Inheritance

Inheritance is one of the cornerstones of object-oriented programming because it allows the creation of hierarchical classifications. Using inheritance, you can create a general class that defines traits common to a set of related items. This class can then be inherited by other, more specific classes, each adding those things that are unique to it. In the terminology of Java, a class that is inherited is called a superclass. The class that does the inheriting is called a subclass. Therefore, a subclass is a specialized version of a superclass. It inherits all of the members defined by the superclass and adds its own, unique elements.

Inheritance Basics

In order to have your class inherit from a superclass, you simply incorporate the definition of the superclass into your class using the extends keyword. To see how, let’s begin with a short example. The following program creates a superclass called A and a subclass called B. Notice how the keyword extends is used to create a subclass of A.

// A simple example of inheritance.
// Create a superclass.
class A {
    int i, j;

    void showij() {
        System.out.println("i and j: " + i + " " + j);
    }
}

// Create a subclass by extending class A.
class B extends A {
    int k;

    void showk() {
        System.out.println("k: " + k);
    }

    void sum() {
        System.out.println("i+j+k: " + (i+j+k));
    }
}
  
class SimpleInheritance {
    public static void main(String args[]) {
        A superOb = new A();
        B subOb = new B();

        // The superclass may be used by itself.
        superOb.i = 10;
        superOb.j = 20;
        System.out.println("Contents of superOb: ");
        superOb.showij();
        System.out.println();

        /* The subclass has access to all public members of
        its superclass. */
        subOb.i = 7;
        subOb.j = 8;
        subOb.k = 9; 
        System.out.println("Contents of subOb: ");
        subOb.showij();
        subOb.showk();
        System.out.println();

        System.out.println("Sum of i, j and k in subOb:");
        subOb.sum();
    }
}

The output from this program is shown here:

Contents of superOb: 
i and j: 10 20

Contents of subOb:
i and j: 7 8
k: 9

Sum of i, j and k in subOb:
i+j+k: 24

As you can see, the subclass B includes all of the members of its superclass, A. This is why subOb can access i and j and call showij(). Also, inside sum(), i and j can be referred to directly, as if they were part of B.

Even though A is a superclass for B, it is also a completely independent, stand-alone class. Being a superclass for a subclass does not mean that the superclass cannot be used by itself. Further, a subclass can be a superclass for another subclass.

The general form of a class declaration that inherits a superclass is shown here:

class subclass-name extends superclass-name {
    // body of class
}

You can only specify one superclass for any subclass that you create. Java does not support the inheritance of multiple superclasses into a single subclass. You can, as stated, create a hierarchy of inheritance in which a subclass becomes a superclass of another subclass. However, no class can be a superclass of itself.

Member Access and Inheritance

Although a subclass includes all of the members of its superclass, it cannot access those members of the superclass that have been declared as private. For example, consider the following simple class hierarchy:

/* In a class hierarchy, private members remain private to their class.
   This program contains an error and will not compile.
*/

// Create a superclass.
class A {
    int i; // public be default
    private int j; // private to A

    void setij(int x, int y) {
        i = x;
        j = y;
    }
}

// A's j is not accessible here.
class B extends A {
    int total;

    void sum() {
        total = i + j; // ERROR, j is not accessible here
    }
}
  
class Access {
    public static void main(String args[]) {
        B subOb = new B();

        subOb.setij(10, 12);

        subOb.sum();
        System.out.println("Total is " + subOb.total);
    }
}

This program will not compile because the use of j inside the sum() method of B causes an access violation. Since j is declared as private, it is only accessible by other members of its own class. Subclasses have no access to it.

REMEMBER A class member that has been declared as private will remain private to its class. It is not accessible by any code outside its class, including subclasses.

A More Practical Example

Let’s look at a more practical example that will help illustrate the power of inheritance. Here, the final version of the Box class developed in the preceding chapter will be extended to include a fourth component called weight. Thus, the new class will contain a box’s width, height, depth, and weight.

The output from this program is shown here:

BoxWeight inherits all of the characteristics of Box and adds to them the weight component. It is not necessary for BoxWeight to re-create all of the features found in Box. It can simply extend Box to meet its own purposes.

A major advantage of inheritance is that once you have created a superclass that defines the attributes common to a set of objects, it can be used to create any number of more specific subclasses. Each subclass can precisely tailor its own classification. For example, the following class inherits Box and adds a color attribute:

Remember, once you have created a superclass that defines the general aspects of an object, that superclass can be inherited to form specialized classes. Each subclass simply adds its own unique attributes. This is the essence of inheritance.

