Cover by Patrick Niemeyer, Daniel Leuck

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Inner Classes

All of the classes we’ve seen so far in this book have been top-level, “freestanding” classes declared at the file and package level. But classes in Java can actually be declared at any level of scope, within any set of curly braces (i.e., almost anywhere that you could put any other Java statement). These inner classes belong to another class or method as a variable would and may have their visibility limited to its scope in the same way. Inner classes are a useful and aesthetically pleasing facility for structuring code. Their cousins, anonymous inner classes, are an even more powerful shorthand that make it seem as if you can create new kinds of objects dynamically within Java’s statically typed environment. In Java, anonymous inner classes play part of the role of closures in other languages, giving the effect of handling state and behavior independently of classes.

However, as we delve into their inner workings, we’ll see that inner classes are not quite as aesthetically pleasing or dynamic as they seem. Inner classes are pure syntactic sugar; they are not supported by the VM and are instead mapped to regular Java classes by the compiler. 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 compiled code and a few potential side effects.

Inner classes are essentially nested classes, for example:

    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 bit of explanation, we’ll start by saying that Java tries to make the meaning, as much as possible, the same as for the other members (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 generally accessible outside of the class Animal without some additional qualification.

Within the body of the inner Brain class and the body of the performBehavior() method, we have direct access to all 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, methods within the Brain class can invoke the performBehavior() method of Animal as well as work with any other methods and variables declared in Animal. The Brain class “sees” all of the methods and variables of the Animal class directly in its scope.

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. All of the usual visibility modifiers apply, and inner classes can also be declared static, as we’ll discuss later.

We’ve said that within the Animal class, we can construct a Brain in the ordinary way, using new Brain(), for example. Although we’d probably never find a need to do it, we can also construct an instance of Brain from outside the class by referencing an instance of Animal. To do this requires that the inner class Brain be accessible and that we use a special form of the new operator designed just for inner classes:

    Animal monkey = new Animal();
    Animal.Brain monkeyBrain = monkey.new Brain();

Here, the Animal instance monkey is used to qualify the new operator on Brain. Again, this is not a very common thing to do and you can probably just forget that we said anything about it. Static inner classes are more useful. We’ll talk about them a bit later.

Inner Classes as Adapters

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, standard interface to a sequence of objects. The java.util.Iterator interface has several methods:

    public interface Iterator {
        boolean hasNext();
        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?

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, in order to 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 hasNext() {
                return  element < employees.length ;
            }

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

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

Now EmployeeList can provide a 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, variables, and initializers, 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 for “local” use within the body of a method. Returning to the Animal class, we can put Brain inside the performBehavior() method if we decide that the class is useful only inside 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 because of the fleeting nature of a method invocation, there are a few limitations and additional restrictions, as described in the following sections. If you are thinking that inner classes within methods sounds arcane, bear with us until we talk about anonymous inner classes, which are tremendously useful.

Limitations on inner classes in methods

performBehavior() is a method, and method invocations have limited lifetimes. When they exit, their local variables normally disappear into the abyss. However, an instance of Brain (like any object created in the method) lives on as long as it is referenced. Java must make sure that any local variables used by instances of Brain created within an invocation of performBehavior() also live on. Furthermore, all 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 or arguments 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. For example:

    void performBehavior( final boolean nocturnal )
    {
        class Brain {
            void sleep() {
                if ( nocturnal ) { ... }
            }
        }
    }

In this code snippet, the argument nocturnal to the performBehavior() method must be marked final so that it can be referenced within the inner class Brain. This is just a technical limitation of how inner classes are implemented, ensuring that it’s OK for the Brain class to keep a copy of the value.

Static inner classes

We mentioned earlier that the inner class Brain of the class Animal can, 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 as we described, given that our Animal.Brain class always requires an instance of an Animal as its enclosing instance, it’s not as common to work with them directly in this way.

However, there is another situation in which we want to 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 may seem strange, it is not inconsistent because 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 minipackage, 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 statement enables us to refer to the class simply as MigrationPattern. We can use all the standard visibility modifiers on inner classes, so a static inner class can have private, protected, default, or public visibility.

Here’s another example. The Java 2D API uses static inner classes to implement specialized shape classes (i.e., the java.awt.geom.Rectangle2D class has two inner classes, Float and Double, that implement two different precisions). These shape classes are actually very simple subclasses; it would have been sad to have to multiply the number of top-level classes in that package by three to accommodate all of them. With inner classes, we can bundle them with their respective classes:

    Rectangle2D.Float rect = new Rectangle2D.Float();

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 earlier iterator example. This is not just a purely aesthetic issue. Naming is an important part of writing readable, maintainable code. We generally want to use the most concise, 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 a class declaration with the allocation of an instance of that class, effectively creating a “one-time only” class and a class instance in one operation. After the new keyword, 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 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 hasNext() {
                return  element < employees.length ;
            }
            Object next() {
                if ( hasNext() )
                    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 implicitly creates 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 getIterator() method contains a single statement, the return statement.

The previous example is a bit extreme and certainly does not improve readability. Inner classes are best used when you want to implement a few lines of code, but 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 cover fully in Chapter 16). 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 such as:

    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 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 shadowed 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 prefixing it with the name of the class. 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, let’s 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 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 the class with the JDK’s javap utility. Starting in Java 5.0, you can refer to the inner class as Animal.Brain, but in earlier versions of Java, you may have to call the class by its real name, Animal$Brain:

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

On a Windows system, it’s not necessary to quote the argument, as we did on this Unix command line.

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 reference to its enclosing instance.

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

    % jar cvf animal.jar Animal*class

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 are 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 VM 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.[19]



[19] Inner classes were added to Java in version 1.1.

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