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Using Constructors that Have Arguments

To create an object with a constructor that has arguments, you invoke the newInstance method on a Constructor object, not a Class object. This technique involves several steps:

1. Create a Class object for the object you want to create.
2. Create a Constructor object by invoking getConstructor on the Class object. The getConstructor method has one parameter: an array of Class objects that correspond to the constructor's parameters.
3. Create the object by invoking newInstance on the Constructor object. The newInstance method has one parameter: an Object array whose elements are the argument values being passed to the constructor.

The sample program that follows creates a Rectangle with the constructor that accepts two integers as parameters. Invoking newInstance on this constructor is analogous to this statement:

Rectangle rectangle = new Rectangle(12, 34);

This constructor's arguments are primitive types, but the argument values passed to newInstance must be objects. Therefore, each of the primitive int types is wrapped in an Integer object.

The sample program hardcodes the argument passed to the getConstructor method. In a real-life application such as a debugger, you would probably let the user select the constructor. To verify the user's selection, you could use the methods described in the section Discovering Class Constructors.

The source code for the sample program follows:

import java.lang.reflect.*;
import java.awt.*;

class SampleInstance {

   public static void main(String[] args) {

      Rectangle rectangle;
      Class rectangleDefinition;
      Class[] intArgsClass = new Class[] {int.class, int.class};
      Integer height = new Integer(12);
      Integer width = new Integer(34);
      Object[] intArgs = new Object[] {height, width};
      Constructor intArgsConstructor;

      try {
        rectangleDefinition = Class.forName("java.awt.Rectangle");
        intArgsConstructor = 
            rectangleDefinition.getConstructor(intArgsClass);
        rectangle = 
            (Rectangle) createObject(intArgsConstructor, intArgs);
      } catch (ClassNotFoundException e) {
          System.out.println(e);
      } catch (NoSuchMethodException e) {
          System.out.println(e);
      }
   }

   public static Object createObject(Constructor constructor, 
                                     Object[] arguments) {

      System.out.println ("Constructor: " + constructor.toString());
      Object object = null;

      try {
        object = constructor.newInstance(arguments);
        System.out.println ("Object: " + object.toString());
        return object;
      } catch (InstantiationException e) {
          System.out.println(e);
      } catch (IllegalAccessException e) {
          System.out.println(e);
      } catch (IllegalArgumentException e) {
          System.out.println(e);
      } catch (InvocationTargetException e) {
          System.out.println(e);
      }
      return object;
   }
}

The sample program prints a description of the constructor and the object that it creates:

Constructor: public java.awt.Rectangle(int,int)
Object: java.awt.Rectangle[x=0,y=0,width=12,height=34]

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