A Superclass Variable Can Reference a Subclass Object

A reference variable of a superclass can be assigned a reference to any subclass derived from that superclass. You will find this aspect of inheritance quite useful in a variety of situations. For example, consider the following:

Here, weightbox is a reference to BoxWeight objects, and plainbox is a reference to Box objects. Since BoxWeight is a subclass of Box, it is permissible to assign plainbox a reference to the weightbox object.

It is important to understand that it is the type of the reference variable—not the type of the object that it refers to—that determines what members can be accessed. That is, when a reference to a subclass object is assigned to a superclass reference variable, you will have access only to those parts of the object defined by the superclass. This is why plainbox can’t access weight even when it refers to a BoxWeight object. If you think about it, this makes sense, because the superclass has no knowledge of what a subclass adds to it. This is why the last line of code in the preceding fragment is commented out. It is not possible for a Box reference to access the weight field, because Box does not define one.

Although the preceding may seem a bit esoteric, it has some important practical applications—two of which are discussed later in this chapter.

Using super

In the preceding examples, classes derived from Box were not implemented as efficiently or as robustly as they could have been. For example, the constructor for BoxWeight explicitly initializes the width, height, and depth fields of Box. Not only does this duplicate code found in its superclass, which is inefficient, but it implies that a subclass must be granted access to these members. However, there will be times when you will want to create a superclass that keeps the details of its implementation to itself (that is, that keeps its data members private). In this case, there would be no way for a subclass to directly access or initialize these variables on its own. Since encapsulation is a primary attribute of OOP, it is not surprising that Java provides a solution to this problem. Whenever a subclass needs to refer to its immediate superclass, it can do so by use of the keyword super.

super has two general forms. The first calls the superclass’s constructor. The second is used to access a member of the superclass that has been hidden by a member of a subclass. Each use is examined here.

Using super to Call Superclass Constructors

A subclass can call a constructor defined by its superclass by use of the following form of super:

super(_arg-list_);

Here, arg-list specifies any arguments needed by the constructor in the superclass. super() must always be the first statement executed inside a subclass’s constructor.

To see how super() is used, consider this improved version of the BoxWeight class:

Here, BoxWeight() calls super() with the arguments w, h, and d. This causes the Box constructor to be called, which initializes width, height, and depth using these values. BoxWeight no longer initializes these values itself. It only needs to initialize the value unique to it: weight. This leaves Box free to make these values private if desired.

In the preceding example, super() was called with three arguments. Since constructors can be overloaded, super() can be called using any form defined by the superclass. The constructor executed will be the one that matches the arguments. For example, here is a complete implementation of BoxWeight that provides constructors for the various ways that a box can be constructed. In each case, super() is called using the appropriate arguments. Notice that width, height, and depth have been made private within Box.

This program generates the following output:

Pay special attention to this constructor in BoxWeight:

Notice that super() is passed an object of type BoxWeight—not of type Box. This still invokes the constructor Box(Box ob). As mentioned earlier, a superclass variable can be used to reference any object derived from that class. Thus, we are able to pass a BoxWeight object to the Box constructor. Of course, Box only has knowledge of its own members.

Let’s review the key concepts behind super(). When a subclass calls super(), it is calling the constructor of its immediate superclass. Thus, super() always refers to the superclass immediately above the calling class. This is true even in a multileveled hierarchy. Also, super() must always be the first statement executed inside a subclass constructor.

A Second Use for super

The second form of super acts somewhat like this, except that it always refers to the superclass of the subclass in which it is used. This usage has the following general form:

super.member

Here, member can be either a method or an instance variable.

This second form of super is most applicable to situations in which member names of a subclass hide members by the same name in the superclass. Consider this simple class hierarchy:

This program displays the following:

Although the instance variable i in B hides the i in A, super allows access to the i defined in the superclass. As you will see, super can also be used to call methods that are hidden by a subclass.

Creating a Multilevel Hierarchy

Up to this point, we have been using simple class hierarchies that consist of only a superclass and a subclass. However, you can build hierarchies that contain as many layers of inheritance as you like. As mentioned, it is perfectly acceptable to use a subclass as a superclass of another. For example, given three classes called A, B, and C, C can be a subclass of B, which is a subclass of A. When this type of situation occurs, each subclass inherits all of the traits found in all of its superclasses. In this case, C inherits all aspects of B and A. To see how a multilevel hierarchy can be useful, consider the following program. In it, the subclass BoxWeight is used as a superclass to create the subclass called Shipment. Shipment inherits all of the traits of BoxWeight and Box, and it adds a field called cost, which holds the cost of shipping such a parcel.

