Object Inheritance

It often happens that we would like to define classes that are closely related to each other: they may store similar data (i.e., have fields of the same names and types), or offer similar functionalities (i.e., have similar methods). Take for example the following two classes:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
    public class Person {
      private String name;
      private int age;
      
      public Person (String name, int age) {
        this.name = name;
        this.age = age;
      }
    
      public String getName () {return name;}
      public int getAge () {return age;}
    
      public void sayGreeting () {
        System.out.println(name + ": Hello there!");
      }
    
      public void greet (Person p) {
        System.out.println(name + ": Hello, " + p.getName() + "!");
      }
    }

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
    public class Student {
      private String name;
      private String institution;
      private int age;
      
      public Student (String name, String major, int age) {
        this.name = name;
        this.major = institution;
        this.age = age;
      }
    
      public String getName () {return name;}
      public String getInstitution () {return institution;}
      public int getAge () {return age;}
    
      public void sayGreeting () {
        System.out.println(name + ": Can't talk now... I'm on my way to " + institution);
      }
    
      public void greet (Student s) {
        System.out.println(name + ": Hey, " + s.getName() + "! Can we talk after the exam?");
      }
    }

Notice that these classes have many features in common:

• fields for name and age
• getter methods for name and age
• methods called sayGreeting() and greet(…)

Since there is a lot of similar and duplicated code between the classes, it would be nice to not have to write this code twice. Thankfully, Java gives us a way of creating new classes by extending and modifying existing classse though inheritance.

Code for this note

Note that PersonTester.java contains the main method.

• ACStudent.java
• Being.java
• Employee.java
• Person.java
• PersonTester.java
• Student.java

Inheritance

Inheritance allows us to define a new class as an extension of a previously defined class. The new class is known as a subclass of the previously defined class, and the the old is a superclass of the new class. A subclass inherits all of the fields and methods of the superclass by default, although private methods and fields are not accessible to a subclass. In order to define a subclass of an existing class, use the extends keyword. For example, we can define Student as a subclass of Person, by using

1
2
3
    public class Student extends Person {
      ...
    }

Once Person is implemented, we need only implement (1) new functionality that a Student should offer that is not already present in Person, and (2) modify any functionality that should be different for a Student than for a Person. Note that Person already has

• fields for name and age
• a constructor and the following methods: getters for name and age, and methods sayGreeting() and greet(Person p).

Thus in our Student class, we only need to do the following:

1. add a field and getter method for institution,
2. add a constructor that allows us to set the institution of a student,
3. modify the sayGreeting() and greet() methods.

There are a couple things we need to be aware of to complete the implementation. The first is that private fields/methods from Person are not accessible to the subclass Student. Thus we need to use getters/setters (or use the modifier protected in the Person class instead of private—we’ll talk about this later). Since we don’t have access to, for example, name and age in Student’s constructor, we can call the constructor for Person explicitly using super(…) where … refers to whatever arguments we want to pass to the Person constructor.

Here is a complete implementation of Student that will give exactly the same functionality as before, but using the fields and methods inherited from the Person class:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
    public class Student extends Person {
      private String institution;
      
      public Student (String name, String institution, int age) {
        super(name, age);    // call the Person constructor with args name and age
        this.institution = institution;
      }
    
      public String getInstitution () {return institution;}
      
      public void sayGreeting () {
        System.out.println(getName() + ": Can't talk now... I'm on my way to " + institution);
      }
    
      public void greet (Person s) {
        System.out.println(getName() + ": Hey, " + s.getName() + "! Can we talk after the exam?");
      }
    }

Exercise. Notice that we used the getter methods getName() in sayGreeting() and greet(Person s), where Person just uses the field name. Modify the code above to use name instead of getName() for the greeting methods. What happens when you compile?

The “is a” rule

Why did we define Student as a subclass of Person and not the other way around? Well, because Student only adds and modifies fields/methods already present in Person. For a more general principle for deciding when and whether to use inheritance, consider the is a rule: Suppose you have to classes ClassA and ClassB. Is it aways the case that a ClassB is a ClassA? If so then it is probably appropriate to define ClassB as a subclass of ClassA. For example every Student is a Person, but not every Person is a Student. So it would make sense to define a Student as a subclass of Person, while the opposite would not make sense.

