Trail: The Reflection API
Lesson: Members
Section: Constructors
Creating New Class Instances
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Creating New Class Instances

There are two reflective methods for creating instances of classes: java.lang.reflect.Constructor.newInstance() and Class.newInstance(). The former is preferred and is thus used in these examples because:

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 java.io.Console. (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 java.nio.charset.Charset.defaultCharset()). The ConsoleCharset 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: 

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


Example output for a UNIX system:

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

Example output for a Windows system:

C:\> java ConsoleCharset
Console charset          :  IBM437
Charset.defaultCharset() :  windows-1252

Another common application of Constructor.newInstance() is to invoke constructors which take arguments. The RestoreAliases 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<String> aliases;
    private EmailAliases(HashMap<String, String> 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<String, String> defaultAliases = new HashMap<String, String>();
    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 Class.getDeclaredConstructor() to find the constructor with a single argument of type java.util.HashMap. Note that it is sufficient to pass HashMap.class since the parameter to any get*Constructor() method requires a class only for type purposes. Due to type erasure, the following expression evaluates to true:

HashMap.class == defaultAliases.getClass()

The example then creates a new instance of the class using this constructor with Constructor.newInstance().

$ java RestoreAliases
Mail keys:
  Duke
  Fang

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