6.1.1 Shadowed Variables

In the section on methods in Chapter 5, we saw that a local variable of the same name as an instance variable hides the instance variable. Similarly, an instance variable in a subclass can shadow an instance variable of the same name in its parent class, as shown in Figure 6.2.

Figure 6.2: The scope of shadowed variables

In Figure 6.2, the variable weight is declared in three places: as a local variable in the method foodConsumption() of the class Mammal, as an instance variable of the class Mammal, and as an instance variable of the class Animal. The actual variable selected depends on the scope in which we are working.

In the above example, all variables were of the same type. About the only reason for declaring a variable with the same type in a subclass is to provide an alternate initializer. A more important use of shadowed variables involves changing their types. We could, for example, shadow an int variable with a double variable in a subclass that needs decimal values instead of integer values. We do this without changing the existing code because, as its name suggests, when we shadow variables, we don't replace them but instead mask them. Both variables still exist; methods of the superclass see the original variable, and methods of the subclass see the new version. The determination of what variables the various methods see is static and happens at compile-time.

Here's a simple example:

class IntegerCalculator { 
    int sum; 
    ... 
} 
 
class DecimalCalculator extends IntegerCalculator { 
    double sum; 
    ... 
} 

In this example, we override the instance variable sum to change its type from int to double.[1] Methods defined in the class IntegerCalculator see the integer variable sum, while methods defined in DecimalCalculator see the decimal variable sum. However, both variables actually exist for a given instance of DecimalCalculator, and they can have independent values. In fact, any methods that DecimalCalculator inherits from IntegerCalculator actually see the integer variable sum.

[1] Note that a better way to design our calculators would be to have an abstract Calculator class with two subclasses: IntegerCalculator and DecimalCalculator.

Since both variables exist in DecimalCalculator, we need to reference the variable inherited from IntegerCalculator. We do that using the super reference:

int s = super.sum; 

Inside of DecimalCalculator, the super keyword used in this manner refers to the sum variable defined in the superclass. I'll explain the use of super more fully in a bit.

Another important point about shadowed variables has to do with how they work when we refer to an object by way of a less derived type. For example, we can refer to a DecimalCalculator object as an IntegerCalculator. If we do so and then access the variable sum, we get the integer variable, not the decimal one:

DecimalCalculator dc = new DecimalCalculator(); 
IntegerCalculator ic = dc; 
 
int s = ic.sum;                  // Accesses IntegerCalculator sum 

After this detailed explanation, you may still be wondering what shadowed variables are good for. Well, to be honest, the usefulness of shadowed variables is limited, but it's important to understand the concepts before we talk about doing the same thing with methods. We'll see a different and more dynamic type of behavior with method shadowing, or more correctly, method overriding.

6.1.2 Overriding Methods

In Chapter 5, we saw we could declare overloaded methods (i.e., methods with the same name but a different number or type of arguments) within a class. Overloaded method selection works the way we described on all methods available to a class, including inherited ones. This means that a subclass can define some overloaded methods that augment the overloaded methods provided by a superclass.

But a subclass does more than that; it can define a method that has exactly the same method signature (arguments and return type) as a method in its superclass. In that case, the method in the subclass overrides the method in the superclass and effectively replaces its implementation, as shown in Figure 6.3. Overriding methods to change the behavior of objects is another form of polymorphism (sub-type polymorphism): the one most people think of when they talk about the power of object-oriented languages.

Figure 6.3: Method overriding

In Figure 6.3, Mammal overrides the reproduce() method of Animal, perhaps to specialize the method for the peculiar behavior of Mammals giving live birth.[2] The Cat object's sleeping behavior is overridden to be different from that of a general Animal, perhaps to accommodate cat naps. The Cat class also adds the more unique behaviors of purring and hunting mice.

[2] We'll ignore the platypus, which is an obscure nonovoviviparous mammal.

From what you've seen so far, overridden methods probably look like they shadow methods in superclasses, just as variables do. But overridden methods are actually more powerful than that. An overridden method in Java acts like a virtual method in C++. When there are multiple implementations of a method in the inheritance hierarchy of an object, the one in the most derived class always overrides the others, even if we refer to the object by way of a less derived type. In other words, if we have a Cat instance assigned to a variable of the more general type Animal and we call its sleep() method, we get the sleep() method implemented in the Cat class, not the one in Animal:

Cat simon = new Cat(); 
Animal creature = simon; 
 
creature.sleep();	       // Accesses Cat sleep(); 

In other respects, the variable creature looks like an Animal. For example, access to a shadowed variable would find the implementation in the Animal class, not the Cat class. However, because methods are virtual, the appropriate method in the Cat class can be located, even though we are dealing with an Animal object. This means we can deal with specialized objects as if they were more general types of objects and still take advantage of their specialized implementations of behavior.

