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Tip of the Day
Language: Java Language
Expertise: Beginner
Feb 1, 1999

Function Pointers

Question:
I'm converting an fsm from C to Java. The C code has a struct and one of the members contains a pointer that contains the address of a function. How can this address issue be resolved in Java?

Answer:
As you already know, Java does not have pointers. As a result, you have to go through some hoops to port C code that relies on function pointers. The standard way to simulate function pointers in Java is to create an interface containing one or more methods. Then you need to create a class that implements the method(s), which effectively acts as a container for the function. In place of of using a function pointer, you would now use a reference to an instantiation of the class. This approach is littered throughout the Java AWT and Swing APIs to implement callback functions. In Java 1.1, inner classes were added partly as a more convenient way of creating arbitrary wrapper classes to serve in place of function pointers. More often than not, you want a method of a particular class to act as a function pointer. Rather than force that class to implement a specific interface, you can create an inner class or an anonymous class that implements the "function pointer" interface and calls the necessary outer class's method.

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