The output of this program is shown here:

Because of inheritance, Shipment can make use of the previously defined classes of Box and BoxWeight, adding only the extra information it needs for its own, specific application. This is part of the value of inheritance; it allows the reuse of code.

This example illustrates one other important point: super() always refers to the constructor in the closest superclass. The super() in Shipment calls the constructor in BoxWeight. The super() in BoxWeight calls the constructor in Box. In a class hierarchy, if a superclass constructor requires arguments, then all subclasses must pass those arguments “up the line.” This is true whether or not a subclass needs arguments of its own.

NOTE In the preceding program, the entire class hierarchy, including Box, BoxWeight, and Shipment, is shown all in one file. This is for your convenience only. In Java, all three classes could have been placed into their own files and compiled separately. In fact, using separate files is the norm, not the exception, in creating class hierarchies.

When Constructors Are Executed

When a class hierarchy is created, in what order are the constructors for the classes that make up the hierarchy executed? For example, given a subclass called B and a superclass called A, is A’s constructor executed before B’s, or vice versa? The answer is that in a class hierarchy, constructors complete their execution in order of derivation, from superclass to subclass. Further, since super() must be the first statement executed in a subclass’s constructor, this order is the same whether or not super() is used. If super() is not used, then the default or parameterless constructor of each superclass will be executed. The following program illustrates when constructors are executed:

The output from this program is shown here:

As you can see, the constructors are executed in order of derivation.

If you think about it, it makes sense that constructors complete their execution in order of derivation. Because a superclass has no knowledge of any subclass, any initialization it needs to perform is separate from and possibly prerequisite to any initialization performed by the subclass. Therefore, it must complete its execution first.

Method Overriding

In a class hierarchy, when a method in a subclass has the same name and type signature as a method in its superclass, then the method in the subclass is said to override the method in the superclass. When an overridden method is called through its subclass, it will always refer to the version of that method defined by the subclass. The version of the method defined by the superclass will be hidden. Consider the following:

The output produced by this program is shown here:

k: 3

When show() is invoked on an object of type B, the version of show() defined within B is used. That is, the version of show() inside B overrides the version declared in A.

If you wish to access the superclass version of an overridden method, you can do so by using super. For example, in this version of B, the superclass version of show() is invoked within the subclass’s version. This allows all instance variables to be displayed.

If you substitute this version of A into the previous program, you will see the following output:

Here, super.show() calls the superclass version of show().

Method overriding occurs only when the names and the type signatures of the two methods are identical. If they are not, then the two methods are simply overloaded. For example, consider this modified version of the preceding example:

The output produced by this program is shown here:

The version of show() in B takes a string parameter. This makes its type signature different from the one in A, which takes no parameters. Therefore, no overriding (or name hiding) takes place. Instead, the version of show() in B simply overloads the version of show() in A.

Dynamic Method Dispatch

While the examples in the preceding section demonstrate the mechanics of method overriding, they do not show its power. Indeed, if there were nothing more to method overriding than a name space convention, then it would be, at best, an interesting curiosity, but of little real value. However, this is not the case. Method overriding forms the basis for one of Java’s most powerful concepts: dynamic method dispatch. Dynamic method dispatch is the mechanism by which a call to an overridden method is resolved at run time, rather than compile time. Dynamic method dispatch is important because this is how Java implements run-time polymorphism.

Let’s begin by restating an important principle: a superclass reference variable can refer to a subclass object. Java uses this fact to resolve calls to overridden methods at run time. Here is how. When an overridden method is called through a superclass reference, Java determines which version of that method to execute based upon the type of the object being referred to at the time the call occurs. Thus, this determination is made at run time. When different types of objects are referred to, different versions of an overridden method will be called. In other words, it is the type of the object being referred to (not the type of the reference variable) that determines which version of an overridden method will be executed. Therefore, if a superclass contains a method that is overridden by a subclass, then when different types of objects are referred to through a superclass reference variable, different versions of the method are executed.

