4.8 Reflection

Reflection in Java 1.1 refers to the ability of Java classes to reflect upon themselves, or to "look inside themselves." The java.lang.Class class has been greatly enhanced in Java 1.1. It now includes methods that return the fields, methods, and constructors defined by a class. These items are returned as objects of type Field, Method, and Constructor, respectively. These new classes are part of the new java.lang.reflect package, and they each provide methods to obtain complete information about the field, method, or constructor they represent. For example, the Method object has methods to query the name, the parameter types, and the return type of the method it represents. Chapter 12, Reflection provides some examples of using the Reflection API.

Besides allowing a program to inspect the members of a class, the java.lang.reflect package also allows a program to manipulate these fields and methods. The Field class defines methods that get and set the value of the represented field for any given object of the appropriate type. Similarly, the Method object defines an invoke() method that allows the represented method to be invoked, and the Constructor class defines a newInstance() method that creates a new object and invokes the represented constructor on it. java.lang.reflect also defines an Array class. It does not represent a specific array, but defines static methods that read and write array elements and dynamically create new arrays.

With the addition of reflection, the Class class has been expanded to represent not just Java classes, but any Java type, including primitive types and array types. There is a special Class object that represents each of the eight Java primitive types, and another special Class object that represents the void type. These special Class objects are available as constants in the wrapper objects for the primitive types. Integer.TYPE is a Class object that represents the int type, for example, and Void.TYPE is a Class object that represents the void type.

Finally, new Java language syntax makes it easier to obtain a Class object that represents a Java class. If you follow the name of a class, interface, or other type with .class, Java evaluates that expression and returns the corresponding Class object. So, for example, the following two expressions are equivalent:

String.class
Class.forName("java.lang.String")

Note that this syntax also works with primitive type names: you can write short.class, for example, which returns the same value as Short.TYPE.