Why inner classes?

At this point you’ve seen a lot of syntax and semantics describing the way inner classes work, but this doesn’t answer the question of why they exist. Why did Sun go to so much trouble to add this fundamental language feature?

Typically, the inner class inherits from a class or implements an interface, and the code in the inner class manipulates the outer class object that it was created within. So you could say that an inner class provides a kind of window into the outer class.

A question that cuts to the heart of inner classes is this: If I just need a reference to an interface, why don’t I just make the outer class implement that interface? The answer is “If that’s all you need, then that’s how you should do it.” So what is it that distinguishes an inner class implementing an interface from an outer class implementing the same interface? The answer is that you can’t always have the convenience of interfaces—sometimes you’re working with implementations. So the most compelling reason for inner classes is:

Each inner class can independently inherit from an implementation. Thus, the inner class is not limited by whether the outer class is already inheriting from an implementation.

Without the ability that inner classes provide to inherit—in effect—from more than one concrete or abstract class, some design and programming problems would be intractable. So one way to look at the inner class is as the rest of the solution of the multiple-inheritance problem. Interfaces solve part of the problem, but inner classes effectively allow “multiple implementation inheritance.” That is, inner classes effectively allow you to inherit from more than one non-interface.

To see this in more detail, consider a situation in which you have two interfaces that must somehow be implemented within a class. Because of the flexibility of interfaces, you have two choices: a single class or an inner class:

//: c08:MultiInterfaces.java
// Two ways that a class can implement multiple interfaces.

interface A {}
interface B {}

class X implements A, B {}

class Y implements A {
  B makeB() {
    // Anonymous inner class:
    return new B() {};
  }
}

public class MultiInterfaces {
  static void takesA(A a) {}
  static void takesB(B b) {}
  public static void main(String[] args) {
    X x = new X();
    Y y = new Y();
    takesA(x);
    takesA(y);
    takesB(x);
    takesB(y.makeB());
  }
} ///:~

Of course, this assumes that the structure of your code makes logical sense either way. However, you’ll ordinarily have some kind of guidance from the nature of the problem about whether to use a single class or an inner class. But without any other constraints, the approach in the preceding example doesn’t really make much difference from an implementation standpoint. Both of them work.

However, if you have abstract or concrete classes instead of interfaces, you are suddenly limited to using inner classes if your class must somehow implement both of the others:

//: c08:MultiImplementation.java
// With concrete or abstract classes, inner
// classes are the only way to produce the effect
// of "multiple implementation inheritance."
package c08;

class D {}
abstract class E {}

class Z extends D {
  E makeE() { return new E() {}; }
}

public class MultiImplementation {
  static void takesD(D d) {}
  static void takesE(E e) {}
  public static void main(String[] args) {
    Z z = new Z();
    takesD(z);
    takesE(z.makeE());
  }
} ///:~

If you didn’t need to solve the “multiple implementation inheritance” problem, you could conceivably code around everything else without the need for inner classes. But with inner classes you have these additional features:

1. The inner class can have multiple instances, each with its own state information that is independent of the information in the outer class object.
2. In a single outer class you can have several inner classes, each of which implement the same interface or inherit from the same class in a different way. An example of this will be shown shortly.
3. The point of creation of the inner class object is not tied to the creation of the outer class object.
4. There is no potentially confusing “is-a” relationship with the inner class; it’s a separate entity.

   Thinking in Java
   Prev	Contents / Index	Next