Here is an example that illustrates dynamic method dispatch:

The output from the program is shown here:

This program creates one superclass called A and two subclasses of it, called B and C. Subclasses B and C override callme() declared in A. Inside the main() method, objects of type A, B, and C are declared. Also, a reference of type A, called r, is declared. The program then in turn assigns a reference to each type of object to r and uses that reference to invoke callme(). As the output shows, the version of callme() executed is determined by the type of object being referred to at the time of the call. Had it been determined by the type of the reference variable, r, you would see three calls to A’s callme() method.

NOTE Readers familiar with C++ or C# will recognize that overridden methods in Java are similar to virtual functions in those languages.

Why Overridden Methods?

As stated earlier, overridden methods allow Java to support run-time polymorphism. Polymorphism is essential to object-oriented programming for one reason: it allows a general class to specify methods that will be common to all of its derivatives, while allowing subclasses to define the specific implementation of some or all of those methods. Overridden methods are another way that Java implements the “one interface, multiple methods” aspect of polymorphism.

Part of the key to successfully applying polymorphism is understanding that the superclasses and subclasses form a hierarchy that moves from lesser to greater specialization. Used correctly, the superclass provides all elements that a subclass can use directly. It also defines those methods that the derived class must implement on its own. This allows the subclass the flexibility to define its own methods, yet still enforces a consistent interface. Thus, by combining inheritance with overridden methods, a superclass can define the general form of the methods that will be used by all of its subclasses.

Dynamic, run-time polymorphism is one of the most powerful mechanisms that object-oriented design brings to bear on code reuse and robustness. The ability of existing code libraries to call methods on instances of new classes without recompiling while maintaining a clean abstract interface is a profoundly powerful tool.

Applying Method Overriding

Let’s look at a more practical example that uses method overriding. The following program creates a superclass called Figure that stores the dimensions of a two-dimensional object. It also defines a method called area() that computes the area of an object. The program derives two subclasses from Figure. The first is Rectangle and the second is Triangle. Each of these subclasses overrides area() so that it returns the area of a rectangle and a triangle, respectively.

The output from the program is shown here:

Through the dual mechanisms of inheritance and run-time polymorphism, it is possible to define one consistent interface that is used by several different, yet related, types of objects. In this case, if an object is derived from Figure, then its area can be obtained by calling area(). The interface to this operation is the same no matter what type of figure is being used.

Using Abstract Classes

There are situations in which you will want to define a superclass that declares the structure of a given abstraction without providing a complete implementation of every method. That is, sometimes you will want to create a superclass that only defines a generalized form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. Such a class determines the nature of the methods that the subclasses must implement. One way this situation can occur is when a superclass is unable to create a meaningful implementation for a method. This is the case with the class Figure used in the preceding example. The definition of area() is simply a placeholder. It will not compute and display the area of any type of object.

As you will see as you create your own class libraries, it is not uncommon for a method to have no meaningful definition in the context of its superclass. You can handle this situation two ways. One way, as shown in the previous example, is to simply have it report a warning message. While this approach can be useful in certain situations—such as debugging—it is not usually appropriate. You may have methods that must be overridden by the subclass in order for the subclass to have any meaning. Consider the class Triangle. It has no meaning if area() is not defined. In this case, you want some way to ensure that a subclass does, indeed, override all necessary methods. Java’s solution to this problem is the abstract method.

You can require that certain methods be overridden by subclasses by specifying the abstract type modifier. These methods are sometimes referred to as subclasser responsibility because they have no implementation specified in the superclass. Thus, a subclass must override them—it cannot simply use the version defined in the superclass. To declare an abstract method, use this general form:

abstract _type name_(_parameter-list_);

As you can see, no method body is present.

Any class that contains one or more abstract methods must also be declared abstract. To declare a class abstract, you simply use the abstract keyword in front of the class keyword at the beginning of the class declaration. There can be no objects of an abstract class. That is, an abstract class cannot be directly instantiated with the new operator. Such objects would be useless, because an abstract class is not fully defined. Also, you cannot declare abstract constructors or abstract static methods. Any subclass of an abstract class must either implement all of the abstract methods in the superclass or be declared abstract itself.

Here is a simple example of a class with an abstract method, followed by a class which implements that method:

Notice that no objects of class A are declared in the program. As mentioned, it is not possible to instantiate an abstract class. One other point: class A implements a concrete method called callmetoo(). This is perfectly acceptable. Abstract classes can include as much implementation as they see fit.

Although abstract classes cannot be used to instantiate objects, they can be used to create object references, because Java’s approach to run-time polymorphism is implemented through the use of superclass references. Thus, it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object. You will see this feature put to use in the next example.