Inheritance only allows you add and modify functionality to a superclass—you cannot remove a field or method present in a superclass.

Keyword super

Above we used a method call super(name, age) to call the constructor of Student’s superclass, Person. The keyword super can be used to access the (overridden) version of method in a superclass. For example, inside Student, we could use super.greet(...) to call the Person version of the greet method (which was overridden in Student).

Question. Suppose we redefine the greet(Person p) method for Student as follows:

1
2
3
4
      public void greet (Person s) {
        super.greet(s);
        System.out.println("Can we talk after the exam?");
      }

What would be the result of the following code?:

1
2
3
    Person alice = new Person("Alice", 23);
    Student bob = new Student("Bob", 19);
    bob.greet(alice);

@Override

In Java, you can use the annotation @Override before overriding a method from a superclass. For example, in our Student class we could have written:

1
2
3
4
5
6
7
8
9
10
11
12
    public class Student extends Person {
      ...
      @Override
      public void sayGreeting () {
        System.out.println(name + ": Can't talk now... I'm on my way to " + institution);
      }
      
      @Override
      public void greet (Person s) {
        System.out.println(name + ": Hey, " + s.getName() + "! Can we talk after the exam?");
      }
    }

The @Override annotation doesn’t affect how the code runs. Instead, the annotation tells the compiler that we intend for the next method defined to override a method in a superclass. The compiler will then verify that there is indeed a method with the same signature (i.e., name and parameter type list) in a superclass, and the compiler will raise an error if no such method is found. This behavior is incredibly helpful in debugging and maintaining your code.

Exercise. In the Person class, rename the method greet(Person s) to great(Person s) and compile. What error do you get? Remove the line @Override and compile again. Do you still get the error? What happens if you define a new method public void greet (Person p, Person s) for Student that greets two Persons and put @Override before the method definition?

Why should you use inheritance?

1. Encapsulation. Using inheritance allows you to cut down redundant code. This means when some functionality needs modification, you only need to change it in one place.
2. Writing less code. Often, using inheritance allows to to get more functionality without having to re-write redundant code.
3. More clarity of code. Using extends makes your intentions more obvious to anyone reading your code (including your future self).

Inheritance Diagrams

Now that we’ve seen how inheritance can help us simplify, clarify, and encapsulate our code, we should be eager to apply inheritance whenever it seems appropriate. Above, we had a single (super)class Person and subclass Student. But there are many different types of Person we could envision, and even many types of Student. We can use inheritance so that a given class may have many subclasses, each of which has subclasses itself. However, each *class* can extend only a single *class*. (Think about why this must be the case!)

For example, we might have a class ACStudent that represents a student at Amherst College. For example, consider the definition:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
    public class ACStudent extends Student {
      public ACStudent (String name, int age) {
        super(name, "Amherst College", age);
      }
    
      @Override
      public void sayGreeting () {
        System.out.println(getName() + ": Can't talk now... I'm on my way to Val.");
      }
      
      @Override
      public void greet (Person s) {
        super.greet(s);
        System.out.println("Go Mammoths!");
      }
      
    }

We also might want to make another subclass of Person for employees:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
    public class Employee extends Person {
      private String title;
    
      public Employee (String name, String title, int age) {
        super(name, age);
        this.title = title;
      }
    
      public void getPaid(int amount) {
        System.out.println(getName() + ": I just got paid $" + amount + "!!!");
      }
    
      @Override
      public void sayGreeting () {
        System.out.println(getName() + ": Hello everyone! I'm a + " + title + "!");
      }
    } 

Question. Suppose we wanted to implement a StudentEmployee class for student employees. What would be the (conceptual) problem with defining a class that extends both the Student and Employee classes? (Hint: how would a StudentEmployee say a greeting?)

We can represent the relationships between the classes Person, Student, ACStudent, and Employee using an inheritance diagram. To produce an inheritance diagram, each class is represented by a node labeled with the class’s name. Whenever one class extends another class, we write the superclass above the subclass and draw a line between the classes. For the classes above we get the following inheritance diagram.

Note that while ACStudent only extends Student, both Student and Person are superclasses of ACStudent. In general the superclass relationship is transitive: if A is a superclass of B and B is a superclass of C, then A is a superclass of C.

Exercise. Write a StudentEmployee class that extends the Student class. It should include all of the functionality of both the Student and Employee classes. Update the inheritance diagrams to include your StudentEmployee class.

