forName() Method - Loading Classes

JVM Tutorials - Herong's Tutorial Examples

∟forName() Method - Loading Classes

This section provides a tutorial example on how to load a given class explicitly using the forName() method and create a new object using the newInstance() method.

Usually, a class can be used loaded implicitly on an on-demand basis when you create the first object of the class using the "new" operator.

But you can also use Class.forName() methods to load a class explicitly:

static Class<?> forName(String className) - Returns the Class object associated with the class or interface with the given string name.
static Class<?> forName(String name, boolean initialize, ClassLoader loader) - Returns the Class object associated with the class or interface with the given string name, using the given class loader.

Once a class is loaded as a Class object, a new instance of the class can be created with the newInstance() method on the Class object. For example:

   Class<?> c = Class.forName("java.util.Date");
   java.util.Date d = null;
   // d = (java.util.Date) c.newInstance(); // deprecated
   d = (java.util.Date) c.getDeclaredConstructor().newInstance();

The above code is equivalent to:

   java.util.Date d = new java.util.Date();

Here is tutorial example program on how to load a class and create a new instance:

/* ClassForName.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class ClassForName {
   public static void main(String[] a) {
      java.io.PrintStream out = System.out;
      Class<?> c = null;
      Object o = null;

      // Loading a class explicitly and create a new instance
      try {
         c = Class.forName("java.util.Random");
         o = c.getDeclaredConstructor().newInstance();
         java.util.Random r = (java.util.Random) o;
         out.println("Random number: "+r.nextInt());
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

When executed, I got this result:

herong> java ClassForName
Random number: -1655424893

herong> java ClassForName
Random number: 1511927719

herong> java ClassForName
Random number: 1247702944

Note that:

Calling the newInstance() method is equivalent to calling the constructor of the class with no arguments.
The object returned from the newInstance() method should be casted back to the original class type.