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Trail: The Reflection API
Lesson: Members
Section: Constructors
Members
Fields
Obtaining Field Types
Retrieving and Parsing Field Modifiers
Getting and Setting Field Values
Troubleshooting
Methods
Obtaining Method Type Information
Retrieving and Parsing Method Modifiers
Invoking Methods
Troubleshooting
Constructors
Finding Constructors
Retrieving and Parsing Constructor Modifiers
Creating New Class Instances
Troubleshooting
Home Page > The Reflection API > Members
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:
> - `Class.newInstance()` can only invoke the zero-argument constructor, while `Constructor.newInstance()` may invoke any constructor, regardless of the number of parameters.
> - `Class.newInstance()` throws any exception thrown by the constructor, regardless of whether it is checked or unchecked. `InvocationTargetException```.
> - `Class.newInstance()` requires that the constructor be visible; `Constructor.newInstance()` 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 `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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