Java Inheritance

In an object-oriented language, inheritance is a mechanism for deriving part of a class definition from another existing class definition. This allows the programmer to “share” code between classes, reducing the amount of code that must be written.

Consider the Student class we created earlier:

public class Student {
    
    private int creditPoints;
    private int creditHours;
    private String first;
    private String last;
    
    // accessor methods for first and last omitted

    public Student(String first, String last) {
        this.first = first;
        this.last = last;
    }
    
    /**
     * Gets the student's grade point average.
     */
    public double getGPA() {
        return ((double) creditPoints) / creditHours;
    }
    
    /**
     * Records a final grade for a course taken by this student.
     * 
     * @param grade       the grade earned by the student
     * @param hours       the number of credit hours in the course
     */
    public void addCourseGrade(Grade grade, int hours) {
        this.creditHours += hours;
        switch(grade) {
            case A:
                this.creditPoints += 4 * hours;
                break;
            case B:
                this.creditPoints += 3 * hours;
                break;
            case C:
                this.creditPoints += 2 * hours;
                break;
            case D:
                this.creditPoints += 1 * hours;
                break;
            case F:
                this.creditPoints += 0 * hours;
                break;
        }
    }
}

This would work well for representing a student. But what if we are representing multiple kinds of students, like undergraduate and graduate students? We’d need separate classes for each, but both would still have names and calculate their GPA the same way. So, it would be handy if we could say “an undergraduate is a student, and has all the properties and methods a student has” and “a graduate student is a student, and has all the properties and methods a student has.” This is exactly what inheritance does for us, and we often describe it as an is-a relationship. We distinguish this from the interface mechanism we looked at earlier by saying it is a strong is-a relationship, as an Undergraduate student is, for all purposes, also a Student.

Let’s define an undergraduate student class:

public class UndergraduateStudent extends Student {
    
    public UndergraduateStudent(String first, String last) {
        super(first, last);
    }

}

In Java, we use the extends keyword to declare that a class is inheriting from another class. So, public class UndergraduateStudent extends Student indicates that UndergraduateStudent inherits from (is a) Student. Thus, it has the attributes first and last that are inherited from Student. Similarly, it inherits the getGPA() and addCourseGrade() methods.

In fact, the only method we need to define in our UndergraduateStudent class is the constructor - and we only need to define this because the base class has a defined constructor taking two parameters, first and last names. This Student constructor must be invoked by the UndergraduateStudent constructor - that’s what the super(first, last) line does - it invokes the Student constructor with the first and last parameters passed into the UndergraduateStudent constructor. In Java, the super() method call must be the first line in the child class’s constructor. It can be omitted if the parent class includes a default (parameter-less) constructor.

Inheritance, State, and Behavior

Let’s define a GraduateStudent class as well. This will look much like an UndergraduateStudent, but all graduates have a bachelor’s degree:

public class GraduateStudent extends Student {

    private String bachelorDegree;
    
    public GraduateStudent(String first, String last, String degree) {
        super(first, last);
        this.bachelorDegree = degree;
    }
    
    public String getBachelorDegree() {
        return this.bachelorDegree;
    }

}

Here we added a property for bachelorDegree. Since the attribute itself is marked as private, it can only be written to by the class, as is done in the constructor. To the outside world, it is treated as read-only through the getter method.

Thus, the GraduateStudent has all the state and behavior encapsulated in Student, plus the additional state of the bachelor’s degree title.

The protected Keyword

What you might not expect is that any fields declared private in the base class are inaccessible in the derived class. Thus, the private fields creditPoints and creditHours cannot be used in a method defined in GraduateStudent. This is again part of the encapsulation and data hiding ideals - we’ve encapsulated and hid those variables within the base class, and any code outside that assembly, even in a derived class, is not allowed to mess with it.

However, we often will want to allow access to such variables in a derived class. Java uses the access modifier protected to allow for this access in derived classes, but not the wider world.

In UML, protected attributes are denoted by a hash symbol # as the visibility of the attribute.

Inheritance and Memory

What happens when we construct an instance of GraduateStudent? First, we invoke the constructor of the GraduateStudent class:

GraduateStudent gradStudent = new GraduateStudent("Willie", "Wildcat", "Computer Science");

This constructor then invokes the constructor of the base class, Student, with the arguments "Willie" and "Wildcat". Thus, we allocate space to hold the state of a student, and populate it with the values set by the constructor. Finally, execution returns to the super class of GraduateStudent, which allocates the additional memory for the reference to the BachelorDegree property. Thus, the memory space of the GraduateStudent contains an instance of the Student, somewhat like nesting dolls.

Because of this, we can treat a GraduateStudent object as a Student object. For example, we can store it in a list of type Student, along with UndergraduateStudent objects:

List<Student> students = new LinkedList<>();
students.Add(gradStudent);
students.Add(new UndergraduateStudent("Dorothy", "Gale"));

Because of their relationship through inheritance, both GraduateStudent class instances and UndergraduateStudent class instances are considered to be of type Student, as well as their supertypes.

Nested Inheritance

We can go as deep as we like with inheritance - each base type can be a superclass of another base type, and has all the state and behavior of all the inherited base classes.

This said, having too many levels of inheritance can make it difficult to reason about an object. In practice, a good guideline is to limit nested inheritance to two or three levels of depth.

Abstract Classes

If we have a base class that only exists to be inherited from (like our Student class in the example), we can mark it as abstract with the abstract keyword. An abstract class cannot be instantiated (that is, we cannot create an instance of it using the new keyword). It can still define fields and methods, but you can’t construct it. If we were to re-write our Student class as an abstract class:

public abstract class Student {
    
    private int creditPoints;
    private int creditHours;
    protected String first;
    protected String last;
    
    // accessor methods for first and last omitted

    public Student(String first, String last) {
        this.first = first;
        this.last = last;
    }
    
    /**
     * Gets the student's grade point average.
     */
    public double getGPA() {
        return ((double) creditPoints) / creditHours;
    }
    
    /**
     * Records a final grade for a course taken by this student.
     * 
     * @param grade       the grade earned by the student
     * @param hours       the number of credit hours in the course
     */
    public void addCourseGrade(Grade grade, int hours) {
        this.creditHours += hours;
        switch(grade) {
            case A:
                this.creditPoints += 4 * hours;
                break;
            case B:
                this.creditPoints += 3 * hours;
                break;
            case C:
                this.creditPoints += 2 * hours;
                break;
            case D:
                this.creditPoints += 1 * hours;
                break;
            case F:
                this.creditPoints += 0 * hours;
                break;
        }
    }
}

Now with Student as an abstract class, attempting to create a Student instance:

Student theWiz = new Student("Wizard", "Oz");

would fail with an exception. However, we can still create instances of the derived classes GraduateStudent and UndergraduateStudent, and treat them as Student instances. It is best practice to make any class that serves only as a base class for derived classes and will never be created directly an abstract class.

Interfaces and Inheritance

A class can use both inheritance and interfaces. In Java, a class can only inherit one base class, and it should always be listed first after the extends keyword. Following that, we can have as many interfaces as we want listed after the implements keyword, all separated from each other and the base class by commas (,):

public class UndergraduateStudent extends Student implements ITeachable, IEmailable {
  // TODO: Implement student class 
}