Learning Java

Inner Classes

Java 1.1 added to the language a large heap of syntactic sugar called inner classes. Simply put, classes in Java can be declared at any level of scope. That is, you can declare a class within any set of curly braces (i.e., almost anywhere that you could put any other Java statement), and its visibility is limited to that scope in the same way that the name of a variable or method would be. Inner classes are a powerful and aesthetically pleasing facility for structuring code. Their even sweeter cousins, anonymous inner classes , are another powerful shorthand that make it seem as if you can create classes dynamically within Java’s statically typed environment.

However, if you delve into the inner workings of Java, inner classes are not quite as aesthetically pleasing or dynamic. We said that they are syntactic sugar; this means that they let you leverage the compiler by writing a few lines of code that trigger a lot of behind-the-scenes work somewhere between the compiler’s front end and the byte-code. Inner classes rely on code generation; they are a feature of the Java language, but not of the Java virtual machine. As a programmer you may never need be aware of this; you can simply rely on inner classes like any other language construct. However, you should know a little about how inner classes work, to better understand the results and a few potential side effects.

To this point, all of our classes have been top-level classes. We have declared them, freestanding, at the package level. Inner classes are essentially nested classes, like this:

Class Animal { 
    Class Brain {  
        ... 
    } 
}

Here the class Brain is an inner class: it is a class declared inside the scope of class Animal. Although the details of what that means require a fair bit of explanation, we’ll start by saying that the Java language tries to make the meaning, as much as possible, the same as for the other Java entities (methods and variables) living at that level of scope. For example, let’s add a method to the Animal class:

Class Animal { 
    Class Brain {  
        ... 
    } 
    void performBehavior( ) { ... } 
}

Both the inner class Brain and the method performBehavior( ) are within the scope of Animal. Therefore, anywhere within Animal we can refer to Brain and performBehavior( ) directly, by name. Within Animal we can call the constructor for Brain (new Brain( )) to get a Brain object, or invoke performBehavior( ) to carry out that method’s function. But neither Brain nor performBehavior( ) are accessible outside of the class Animal without some additional qualification.

Within the body of the Brain class and the body of the performBehavior( ) method, we have direct access to all of the other methods and variables of the Animal class. So, just as the performBehavior( ) method could work with the Brain class and create instances of Brain, code within the Brain class can invoke the performBehavior( ) method of Animal as well as work with any other methods and variables declared in Animal.

That last bit has important consequences. From within Brain we can invoke the method performBehavior( ); that is, from within an instance of Brain we can invoke the performBehavior( ) method of an instance of Animal. Well, which instance of Animal? If we have several Animal objects around (say, a few Cats and Dogs), we need to know whose performBehavior( ) method we are calling. What does it mean for a class definition to be “inside” another class definition? The answer is that a Brain object always lives within a single instance of Animal: the one that it was told about when it was created. We’ll call the object that contains any instance of Brain its enclosing instance.

A Brain object cannot live outside of an enclosing instance of an Animal object. Anywhere you see an instance of Brain, it will be tethered to an instance of Animal. Although it is possible to construct a Brain object from elsewhere (i.e., another class), Brain always requires an enclosing instance of Animal to “hold” it. We’ll also say now that if Brain is to be referred to from outside of Animal, it acts something like an Animal.Brain class. And just as with the performBehavior( ) method, modifiers can be applied to restrict its visibility. There is even an interpretation of the static modifier, which we’ll talk about a bit later. However, the details are somewhat boring and not immediately useful. For more information, consult a full language reference, such as Java Language Reference, Second Edition, by Mark Grand (O’Reilly & Associates). Before we get too far afield, let’s turn to a more compelling example.

