Java Cookbook

Finding and Using Methods and Fields

Problem

You need more to find arbitrary method or field names in arbitrary classes.

Solution

Use the reflection package java.lang.reflect .

Discussion

If you just wanted to find fields and methods in one particular class, you wouldn’t need this; you could simply create an instance of the class using new and refer to its fields and methods directly. This allows you to find methods and fields in any class, even classes that have not yet been written! Given a class object created as in Section 25.2, you can obtain a list of constructors, a list of methods, or a list of fields. The method getMethods( ) lists the methods available for a given class as an array of Method objects. Since constructor methods are treated specially by Java, there is also a getConstructors( ) method, which returns an array of Constructor objects. Even though “class” is in the package java.lang, the Constructor, Method, and Field objects it returns are in java.lang.reflect, so you need an import of this package. The ListMethods class (Example 25-1) shows how to do this.

Example 25-1. ListMethods.java

import java.lang.reflect.*; /** * List the Constructors and methods */ public class ListMethods { public static void main(String[] argv) throws ClassNotFoundException { if (argv.length == 0) { System.err.println("Usage: ListMethods className"); return; } Class c = Class.forName(argv[0]); Constructor[] cons = c.getConstructors( ); printList("Constructors", cons); Method[] meths ...