Using an abstract class, you can improve the Figure class shown earlier. Since there is no meaningful concept of area for an undefined two-dimensional figure, the following version of the program declares area() as abstract inside Figure. This, of course, means that all classes derived from Figure must override area().

As the comment inside main() indicates, it is no longer possible to declare objects of type Figure, since it is now abstract. And, all subclasses of Figure must override area(). To prove this to yourself, try creating a subclass that does not override area(). You will receive a compile-time error.

Although it is not possible to create an object of type Figure, you can create a reference variable of type Figure. The variable figref is declared as a reference to Figure, which means that it can be used to refer to an object of any class derived from Figure. As explained, it is through superclass reference variables that overridden methods are resolved at run time.

Using final with Inheritance

The keyword final has three uses. First, it can be used to create the equivalent of a named constant. This use was described in the preceding chapter. The other two uses of final apply to inheritance. Both are examined here.

Using final to Prevent Overriding

While method overriding is one of Java’s most powerful features, there will be times when you will want to prevent it from occurring. To disallow a method from being overridden, specify final as a modifier at the start of its declaration. Methods declared as final cannot be overridden. The following fragment illustrates final:

Because meth() is declared as final, it cannot be overridden in B. If you attempt to do so, a compile-time error will result.

Methods declared as final can sometimes provide a performance enhancement: The compiler is free to inline calls to them because it “knows” they will not be overridden by a subclass. When a small final method is called, often the Java compiler can copy the bytecode for the subroutine directly inline with the compiled code of the calling method, thus eliminating the costly overhead associated with a method call. Inlining is an option only with final methods. Normally, Java resolves calls to methods dynamically, at run time. This is called late binding. However, since final methods cannot be overridden, a call to one can be resolved at compile time. This is called early binding.

Using final to Prevent Inheritance

Sometimes you will want to prevent a class from being inherited. To do this, precede the class declaration with final. Declaring a class as final implicitly declares all of its methods as final, too. As you might expect, it is illegal to declare a class as both abstract and final since an abstract class is incomplete by itself and relies upon its subclasses to provide complete implementations.

Here is an example of a final class:

As the comments imply, it is illegal for B to inherit A since A is declared as final.

NOTE Beginning with JDK 17, the ability to seal a class was added to Java. Sealing offers fine-grained control over inheritance. Sealing is described in Chapter 17.

Local Variable Type Inference and Inheritance

As explained in Chapter 3, JDK 10 added local variable type inference to the Java language, which is supported by the context-sensitive keyword var. It is important to have a clear understanding of how type inference works within an inheritance hierarchy. Recall that a superclass reference can refer to a derived class object, and this feature is part of Java’s support for polymorphism. However, it is critical to remember that when using local variable type inference, the inferred type of a variable is based on the declared type of its initializer. Therefore, if the initializer is of the superclass type, that will be the inferred type of the variable. It does not matter if the actual object being referred to by the initializer is an instance of a derived class. For example, consider this program:

In the program, a hierarchy is created that consists of three classes, at the top of which is MyClass. FirstDerivedClass is a subclass of MyClass, and SecondDerivedClass is a subclass of FirstDerivedClass. The program then uses type inference to create three variables, called mc, mc2, and mc3, by calling getObj(). The getObj() method has a return type of MyClass (the superclass) but returns objects of type MyClass, FirstDerivedClass, or SecondDerivedClass, depending on the argument that it is passed. As the output shows, the inferred type is determined by the return type of getObj(), not by the actual type of the object obtained. Thus, all three variables will be of type MyClass.

The Object Class

There is one special class, Object, defined by Java. All other classes are subclasses of Object. That is, Object is a superclass of all other classes. This means that a reference variable of type Object can refer to an object of any other class. Also, since arrays are implemented as classes, a variable of type Object can also refer to any array.

Object defines the following methods, which means that they are available in every object.

The methods getClass(), notify(), notifyAll(), and wait() are declared as final. You may override the others. These methods are described elsewhere in this book. However, notice two methods now: equals() and toString(). The equals() method compares two objects. It returns true if the objects are equal, and false otherwise. The precise definition of equality can vary, depending on the type of objects being compared. The toString() method returns a string that contains a description of the object on which it is called. Also, this method is automatically called when an object is output using println(). Many classes override this method. Doing so allows them to tailor a description specifically for the types of objects that they create.

One last point: Notice the unusual syntax in the return type for getClass(). This relates to Java’s generics feature, which is described in Chapter 14.