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 = 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 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 s = new HashSet(); 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 .
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 (that is, the same as double[].class ). The cStringArray variable will contain the Class corresponding to a two-dimensional array of String (that is, 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.
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 .
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();
import java.lang.reflect.Field; Field f = System.class.getField("out"); Class c = f.getDeclaringClass(); public class MyClass < static Object o = new Object() < public void m() <>>; static Class = 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();
public class MyClass < static Object o = new Object() < public void m() <>>; static Class = o.getClass().getEnclosingClass(); >