A particularly important use of inner classes is to make adapter classes. An adapter class is a "helper” class that ties one class to another in a very specific way. Using adapter classes, you can write your classes more naturally, without having to anticipate every conceivable user’s needs in advance. Instead, you provide adapter classes that marry your class to a particular interface. As an example, let’s say that we have an EmployeeList object:

public class EmployeeList { 
    private Employee [] employees = ... ; 
    ... 
}

EmployeeList holds information about a set of employees. Let’s say that we would like to have EmployeeList provide its elements via an iterator. An iterator is a simple interface to a list of objects. The java.util.Iterator interface has several methods:

public interface Iterator { 
    boolean hasMore ( ); 
    Object next( );
	void remove( ); 
}

It lets us step through its elements, asking for the next one and testing to see if more remain. The iterator is a good candidate for an adapter class because it is an interface that our EmployeeList can’t readily implement itself. Why can’t the list implement the iterator directly? Because an iterator is a “one-way,” disposable view of our data. It isn’t intended to be reset and used again. It may also be necessary for there to be multiple iterators walking through the list at different points. We must therefore keep the iterator implementation separate from the EmployeeList itself. This is crying out for a simple class to provide the iterator capability. But what should that class look like?

Well, before we knew about inner classes, our only recourse would have been to make a new “top-level” class. We would probably feel obliged to call it EmployeeListIterator:

class EmployeeListIterator implements Iterator { 
    // lots of knowledge about EmployeeList 
    ... 
}

Here we have a comment representing the machinery that the EmployeeListIterator requires. Think for just a second about what you’d have to do to implement that machinery. The resulting class would be completely coupled to the EmployeeList and unusable in other situations. Worse, to function it must have access to the inner workings of EmployeeList. We would have to allow EmployeeListIterator access to the private array in EmployeeList, exposing this data more widely than it should be. This is less than ideal.

This sounds like a job for inner classes. We already said that EmployeeListIterator was useless without an EmployeeList; this sounds a lot like the “lives inside” relationship we described earlier. Furthermore, an inner class lets us avoid the encapsulation problem, because it can access all the members of its enclosing instance. Therefore, if we use an inner class to implement the iterator, the array employees can remain private, invisible outside the EmployeeList. So let’s just shove that helper class inside the scope of our EmployeeList:

public class EmployeeList { 
    private Employee [] employees = ... ; 
    ... 

    class Iterator implements java.util.Iterator { 
        int element = 0; 

        boolean hasMore( ) { 
            return  element < employees.length ; 
        } 

        Object next( ) { 
            if ( hasMoreElements( ) ) 
                return employees[ element++ ]; 
            else  
                throw new NoSuchElementException( ); 
        } 

        void remove( ) {
             throw new UnsupportedOperationException( );
        }
    } 
}

Now EmployeeList can provide an accessor method like the following to let other classes work with the list:

Iterator getIterator( ) { 
        return new Iterator( ); 
    }

One effect of the move is that we are free to be a little more familiar in the naming of our iterator class. Since it is no longer a top-level class, we can give it a name that is appropriate only within the EmployeeList. In this case, we’ve named it Iterator to emphasize what it does—but we don’t need a name like EmployeeIterator that shows the relationship to the EmployeeList class because that’s implicit. We’ve also filled in the guts of the Iterator class. As you can see, now that it is inside the scope of EmployeeList, Iterator has direct access to its private members, so it can directly access the employees array. This greatly simplifies the code and maintains compile-time safety.

Before we move on, we should note that inner classes can have constructors, even though we didn’t need one in this example. They are in all respects real classes.

Inner Classes Within Methods

Inner classes may also be declared within the body of a method. Returning to the Animal class, we could put Brain inside the performBehavior( ) method if we decided that the class was useful only inside of that method:

Class Animal { 
    void performBehavior( ) {  
        Class Brain {  
            ... 
        } 
    } 
}

In this situation, the rules governing what Brain can see are the same as in our earlier example. The body of Brain can see anything in the scope of performBehavior( ) and above it (in the body of Animal). This includes local variables of performBehavior( ) and its arguments. But there are a few limitations and additional restrictions, as described in the following sections.

Limitations on inner classes

performBehavior( ) is a method, and methods have limited lifetimes. When they exit, their local variables normally disappear into the stacky abyss. But an instance of Brain (like any object) lives on as long as it is referenced. So Java must make sure that any local variables used by instances of Brain created within an invocation of performBehavior( ) also live on. Furthermore, all of the instances of Brain that we make within a single invocation of performBehavior( ) must see the same local variables. To accomplish this, the compiler must be allowed to make copies of local variables. Thus, their values cannot change once an inner class has seen them. This means that any of the method’s local variables that are referenced by the inner class must be declared final. The final modifier means that they are constant once assigned. This is a little confusing and easy to forget, but the compiler will graciously remind you.

