Learning Java

The Class Class

The last method of Object we need to discuss is getClass( ) . This method returns a reference to the Class object that produced the Object instance.

A good measure of the complexity of an object-oriented language is the degree of abstraction of its class structures. We know that every object in Java is an instance of a class, but what exactly is a class? In C++, objects are formulated by and instantiated from classes, but classes are really just artifacts of the compiler. Thus, you see classes only mentioned in C++ source code, not at runtime. By comparison, classes in Smalltalk are real, runtime entities in the language that are themselves described by “metaclasses” and “metaclass classes.” Java strikes a happy medium between these two languages with what is effectively a two-tiered system that uses Class objects.

Classes in Java source code are represented at runtime by instances of the java.lang.Class class. There’s a Class object for every class you use; this Class object is responsible for producing instances for its class. You don’t have to worry about any of this unless you are interested in loading new kinds of classes dynamically at runtime. The Class object is also the basis for “reflecting” on a class to find out its methods and other properties; we’ll discuss this feature in the next section.

We get the Class associated with a particular object with the getClass( ) method:

String myString = "Foo!" 
Class c = myString.getClass( );

We can also get the Class reference for a particular class statically, using the special .class notation:

Class c = String.class;

The .class reference looks like a static field that exists in every class. However, it is really resolved by the compiler.

One thing we can do with the Class object is ask for the name of the object’s class:

String s = "Boofa!";  
Class mycls= s.getClass( );  
System.out.println( mycls.getName( ) );   // "java.lang.String"

Another thing that we can do with a Class is to ask it to produce a new instance of its type of object. Continuing with the previous example:

try {  
    String s2 = (String)strClass.newInstance( );  
}  
catch ( InstantiationException e ) { ... }  
catch ( IllegalAccessException e ) { ... }

newInstance( ) has a return type of Object, so we have to cast it to a reference of the appropriate type. (newInstance( ) has to be able to return any kind of constructed object.) A couple of problems can occur here. An InstantiationException indicates we’re trying to instantiate an abstract class or an interface. IllegalAccessException is a more general exception that indicates we can’t access a constructor for the object. Note that newInstance( ) can create only an instance of a class that has an accessible default constructor. It doesn’t allow us to pass any arguments to a constructor. (But see Section 7.3.4 later in this chapter.)

All this becomes more meaningful when we add the capability to look up a class by name. forName( ) is a static method of Class that returns a Class object given its name as a String:

try {  
    Class sneakersClass = Class.forName("Sneakers");  
}   
catch ( ClassNotFoundException e ) { ... }

A ClassNotFoundException is thrown if the class can’t be located.

Combining these tools, we have the power to load new kinds of classes dynamically. When combined with the power of interfaces, we can use new data types by name in our applications:

interface Typewriter {  
    void typeLine( String s );  
    ...  
}  
  
class Printer implements Typewriter {  
    ...  
}  
  
class MyApplication {  
    ...  
    String outputDeviceName = "Printer";  
  
    try {  
        Class newClass = Class.forName( outputDeviceName );  
        Typewriter device = (Typewriter)newClass.newInstance( );  
        ...  
        device.typeLine("Hello...");  
    }  
    catch ( Exception e ) { ... }
}

Here we have an application loading a class implementation (Printer which implements the Typewriter interface) knowing only its name. Imagine the name was entered by the user or looked up from a configuration file.