examples of subclasses of a generalized account class. The account class is generalized, or abstract, in the sense that it makes no sense to have a corresponding instantiated object: whenever a physical account is opened, it must be a savings, checking, or long-term deposit. Nevertheless, we wish to have an account class that will have defined variables and methods common to all subclasses. Such a class is defined using the abstract keyword in Java. An abstract class cannot be instantiated. The abstract class can define variables and methods that subclasses can use or, if required, can override. An abstract class can optionally consist of abstract methods. An abstract method consists of the method declaration only; the method body is omitted. For any subclass of the abstract class, a method must be declared with the same number and types of arguments (or signature) as the abstract method, or the subclass will fail to compile. This is of practical use in large software engineering projects where we want all subclasses of an abstract class to use the same method signature.
The Abstract Account example shows the code for an abstract Account class. Abstract Account 1 abstract class Account { 2 3 int accountNo; 4 String accountName; 5 double balance; 6 7 public Account(int no, String name, double bal){ 8 accountNo = no; 9 accountName = name; 10 balance = bal; 11 } 12 13 public abstract void deposit(double amount); 14 15 } Abstract Account accountNo, accountName, and balance are variables that are inherited by any subclass of Account. Note the abstract class does have a constructor, but it cannot be directly instantiated. We would need to instantiate a subclass, and the subclass constructor would, in turn, invoke the abstract class constructor using the super keyword. deposit is an example of an abstract method. We assume that all subclasses will have a deposit method with one double argument, amount. We assume that not all subclasses will have a withdraw method; LongTermDeposit, for example, may not allow any withdrawals.
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The Morgan Kaufmann Practical Guide
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Senior Editor Rick Adams Publishing
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Switch Statement 24 Relational and
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DataOutputStream Example 95 DataInp
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Finite Applet Thread Example 161 In
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handling. This concise, essentials-
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Iteration Statements 28 while Loop
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Writing to a Random Access File 99
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Appendix B Swing Events 169 Index 1
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The program consists of a class, Mu
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The second kind of development envi
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This approach means that Java progr
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2.1 Comments Comments are included
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7.7.1 Controlling Serialization Wri
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chapter 8 Developing GUIs From its
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8.2.1 Button This is one of the sim
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Figure 8.3: Swing component inherit
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8.2.4 Radio Button A radio button g
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8.2.8 List A list component display
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8.3.2 Background and Foreground Col
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cp = this.getContentPane(); cp.add(
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JPanel mypanel; declares a mypanel
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To specify an alignment, use the co
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classes are not part of the Java la
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8.7.1 ActionListener ActionListener
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itemStateChanged 1 public void item
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The listener registration is perfor
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8.9 CustomerDetails Example In this
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92 if (e.getSource() instanceof JCo
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creates a scrollpane object, addres
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HEIGHT = 200 gives the initial heig
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Figure 8.9: Applet Viewer invoking
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24 class Panel extends JPanel { 25
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chapter 9 Collections The Java Coll
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10 c.add(''Brown"); 11 c.add("Able"
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c2.add("Able"); The statement c.add
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23 return empNumber == e.empNumber
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Suppose we want to sort by name and
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The List interface also provides it
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There is no iterator object that it
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Details of all the Collections clas
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PrintNumbersThread 1 public class P
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10.3 Thread Priorities By default,
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The code for the corresponding HTML
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22 } catch (InterruptedException e)
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We can create a synchronized block
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appendix B Swing Events The followi
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