Much of what you'll be doing when you're writing a Java applet or application is overriding methods defined by various classes in the Java API. For example, think back to the applets we developed in the tutorial in Chapter 2. Almost all of the methods we implemented for those applets were overridden methods. Recall that we created a subclass of Applet for each of the examples. Then we overrode init() to set up our applet and paint() to draw our applet.

A common programming error in Java (at least for me) is to miss and accidentally overload a method when trying to override it. Any difference in the number or type of arguments or the return type of a method produces two overloaded methods instead of a single, overridden method. Make it a habit to look twice when overriding methods.

static final boolean DEBUG = false;
...
static void debug (String message) { 
    if (DEBUG) {
        System.err.println(message); 
        // do other stuff
        ...
    }
}

// A more refined exception 
class MeatInedibleException extends InedibleException { ...
}
        
class Animal {
    void eat( Food f ) throws InedibleException { ...
}
class Herbivore extends Animal {
    void eat( Food f ) throws InedibleException { 
        if ( f instanceof Meat )
            throw new MeatInedibleException();
        ....
    }
}

Animal creature = ...
try {
    creature.eat( food );
} catch ( MeatInedibleException ) {
    // creature can't eat this food because it's meat
} catch ( InedibleException ) {
    // creature can't eat this food
}

6.1.3 this and super

The special references this and super allow you to refer to the members of the current object instance or those of the superclass, respectively. We have seen this used elsewhere to pass a reference to the current object and to refer to shadowed instance variables. The reference super does the same for the parents of a class. You can use it to refer to members of a superclass that have been shadowed or overridden. A common arrangement is for an overridden method in a subclass to do some preliminary work and then defer to the method of the superclass to finish the job.

class Animal { 
    void eat( Food f ) throws InedibleException { 
        // consume food
    }
} 
 
class Herbivore extends Animal { 
    void eat( Food f ) throws MeatInedibleException { 
        // check if edible
        ...
        super.eat( f ); 
    } 
} 

In the above example, our Herbivore class overrides the Animal eat() method to first do some checking on the food object. After doing its job it simply calls the (otherwise overridden) implementation of eat() in its superclass, using super.

super prompts a search for the method or variable to begin in the scope of the immediate superclass rather than the current class. The inherited method or variable found may reside in the immediate superclass, or in a more distant one. The usage of the super reference when applied to overridden methods of a superclass is special; it tells the method resolution system to stop the dynamic method search at the superclass, instead of in the most derived class (as it otherwise does). Without super, there would be no way to access overridden methods.

6.1.4 Casting

As in C++, a cast explicitly tells the compiler to change the apparent type of an object reference. Unlike in C++, casts in Java are checked both at compile-time and at run-time to make sure they are legal. Attempting to cast an object to an incompatible type at run-time results in a ClassCastException. Only casts between objects in the same inheritance hierarchy (and as we'll see later, to appropriate interfaces) are legal in Java and pass the scrutiny of the compiler and the run-time system.

Casts in Java affect only the treatment of references; they never change the form of the actual object. This is an important rule to keep in mind. You never change the object pointed to by a reference by casting it; you change only the compiler's (or run-time system's) notion of it.

A cast can be used to narrow the type of a reference--to make it more specific. Often, we'll do this when we have to retrieve an object from a more general type of collection or when it has been previously used as a less derived type. (The prototypical example is using an object in a Vector or Hashtable, as you'll see in Chapter 9.) Continuing with our Cat example:

Animal creature = ...
Cat simon = ...
 
creature = simon;        // Okay
// simon = creature;     // Compile time error, incompatible type 
simon = (Cat)creature;   // Okay 

We can't reassign the reference in creature to the variable simon even though we know it holds an instance of a Cat (Simon). We have to perform the indicated cast. This is also called downcasting the reference.

Note that an implicit cast was performed when we went the other way to widen the reference simon to type Animal during the first assignment. In this case, an explicit cast would have been legal, but superfluous.

If casting seems complicated, here's a simple way to think about it. Basically, you can't lie about what an object is. If you have a Cat object, you can cast it to a less derived type (i.e., a type above it in the class hierarchy) such as Animal or even Object, since all Java classes are a subclass of Object. If you have an Object you know is a Cat, you can downcast the Object to be an Animal or a Cat. However, if you aren't sure if the Object is a Cat or a Dog at run-time, you should check it with instanceof before you perform the cast. If you get the cast wrong, Java throws a ClassCastException.

