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Tip of the Day
Language: Java
Expertise: Advanced
Dec 21, 1998

Java's Alternative to Function Pointers

Although Java borrows a lot of concepts and syntax from C++, it also leaves out some of C++'s main features to meet its goal of simplicity. For example, Java excluded memory pointers and function pointers from its arsenal. Function pointers allow flexibility in method invocation at run time. A pointer to the function that needs to be called can be passed in as an argument to the calling function. A method can "call back" another method that is specified as one of its arguments.

The inner class construct in JDK 1.1 provides a novel approach to achieve the same goal that function pointers achieve in C++. Anonymous classes in method invocations allow a dynamically defined class to be passed in as a parameter. As a result, the calling class can invoke a method on this class. For example:

1.     interface CallbackIfc {
2.       public void callMe();
3.     }
5.     // This class calls a method on class B.
6.     public class CallbackTester {
7.       CallerClass cc = new CallerClass();
9.       public void sendCallback() {
10.         cc.callback (new CallbackIfc() {     
11.                        public void callMe() {
12.                          // Implementation code here
13.                        }
14.                      }
15.                    );
16.         }
17.     }
19. // This class calls back a method on class CallbackIfc.
20. class CallerClass {
21.   public void callback (CallbackIfc c) {
22.     c.callMe();
23.   }
24. }
Lines 1-3 define a new interface called CallbackIfc to be used as a callback class. CallbackIfc defines a single method (callMe()). Lines 20-24 define a class called CallerClass that will exercise the callback. CallerClass has a single method (callback()) that calls a method (callMe()) on an object of the class CallbackIfc. This object is passed in as an argument. Lines 5-17 define the class that invokes callback() on the class CallerClass. It defines a single method (sendCallback()) that calls the method callback() on an object of the class CallerClass. Note that it defines an instance of the interface CallbackIfc within the parentheses. This defines an anonymous class that implements the interface CallbackIfc.
Ajit Sagar
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