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Retrieving Class Objects

The entry point for all reflection operations is java.lang.Class. With the exception of java.lang.reflect.ReflectPermission, none of the classes in java.lang.reflect have public constructors. To get to these classes, it is necessary to invoke appropriate methods on Class. There are several ways to get a Class depending on whether the code has access to an object, the name of class, a type, or an existing Class.

Object.getClass()

If an instance of an object is available, then the simplest way to get its Class is to invoke Object.getClass(). Of course, this only works for reference types which all inherit from Object. Some examples follow.

Class c = "foo".getClass();

Returns the Class for String

Class c = System.console().getClass();

There is a unique console associated with the virtual machine which is returned by the static method System.console(). The value returned by getClass() is the Class corresponding to java.io.Console.

enum E { A, B }
Class c = A.getClass();

A is is an instance of the enum E; thus getClass() returns the Class corresponding to the enumeration type E.

byte[] bytes = new byte[1024];
Class c = bytes.getClass();

Since arrays are Objects, it is also possible to invoke getClass() on an instance of an array. The returned Class corresponds to an array with component type byte.

import java.util.HashSet;
import java.util.Set;

Set<String> s = new HashSet<String>();
Class c = s.getClass();

In this case, java.util.Set is an interface to an object of type java.util.HashSet. The value returned by getClass() is the class corresponding to java.util.HashSet.

The .class Syntax

If the type is available but there is no instance then it is possible to obtain a Class by appending ".class" to the name of the type. This is also the easiest way to obtain the Class for a primitive type.

boolean b;
Class c = b.getClass();   // compile-time error

Class c = boolean.class;  // correct

Note that the statement boolean.getClass() would produce a compile-time error because a boolean is a primitive type and cannot be dereferenced. The .class syntax returns the Class corresponding to the type boolean.

Class c = java.io.PrintStream.class;

The variable c will be the Class corresponding to the type java.io.PrintStream.

Class c = int[][][].class;

The .class syntax may be used to retrieve a Class corresponding to a multi-dimensional array of a given type.

Class.forName()

If the fully-qualified name of a class is available, it is possible to get the corresponding Class using the static method Class.forName(). This cannot be used for primitive types. The syntax for names of array classes is described by Class.getName(). This syntax is applicable to references and primitive types.

Class c = Class.forName("com.duke.MyLocaleServiceProvider");

This statement will create a class from the given fully-qualified name.

Class cDoubleArray = Class.forName("[D");

Class cStringArray = Class.forName("[[Ljava.lang.String;");

The variable cDoubleArray will contain the Class corresponding to an array of primitive type double (i.e. the same as double[].class). The cStringArray variable will contain the Class corresponding to a two-dimensional array of String (i.e. identical to String[][].class).

TYPE Field for Primitive Type Wrappers

The .class syntax is a more convenient and the preferred way to obtain the Class for a primitive type; however there is another way to acquire the Class. Each of the primitive types and void has a wrapper class in java.lang that is used for boxing of primitive types to reference types. Each wrapper class contains a field named TYPE which is equal to the Class for the primitive type being wrapped.

Class c = Double.TYPE;

There is a class java.lang.Double which is used to wrap the primitive type double whenever an Object is required. The value of Double.TYPE is identical to that of double.class.

Class c = Void.TYPE;

Void.TYPE is identical to void.class.

Methods that Return Classes

There are several Reflection APIs which return classes but these may only be accessed if a Class has already been obtained either directly or indirectly.

Class.getSuperclass()

Returns the super class for the given class.

Class c = javax.swing.JButton.class.getSuperclass();

The super class of javax.swing.JButton is javax.swing.AbstractButton.

Class.getClasses()

Returns all the public classes, interfaces, and enums that are members of the class including inherited members.

Class<?>[] c = Character.class.getClasses();

Character contains two member classes Character.Subset and Character.UnicodeBlock.

Class.getDeclaredClasses()

Returns all of the classes interfaces, and enums that are explicitly declared in this class.

Class<?>[] c = Character.class.getDeclaredClasses();

Character contains two public member classes Character.Subset and Character.UnicodeBlock and one private class Character.CharacterCache.

{ Class, java.lang.reflect. { Field, Method, Constructor } }.getDeclaringClass()

Returns the Class in which these members were declared. Anonymous classes will not have a declaring class but will have an enclosing class.

import java.lang.reflect.Field;

Field f = System.class.getField("out");
Class c = f.getDeclaringClass();

The field

out
is declared in

System.

public class MyClass {
    static Object o = new Object() { public void m() {} };
    static Class<c> = o.getClass().getEnclosingClass();
}


The declaring class of the anonymous class defined by o is
null.





Class.getEnclosingClass()
 Returns the immediately enclosing class of the class.
Class c = Thread.State.class().getEnclosingClass();


The enclosing class of the enum
Thread.State
is
Thread.

public class MyClass {
    static Object o = new Object() { public void m() {} };
    static Class<c> = o.getClass().getEnclosingClass();
}


The anonymous class defined by o is enclosed by
MyClass.


        
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