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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 ofConstructor
objects. You can use the methods provided by theConstructor
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 theConstructor
object's class with theConstructor.newInstance
method. You'll learn how to invokeConstructor.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:
- It retrieves an array of
Constructor
objects from theClass
object by callinggetConstructors
.- For every element in the
Constructor
array, it creates an array ofClass
objects by invokinggetParameterTypes
. TheClass
objects in the array represent the parameters of the constructor.- The program calls
getName
to fetch the class name for every parameter in theClass
array created in the preceding step.It's not as complicated as it sounds. Here's the source code for the sample program:
In the first line of output generated by the sample program, no parameter types appear because that particularimport 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(")"); } } }Constructor
object represents a no-argument constructor. In subsequent lines, the parameters listed are eitherint
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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