Static inner classes

We mentioned earlier that the inner class Brain of the class Animal could in some ways be considered an Animal.Brain class. That is, it is possible to work with a Brain from outside the Animal class, using just such a qualified name: Animal.Brain. But given that our Animal.Brain class always requires an instance of an Animal as its enclosing instance, some explicit setup is needed.^[23]

But there is another situation in which we might use inner classes by name. An inner class that lives within the body of a top-level class (not within a method or another inner class) can be declared static. For example:

class Animal  { 
    static class MigrationPattern {  
        ... 
    } 
    ... 
}

A static inner class such as this acts just like a new top-level class called Animal.MigrationPattern. We can use it just like any other class, without regard to any enclosing instances. Although this seems strange, it is not inconsistent, since a static member never has an object instance associated with it. The requirement that the inner class be defined directly inside a top-level class ensures that an enclosing instance won’t be needed. If we have permission, we can create an instance of the class using the qualified name:

Animal.MigrationPattern stlToSanFrancisco =
    new Animal.MigrationPattern( );

As you see, the effect is that Animal acts something like a mini-package, holding the MigrationPattern class. Here we have used the fully qualified name, but we could also import it like any other class:

Import Animal.MigrationPattern;

This enables us to refer to it simply as MigrationPattern. We can use all of the standard visibility modifiers on inner classes, so a static inner class could be private, protected, default, or publicly visible.

Another example: the Java 2D API uses static inner classes to implement specialized shape classes. For example, the java.awt.geom.Rectangle2D class has two inner classes, Float and Double, that implement two different precisions. These are actually trivial subclasses; it would have been sad to have to multiply the number of top-level classes by three to accommodate them.

Anonymous inner classes

Now we get to the best part. As a general rule, the more deeply encapsulated and limited in scope our classes are, the more freedom we have in naming them. We saw this in our previous iterator example. This is not just a purely aesthetic issue. Naming is an important part of writing readable and maintainable code. We generally want to give things the most concise and meaningful names possible. A corollary to this is that we prefer to avoid doling out names for purely ephemeral objects that are going to be used only once.

Anonymous inner classes are an extension of the syntax of the new operation. When you create an anonymous inner class, you combine the class’s declaration with the allocation of an instance of that class. After the new operator, you specify either the name of a class or an interface, followed by a class body. The class body becomes an inner class, which either extends the specified class or, in the case of an interface, is expected to implement the specified interface. A single instance of the class is created and returned as the value.

For example, we could do away with the declaration of the Iterator class in the EmployeeList example by using an anonymous inner class in the getIterator( ) method:

Iterator getIterator( ) { 
    return new Iterator( ) { 
        int element = 0; 
        boolean hasMore( ) { 
            return  element < employees.length ; 
        } 
        Object next( ) { 
            if ( hasMoreElements( ) ) 
                return employees[ element++ ]; 
            else  
                throw new NoSuchElementException( ); 
        } 
        void remove( ) {
             throw new UnsupportedOperationException( );
        }
    }; 
}

Here we have simply moved the guts of Iterator into the body of an anonymous inner class. The call to new implies a class that implements the Iterator interface and returns an instance of the class as its result. Note the extent of the curly braces and the semicolon at the end. The getIteratgor( ) method contains a single return statement.

But the previous code certainly does not improve readability. Inner classes are best used when you want to implement a few lines of code, when the verbiage and conspicuousness of declaring a separate class detracts from the task at hand.

Here’s a better example. Suppose that we want to start a new thread to execute the performBehavior( ) method of our Animal:

new Thread( ) { 
    public void run() {  performBehavior( );  } 
}.start( );

Here we have gone over to the terse side. We’ve allocated and started a new Thread, using an anonymous inner class that extends the Thread class and invokes our performBehavior( ) method in its run( ) method. The effect is similar to using a method pointer in some other language. However, the inner class allows the compiler to check type consistency, which would be more difficult (or impossible) with a true method pointer. At the same time, our anonymous adapter class with its three lines of code is much more efficient and readable than creating a new, top-level adapter class named AnimalBehaviorThreadAdapter.

