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Discovering Class Constructors

To create an instance of a class, you invoke a special method called a constructor. Like methods, constructors can be overloaded and are distinguished from one another by their signatures.

You can get information about a class's constructors by invoking the getConstructors method, which returns an array of Constructor objects. You can use the methods provided by the Constructor class to determine the constructor's name, set of modifiers, parameter types, and set of throwable exceptions. You can also create a new instance of the Constructor object's class with the Constructor.newInstance method. You'll learn how to invoke Constructor.newInstance in the section Manipulating Objects.

The sample program that follows prints out the parameter types for each constructor in the Rectangle class. The program performs the following steps:

1. It retrieves an array of Constructor objects from the Class object by calling getConstructors.
2. For every element in the Constructor array, it creates an array of Class objects by invoking getParameterTypes. The Class objects in the array represent the parameters of the constructor.
3. The program calls getName to fetch the class name for every parameter in the Class array created in the preceding step.

It's not as complicated as it sounds. Here's the source code for the sample program:

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

class SampleConstructor {

   public static void main(String[] args) {
      Rectangle r = new Rectangle();
      showConstructors(r);
   }

   static void showConstructors(Object o) {
      Class c = o.getClass();
      Constructor[] theConstructors = c.getConstructors();
      for (int i = 0; i < theConstructors.length; i++) {
         System.out.print("( ");
         Class[] parameterTypes = 
            theConstructors[i].getParameterTypes();
         for (int k = 0; k < parameterTypes.length; k ++) {
            String parameterString = parameterTypes[k].getName();
            System.out.print(parameterString + " ");
            }
         System.out.println(")");
         }
   }
}

In the first line of output generated by the sample program, no parameter types appear because that particular Constructor object represents a no-argument constructor. In subsequent lines, the parameters listed are either int types or fully qualified object names. The output of the sample program is:

( )
( int int )
( int int int int )
( java.awt.Dimension )
( java.awt.Point )
( java.awt.Point java.awt.Dimension )
( java.awt.Rectangle )

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