Manipulating Objects

Invoking Methods

Suppose you are writing a debugger which allows the user to select and then invoke methods during a debugging session. Since you don't know at compile time which methods the user will invoke, you cannot hardcode the method name in your source code. Instead, you must follow these steps:

1. Create a Class object that corresponds to the object whose method you want to invoke. See Retrieving Class Objects for more information.
2. Create a Method object by invoking getMethod upon the Class object. The getMethod method has two arguments: a String containing the method name, and an array of Class objects. Each element in the array corresponds to a parameter of the method you want to invoke. For more information on retrieving Method objects, see Obtaining Method Information
3. Invoke the method by calling invoke. The invoke method has two arguments: an array of argument values to be passed to the invoked method, and an object whose class declares or inherits the method.

The sample program that follows shows you how to invoke a method dynamically. The program retrieves the Method object for the String.concat method and then uses invoke to concatenate two String objects.

import java.lang.reflect.*;

class SampleInvoke {

   public static void main(String[] args) {
      String firstWord = "Hello ";
      String secondWord = "everybody.";
      String bothWords = append(firstWord, secondWord);
      System.out.println(bothWords);
   }

   public static String append(String firstWord, String secondWord) {
      String result = null;
      Class c = String.class;
      Class[] parameterTypes = new Class[] {String.class};
      Method concatMethod;
      Object[] arguments = new Object[] {secondWord};
      try {
        concatMethod = c.getMethod("concat", parameterTypes);
        result = (String) concatMethod.invoke(firstWord, arguments);
      } catch (NoSuchMethodException e) {
          System.out.println(e);
      } catch (IllegalAccessException e) {
          System.out.println(e);
      } catch (InvocationTargetException e) {
          System.out.println(e);
      }
      return result;
   }
}

The output of the preceding program is:

Hello everybody.

Manipulating Objects