Learning Java, 4th Edition

Arrays and the Class Hierarchy

Now we’re going to shift gears a bit and return to the topic of arrays, considering them from the object point of view. At the end of Chapter 4, we mentioned that arrays have a place in the Java class hierarchy, but we didn’t give you any details. Now that we’ve discussed the object-oriented aspects of Java, we can give you the whole story.

Array classes live in a parallel Java class hierarchy under the Object class. If a class is a direct subclass of Object, an array class for that base type also exists as a direct subclass of Object. Arrays of more derived classes are subclasses of the corresponding array classes. For example, consider the following class types:

    class Animal { ... }
    class Bird extends Animal { ... }
    class Penguin extends Bird { ... }

Figure 6-8 illustrates the class hierarchy for arrays of these classes. Arrays of the same dimension are related to one another in the same manner as their base type classes. In our example, Bird is a subclass of Animal, which means that the Bird[] type is a subtype of Animal[]. In the same way a Bird object can be used in place of an Animal object, a Bird[] array can be assigned to a variable of type Animal[]:

    Animal [][] animals;
    Bird [][] birds = new Bird [10][10];
    birds[0][0] = new Bird();

    // make animals and birds reference the same array object
    animals = birds;
    observe( animals[0][0] );               // processes Bird object

Because arrays are part of the class hierarchy, we can use instanceof to check the type ...