Creating New Class Instances

There are two reflective methods for creating instances of classes:  and . The former is preferred and is thus used in these examples because:

•  can only invoke the zero-argument constructor, while  may invoke any constructor, regardless of the number of parameters.
•  throws any exception thrown by the constructor, regardless of whether it is checked or unchecked.  always wraps the thrown exception with an .
•  requires that the constructor be visible;  may invoke private constructors under certain circumstances.

Sometimes it may be desirable to retrieve internal state from an object which is only set after construction. Consider a scenario where it is necessary to obtain the internal character set used by . (The Console character set is stored in an private field and is not necessarily the same as the Java virtual machine default character set returned by ). The  example shows how this might be achieved:

import java.io.Console;import java.nio.charset.Charset;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import static java.lang.System.out;public class ConsoleCharset {    public static void main(String... args) {	Constructor[] ctors = Console.class.getDeclaredConstructors();	Constructor ctor = null;	for (int i = 0; i < ctors.length; i++) {	    ctor = ctors[i];	    if (ctor.getGenericParameterTypes().length == 0)		break;	}	try {	    ctor.setAccessible(true); 	    Console c = (Console)ctor.newInstance();	    Field f = c.getClass().getDeclaredField("cs");	    f.setAccessible(true);	    out.format("Console charset         :  %s%n", f.get(c));	    out.format("Charset.defaultCharset():  %s%n",		       Charset.defaultCharset());        // production code should handle these exceptions more gracefully	} catch (InstantiationException x) {	    x.printStackTrace(); 	} catch (InvocationTargetException x) { 	    x.printStackTrace();	} catch (IllegalAccessException x) {	    x.printStackTrace();	} catch (NoSuchFieldException x) {	    x.printStackTrace();	}    }}

Note: 

 will only succeed if the constructor is has zero arguments and is already accessible. Otherwise, it is necessary to use as in the above example.

Example output for a UNIX system:

$ java ConsoleCharsetConsole charset          :  ISO-8859-1Charset.defaultCharset() :  ISO-8859-1

Example output for a Windows system:

C:\> java ConsoleCharsetConsole charset          :  IBM437Charset.defaultCharset() :  windows-1252

Another common application of  is to invoke constructors which take arguments. The  example finds a specific single-argument constructor and invokes it:

import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import java.util.HashMap;import java.util.Map;import java.util.Set;import static java.lang.System.out;class EmailAliases {    private Set
    
      aliases;    private EmailAliases(HashMap
     
       h) {	aliases = h.keySet();    }    public void printKeys() {	out.format("Mail keys:%n");	for (String k : aliases)	    out.format("  %s%n", k);    }}public class RestoreAliases {    private static Map
      
        defaultAliases = new HashMap
       
        ();    static {	defaultAliases.put("Duke", "duke@i-love-java");	defaultAliases.put("Fang", "fang@evil-jealous-twin");    }    public static void main(String... args) {	try {	    Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class);	    ctor.setAccessible(true);	    EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases);	    email.printKeys();        // production code should handle these exceptions more gracefully	} catch (InstantiationException x) {	    x.printStackTrace();	} catch (IllegalAccessException x) {	    x.printStackTrace();	} catch (InvocationTargetException x) {	    x.printStackTrace();	} catch (NoSuchMethodException x) {	    x.printStackTrace();	}    }}
       
      
     
    

This example uses  to find the constructor with a single argument of type . Note that it is sufficient to passHashMap.class since the parameter to any get*Constructor() method requires a class only for type purposes. Due to , the following expression evaluates to true:

HashMap.class == defaultAliases.getClass()

The example then creates a new instance of the class using this constructor with .

$ java RestoreAliasesMail keys:  Duke  Fang