While we’re getting a bit ahead of the story, anonymous adapter classes are a perfect fit for event handling (which we’ll cover fully in Chapter 13). Skipping a lot of explanation, let’s say you want the method handleClicks( ) to be called whenever the user clicks the mouse. You would write code like this:

addMouseListener(new MouseInputAdapter( ) { 
    public void mouseClicked(MouseEvent e) { handleClicks(e); } 
});

In this case, the anonymous class extends the MouseInputAdapter class by overriding its mouseClicked( ) method to call our method. A lot is going on in a very small space, but the result is clean, readable code. You get to assign method names that are meaningful to you, while allowing Java to do its job of type checking.

Scoping of the “this” reference

Sometimes an inner class may want to get a handle on its “parent” enclosing instance. It might want to pass a reference to its parent, or to refer to one of the parent’s variables or methods that has been hidden by one of its own. For example:

class Animal { 
    int size; 
    class Brain { 
        int size; 
    } 
}

Here, as far as Brain is concerned, the variable size in Animal is hidden by its own version.

Normally an object refers to itself using the special this reference (implicitly or explicitly). But what is the meaning of this for an object with one or more enclosing instances? The answer is that an inner class has multiple this references. You can specify which this you want by prepending the name of the class. So, for instance (no pun intended), we can get a reference to our Animal from within Brain like so:

class Brain { 
    Animal ourAnimal = Animal.this; 
    ...
}

Similarly, we could refer to the size variable in Animal:

class Brain { 
    int animalSize = Animal.this.size; 
    ...
}

How do inner classes really work?

Finally, we’ll get our hands dirty and take a look at what’s really going on when we use an inner class. We’ve said that the compiler is doing all of the things that we had hoped to forget about. Let’s see what’s actually happening. Try compiling this trivial example:

class Animal { 
    class Brain { 
    } 
}

What you’ll find is that the compiler generates two .class files: Animal.class and Animal$Brain.class.

The second file is the class file for our inner class. Yes, as we feared, inner classes are really just compiler magic. The compiler has created the inner class for us as a normal, top-level class and named it by combining the class names with a dollar sign. The dollar sign is a valid character in class names, but is intended for use only by automated tools. (Please don’t start naming your classes with dollar signs.) Had our class been more deeply nested, the intervening inner-class names would have been attached in the same way to generate a unique top-level name.

Now take a look at it with the SDK’s javap utility (don’t quote the argument on a Windows system):

% javap 'Animal$Brain' 
class Animal$Brain extends java.lang.Object { 
    Animal$Brain(Animal); 
}

You’ll see that the compiler has given our inner class a constructor that takes a reference to an Animal as an argument. This is how the real inner class gets the handle on its enclosing instance.

The worst thing about these additional class files is that you need to know they are there. Utilities like jar don’t automatically find them; when you’re invoking a utility like jar, you need to specify these files explicitly or use a wildcard that finds them.

Security implications

Given what we just saw—that the inner class really does exist as an automatically generated top-level class—how does it get access to private variables? The answer, unfortunately, is that the compiler is forced to break the encapsulation of your object and insert accessor methods so that the inner class can reach them. The accessor methods will be given package-level access, so your object is still safe within its package walls, but it is conceivable that this difference could be meaningful if people were allowed to create new classes within your package.

The visibility modifiers on inner classes also have some problems. Current implementations of the virtual machine do not implement the notion of a private or protected class within a package, so giving your inner class anything other than public or default visibility is only a compile-time guarantee. It is difficult to conceive of how these security issues could be abused, but it is interesting to note that Java is straining a bit to stay within its original design.

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^[23] Specifically, we would have to follow a design pattern and pass a reference to the enclosing instance of Animal into the Animal.Brain constructor. See a Java language reference for more information. We don’t expect you to run into this situation very often.