Method Calls & Inheritance

When we use inheritance, it often happens that we have many methods with the same signature—namely the methods that are overridden. For example, Person, Student, and ACStudent all have methods called sayGreeting(). Yet for each type the expected behavior is slightly different. Consider the following example:

1
2
3
4
5
6
7
    Person alice = new Person("Alice", 23);
    Student bob = new Student("Bob", "UMass", 19);
    ACStudent eve = new ACStudent("Eve", 20);
    
    alice.sayGreeting(); // prints "Alice: Hello there!"
    bob.sayGreeting();   // prints "Bob: Can't talk now... I'm on my way to UMass"
    eve.sayGreeting();   // prints "Eve: Can't talk now... I'm on my way to Val. "

That is, the program knows which version of sayGreeting() to call based on the class of each instance. All of the (non-overridden) methods for superclasses are also available to to subclasses. For example:

1
2
3
4
    String inst;
    inst = alice.getInstitution(); // ERROR: no such method for Person or superclass
    inst = bob.getInstitution();   // calls method defined in Student
    inst = eve.getInstitution();   // calls method defined in Student (superclass)

Also:

1
2
3
4
    String name;
    name = alice.getName(); // calls method defined in Person
    name = bob.getName();   // calls method defined in Person (superclass of Student)
    name = eve.getName();   // calls method defined in Person (superclass of ACStudent)

Since the there may be many methods with the same signature accessible to an instance of a subclass, Java does a sensible thing to resolve ambiguity: it always calls the method defined in the “closest” superclass with the correct signature. More specifically, consider the call

1
    eve.sayGreeting();

Since eve is an instance of ACStudent which is a subclass of Student and Person, there are three different methods sayGreeting() visible to to an ACStudent. Since ACStudent itself has a method sayGreeting(), this is the method that actually gets called. If sayGreeting() had not been overridden in ACStudent, the method call above would resolve to the version of sayGreeting() defined in Student because ACStudent extends Student. The version of sayGreeting() for Person would only get executed by eve.sayGreeting() if sayGreeting() was defined neither for ACStudent nor Student.

Exercise. Verify this! Delete the method sayGreeting() for ACStudent. What is the output of eve.sayGreeting()?

Abstract Classes

Sometimes we would like to define a class that captures and consolidates the features of many classes, but is so general that it doesn’t make sense to implement all of its methods. For example, every person is a being. But beings include all sorts of other entities: dogs, cats, dragons, etc. All beings have, say, a name and an age. Also, all beings are capable of giving some form of greeting, but printing text to the screen might not be an appropriate representation of all beings’ greetings. For example, a dog might say a greeting by wagging its tail. Since this is very different from a typical greeting for a person, it doesn’t make sense to implement a single greeting method that is applicable to all beings. Yet all beings should offer some form a greeting.

Abstract classes allow us to define a class and declare its methods without implementing them. For example, we can define an abstract class for a Being that declares a sayGreeting() method, but offers no implementation of sayGreeting(). All (non-abstract) subclasses are required to implement the sayGreeting() method.

We use the abstract modifier to declare a method in a class without implementation. In order to use abstract inside a class, we also must declare the class itself as abstract. For example:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
    public abstract class Being {
      private String name;
      private int age;
    
      public Being(String name, int age) {
        this.name = name;
        this.age = age;
      }
    
      public String getName () { return name; }
      public int getAge () { return age; }
    
      // a method declaration without an implementation
      public abstract String sayGreeting ();
    }

Exercise. Implement Person as a subclass of Being:

1
2
3
    public class Person extends Being {
      // you provide the details here!!!
    }

Exercise. Try compiling Being.java and your new version of Person.java with Person’s implementation of sayGreeting() commented out. What error do you get?

Since abstract classes are missing implementations of methods they cannot be instantiated—we can only create instances of subclasses of abstract classes that are not themselves abstract.

Exercise. Try creating an instance of a Being using Being gaia = new Being("Gaia", 100000000); What error do you get?

Vocabulary

• inheritance
• subclass
• superclass
• extends keyword
• override
• super keyword
  • for calling a superclass constructor
  • for calling superclass methods
• @Override annotation
• “is a” rule
• inheritance diagram
• abstract class
• abstract keyword