Java/Reflection/Constructor

Class Reflection: find out the constructor information

   <source lang="java">
 

/* From http://java.sun.ru/docs/books/tutorial/index.html */ /*

* Copyright (c) 1995-1998 Sun Microsystems, Inc. All Rights Reserved.
* 
* Permission to use, copy, modify, and distribute this software and its
* documentation for NON-COMMERCIAL purposes and without fee is hereby granted
* provided that this copyright notice appears in all copies. Please refer to
* the file "copyright.html" for further important copyright and licensing
* information.
* 
* SUN MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF THE
* SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED
* WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR
* NON-INFRINGEMENT. SUN SHALL NOT BE LIABLE FOR ANY DAMAGES SUFFERED BY
* LICENSEE AS A RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS
* DERIVATIVES.
*/

import java.awt.Rectangle; import java.lang.reflect.Constructor; public 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(")");
   }
 }

}

 </source>
   
  
 
  

Constructor Reflection

   <source lang="java">
 

import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Method; import java.lang.reflect.Modifier; public class ReflectionTest {

 public static void main(String[] args) {
   String name = "java.util.Date";
   try {
     Class cl = Class.forName(name);
     Class supercl = cl.getSuperclass();
     System.out.print("class " + name);
     if (supercl != null && !supercl.equals(Object.class))
       System.out.println(" extends " + supercl.getName());
     System.out.print("Its constructors:");
     printConstructors(cl);
     System.out.println();
   } catch (ClassNotFoundException e) {
     System.out.println("Class not found.");
   }
 }
 public static void printConstructors(Class cl) {
   Constructor[] constructors = cl.getDeclaredConstructors();
   for (int i = 0; i < constructors.length; i++) {
     Constructor c = constructors[i];
     Class[] paramTypes = c.getParameterTypes();
     String name = c.getName();
     System.out.print(Modifier.toString(c.getModifiers()));
     System.out.print(" " + name + "(");
     for (int j = 0; j < paramTypes.length; j++) {
       if (j > 0)
         System.out.print(", ");
       System.out.print(paramTypes[j].getName());
     }
     System.out.println(");");
   }
 }

}

 </source>
   
  
 
  

Create object using Constructor object

   <source lang="java">
 

import java.lang.reflect.Constructor; public class Main {

 public static void main(String[] args) throws Exception {
   Class clazz = String.class;
   Constructor constructor = clazz.getConstructor(new Class[] { String.class });
   String object = (String) constructor.newInstance(new Object[] { "Hello World!" });
   System.out.println("String = " + object);
   constructor = clazz.getConstructor(new Class[] { StringBuilder.class });
   object = (String) constructor
       .newInstance(new Object[] { new StringBuilder("Hello Universe!") });
   System.out.println("String = " + object);
 }

}

 </source>
   
  
 
  

Creating an Object Using a Constructor Object

   <source lang="java">
 

import java.lang.reflect.Constructor; public class Main {

 public static void main(String[] argv) throws Exception {
   Constructor con = java.awt.Point.class.getConstructor(new Class[] { int.class, int.class });
   java.awt.Point obj = (java.awt.Point) con.newInstance(new Object[] { new Integer(123),
       new Integer(123) });
 }

}

 </source>
   
  
 
  

Get all construtors from a class

   <source lang="java">
 

import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Method; public class Main{

 public static void main(String args[]) throws Exception {
   Class c = Class.forName("MyClass");
   System.out.println("Constructors:");
   Constructor constructors[] = c.getDeclaredConstructors();
   for (Constructor cons : constructors)
     System.out.println(" " + cons);
 }

} class MyClass {

 private int count;
 MyClass(int c) {
   count = c;
 }
 MyClass() {
   count = 0;
 }
 void setCount(int c) {
   count = c;
 }
 int getCount() {
   return count;
 }
 void showcount() {
   System.out.println("count is " + count);
 }

}

 </source>
   
  
 
  

Get constructors of a class object

   <source lang="java">
 

import java.lang.reflect.Constructor; public class Main {

 public static void main(String[] args) throws Exception {
   Class clazz = String.class;
   Constructor[] constructors = clazz.getDeclaredConstructors();
   for (Constructor constructor : constructors) {
     String name = constructor.getName();
     System.out.println("Constructor name= " + name);
     Class[] paramterTypes = constructor.getParameterTypes();
     for (Class c : paramterTypes) {
       System.out.println("Param type name = " + c.getName());
     }
   }
   Constructor constructor = String.class.getConstructor(new Class[] { String.class });
   System.out.println("Constructor     = " + constructor.getName());
 }

}

 </source>
   
  
 
  

Getting a Constructor of a Class Object: By obtaining a list of all Constructors object

   <source lang="java">
 

import java.lang.reflect.Constructor; public class Main {

 public static void main(String[] argv) throws Exception {
   Constructor[] cons = String.class.getDeclaredConstructors();
   for (int i = 0; i < cons.length; i++) {
     Class[] paramTypes = cons[i].getParameterTypes();
     System.out.println(cons[i]);
   }
 }

}

 </source>
   
  
 
  

Getting a Constructor of a Class Object: By obtaining a particular Constructor object.

   <source lang="java">
 

import java.lang.reflect.Constructor; public class Main {

 public static void main(String[] argv) throws Exception {
   Constructor con = java.awt.Point.class.getConstructor(new Class[] { int.class, int.class });
   System.out.println(con);
 }

}

 </source>
   
  
 
  

Has Declared Constructor

   <source lang="java">

import java.lang.reflect.Constructor; public class ReflectionUtils {

 public static boolean hasDeclaredConstructor(Class targetClass, Class[] partypes) {
   Constructor constructor = null;
   try {
     constructor = targetClass.getConstructor(partypes);
   }catch (Exception e) {
     e.printStackTrace();
   }
   return constructor != null;
 }
 

}

 </source>
   
  
 
  

Object Reflection: invoke constructor with parameters

   <source lang="java">
 

/* From http://java.sun.ru/docs/books/tutorial/index.html */ /*

* Copyright (c) 1995-1998 Sun Microsystems, Inc. All Rights Reserved.
* 
* Permission to use, copy, modify, and distribute this software and its
* documentation for NON-COMMERCIAL purposes and without fee is hereby granted
* provided that this copyright notice appears in all copies. Please refer to
* the file "copyright.html" for further important copyright and licensing
* information.
* 
* SUN MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF THE
* SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED
* WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR
* NON-INFRINGEMENT. SUN SHALL NOT BE LIABLE FOR ANY DAMAGES SUFFERED BY
* LICENSEE AS A RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS
* DERIVATIVES.
*/

import java.awt.Rectangle; import java.lang.reflect.Constructor; import java.lang.reflect.InvocationTargetException; public 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;
 }

}

 </source>
   
  
 
  

Passing a parameter to the constructor and calling a method dynamically

   <source lang="java">
 

public class Main {

 public static void main(String args[]) throws Exception{
     String name = "java.lang.String";
     String methodName = "toLowerCase";
     Class cl = Class.forName(name);
     java.lang.reflect.Constructor constructor = cl.getConstructor(new Class[] { String.class });
     Object invoker = constructor.newInstance(new Object[] { "AAA" });
     Class arguments[] = new Class[] {};
     java.lang.reflect.Method objMethod = cl.getMethod(methodName, arguments);
     Object result = objMethod.invoke(invoker, (Object[]) arguments);
     System.out.println(result);
 }

}

 </source>