Creating object instances out of a class name

Basic means

Frequently, you have to create an object instance out of just a class name, especially in J2EE applications, where you use interfaces, against you program the application, and want to configure some implementation classes of these interfaces in an XML file. At application startup, you read this configuration and create the specified objects.

The basic mean for this behaviour is the class Class which helps us with the creation of object instances.

Example

The following example assumes that there is an interface IPlugin which describes a plugin interface with a method execute(String):

public interface IPlugin {
  public String execute(String pstrArgs);
}

java Test de.kksoftware.krims.MyPlugin "Hi Ingo"

public class ClassTest {
  public static void main(String args[]){

    // get parameter
    if (args.length != 2){
      System.out.println("usage: java Test  ");
      System.exit(0);
    }
    String strClassName = args[0];
    String arg = args[1];

    // first, get the class template for the requested object
    Class oclsPlugin = null;
    try {
      oclsPlugin = Class.forName(strClassName);
    } catch (ClassNotFoundException e){
      System.out.println("class not found: " + strClassName);
      System.exit(1);
    }

    // then create an object instance out of the class template and try
    // to cast it to the desired interface
    IPlugin oPlugin = null;
    try {
      Object o = oclsPlugin.newInstance();
      if (o instanceof IPlugin){
        oPlugin = (IPlugin)o;
      } else {
        System.out.println(strClassName + " does not support IPLugin interface");
        System.exit(1);
      }
    } catch (InstantiationException e){
      System.out.println("class not instantiated: " + strClassName);
      System.exit(1);
    } catch (IllegalAccessException e){
      System.out.println("class not accessible: "+ strClassName);
      System.exit(1);
    }

    // execute a test method on the created object
    String strResult = oPlugin.execute(arg);
    System.out.println("Ergebnis: " + strResult);

  }
}

• The first one is shown in the example, we just have the class name and use the method forName(String) of the Class object to get the class template itself;
• we can get a class itself by using a construction like

  Class oclsPlugin = de.kksoftware.krims.MyPlugin.class;
  
  // or from an object instance
  MyPlugin oPlugin = new MyPlugin();
  Class oclsPlugin = oPlugin.getClass();
  

  This way is preferrable if you do not want to configure the class, but hard-code it. This is very appropriate if you have, e.g. a method call whose return type you want to specifiy, as in

  List getAllX(Class pclsReturnType){
    // create a List of elements which have the specified class
  }
  

  In this way, you can easily get a List with elements of a specified class.

Class information

If you have a Class object, you can examine it for debugging purposes by simply printing it to an output stream. For the following code fragmet, you get an output like this:

Myplugin p = new MyPlugin();
System.out.println("p="+p);
System.out.println("p="+p.getClass().getName());

// yields:
// p=MyPlugin@78c37
// p=MyPlugin

In some cases, the output is more complex, as in the example below:

String sa[] = new String[3];
System.out.println("sa="+sa.getClass().getName());

int ia[] = new int[3];
System.out.println("ia="+ia.getClass().getName());

// yields:
// sa=[Ljava.lang.String
// ia=[I

Calling methods on a Class object

With an object of class Class, you can do very exciting things such as calling a method if you only have the textual (String) representation of the method name. You do this in the following manner:

try {
  // get an object and its class
  MyPlugin p = new MyPlugin();
  Class oclsPlugin = MyPlugin.class;

  // prepare the call
  Class[] oclsSignature = new Class[]{ String.class };
  java.lang.reflect.Method oM = oclsPlugin.getMethod("execute", oclsSignature);

  // execute the method
  System.out.println("Result is " + oM.invoke(p, new Object[]{ "Ingo" }));

} catch (java.lang.reflect.InvocationTargetException te){
	System.out.println("invocation target exception");
} catch (NoSuchMethodException me){
	System.out.println("No such method");
} catch (IllegalAccessException ie){
	System.out.println("Illegal access");
}

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