As I mentioned earlier, casting can affect the selection of compile-time items like variables and overloaded methods, but not the selection of overridden methods. Figure 6.4 shows the difference. As shown in the top half of the diagram, casting the reference simon to type Animal (widening it) affects the selection of the shadowed variable weight within it. However, as the lower half of the diagram indicates, the cast doesn't affect the selection of the overridden method sleep().

Figure 6.4: Casting and its effect on method and variable selection

6.1.5 Using Superclass Constructors

When we talked earlier about constructors, we discussed how the special statement this() invokes an overloaded constructor upon entry to another constructor. Similarly, the statement super() explicitly invokes the constructor of a superclass. Of course, we also talked about how Java makes a chain of constructor calls that includes the superclass's constructor, so why use super() explicitly? When Java makes an implicit call to the superclass constructor, it calls the default constructor. So, if we want to invoke a superclass constructor that takes arguments, we have to do so explicitly using super().

If we are going to call a superclass constructor with super(), it must be the first statement of our constructor, just as this() must be the first call we make in an overloaded constructor. Here's a simple example:

class Person { 
    Person ( String name ) { 
        //  setup based on name 
        ... 
    } 
    ... 
} 
 
class Doctor extends Person { 
    Doctor ( String name, String specialty ) { 
        super( name ); 
        // setup based on specialty 
        ... 
    } 
    ... 
} 

In this example, we use super() to take advantage of the implementation of the superclass constructor and avoid duplicating the code to set up the object based on its name. In fact, because the class Person doesn't define a default (no arguments) constructor, we have no choice but to call super() explicitly. Otherwise, the compiler would complain that it couldn't find an appropriate default constructor to call. Said another way, if you subclass a class that has only constructors that take arguments, you have to invoke one of the superclass's constructors explicitly from your subclass constructor.

Instance variables of the class are initialized upon return from the superclass constructor, whether that's due to an explicit call via super() or an implicit call to the default superclass constructor.

We can now give the full story of how constructors are chained together and when instance variable initialization occurs. The rule has three parts and is applied repeatedly for each successive constructor invoked.

• If the first statement of a constructor is an ordinary statement--i.e., not a call to this() or super()--Java inserts an implicit call to super() to invoke the default constructor of the superclass. Upon returning from that call, Java initializes the instance variables of the current class and proceeds to execute the statements of the current constructor.

• If the first statement of a constructor is a call to a superclass constructor via super(), Java invokes the selected superclass constructor. Upon its return, Java initializes the current class's instance variables and proceeds with the statements of the current constructor.

• If the first statement of a constructor is a call to an overloaded constructor via this(), Java invokes the selected constructor and upon its return simply proceeds with the statements of the current constructor. The call to the superclass's constructor has happened within the overloaded constructor, either explicitly or implicitly, so the initialization of instance variables has already occurred.

6.1.6 Abstract Methods and Classes

A method in Java can be declared with the abstract modifier to indicate that it's just a prototype. An abstract method has no body; it's simply a signature definition followed by a semicolon. You can't directly use a class that contains an abstract method; you must instead create a subclass that implements the abstract method's body.

abstract void vaporMethod( String name ); 

In Java, a class that contains one or more abstract methods must be explicitly declared as an abstract class, also using the abstract modifier:

abstract class vaporClass { 
    ... 
    abstract void vaporMethod( String name ); 
    ... 
} 

An abstract class can contain other, nonabstract methods and ordinary variable declarations; however, it can't be instantiated. To be used, it must be subclassed and its abstract methods must be overridden with methods that implement a body. Not all abstract methods have to be implemented in a single subclass, but a subclass that doesn't override all its superclass's abstract methods with actual, concrete implementations must also be declared abstract.

Abstract classes provide a framework for classes that are to be "filled in" by the implementor. The java.io.InputStream class, for example, has a single abstract method called read(). Various subclasses of InputStream implement read() in their own ways to read from their own sources. The rest of the InputStream class, however, provides extended functionality built on the simple read() method. A subclass of InputStream inherits these nonabstract methods that provide functionality based on the simple read() method that the subclass implements.

It's often desirable to specify the prototypes for a set of methods and provide no implementation. In Java, this is called an interface. An interface defines a set of methods a class must implement (i.e., the behavior of a class). A class in Java can simply say that it implements an interface and go about implementing those methods. A class that implements an interface doesn't have to inherit from any particular part of the inheritance hierarchy or use a particular implementation.