Inheritance and polymorphism

Objectives
    • Apply the concept of inheritance to design an application.
    • Design a class that makes use of an interface.
1 - Inheritance and polymorphism
    • A class (child) is said to be derived from another if it inherits from that class (parent).
    • The key word extends is used to signifiy that a class inherits from the given class.
    • When a class is derived from another, this class can access all the methods or instance variables that are declared public or protected.
    • Inheritance makes use of polymorphism. It is possible to call a method of a parent class using the key word super.
The word polymorphism derives from the words poly meaning "many", and morphism meaning "behaviour". In Java, polymorphism means that an instance of a class (an object) can be used as if it were of different types, thus having many behaviours. Here, a type means a class is derived from another class or an interface.
2 - Interfaces
You can use interfaces in Java as a way to achieve polymorphism.
public interface GreetingsInterface{

    // An interface cannot contain an implementation
    // of the method, only the signature
    public abstract void sayGreetings();
}
Although the language allows you to declare constants in an interface, the general best practice is to avoid placing constants in interfaces and to only declare abstract methods.
The methods however need to be implemented by some class before you can execute them. All the methods in an interface are public and abstract, even if you leave out the "public" and "abstract" keywords.
To use an interface you must implement that interface in some Java class.
// The key word implements signals to the compiler
// that the SpanishGreetings class implements the
// GreetingsInterface interface
public class SpanishGreetings implements GreetingsInterface{

    // A class that implements an interface must implement
    // all the methods declared in the interface
    public void sayGreetings() {
        System.out.println("Hola");
    }
}

public class JapaneseGreetings implements GreetingsInterface{
    public void sayGreetings() {
        System.out.println(“Konnichiwa”);
    }
}
Once a class implements an interface you safe the reference of an instance of that class in a reference variable whose type is the name of the interface. Here is an example of a main method:
public static void main(String[] args){

// The variable is declared of interface type GreetingsInterface,
// the object created is of type SpanishGreetings
GreetingsInterface myGreetings = new SpanishGreetings();
myGreetings.sayGreetings();

// The variable is declared of interface type GreetingsInterface,
// the object created is of type JapaneseGreetings
myGreetings = new JapaneseGreetings();
myGreetings.sayGreetings();

}
Question 2.1 :
What gets printed to the console after executing the method main?
Answer
If you omit implementing one of the methods that is part of the interface, an error message will occur. Let us take the example from above but this time SpanishGreetings does not implement sayGreetings() method.
public class SpanishGreetings implements GreetingsInterface{
    /*public void sayGreetings() {
        System.out.println(GreetingsInterface.greetings);
    }*/
}
After compiling the SpanishGreetings class, we get the following:
SpanishGreetings.java:1: error:
SpanishGreetings is not abstract and does not override abstract method sayGreetings() in GreetingsInterface
public class SpanishGreetings implements GreetingsInterface{
       ^
1 error
The error message shows us that we haven't defined the method sayGreetings() in GreetingsInterface. Again, once more time, we remind you the imporance to read carefully the error messages. This last error is quite precises as to the nature of your error.
To continue, a class can implement multiple Java interfaces.
public interface PartingsInterface{
    public void sayPartings();
}

// List the names of the interfaces
// to implement separated by a comma.
public class SchwarzeneggerPhrases implements GreetingInterface, PartingsInterface{

        public void sayGreeting() {
                System.out.println("I’ll be back");
        }

        // The class must still implement all the
        // methods declared in all the interfaces
        public void sayParting() {
                System.out.println("Hasta la vista");
        }

        public static void main(String[] args){

                GreetingsInterface myGreetings = new SchwarzeneggerPhrases();
                myGreetings.sayGreetings();

                PartingsInterface myPartings = new SchwarzeneggerPhrases();
                myPartings.sayPartings();
        }
}
Question 2.2:
What gets printed to the console after executing the method main?
Answer
It is possible for an interface to inherit from another interface, just like classes can inherit from other classes. You specify inheritance using the "extends" keyword to declare your interface derives from another. Unlike classes, interfaces can inherit from multiple superinterfaces. You specify that by listing the names of all parent interfaces to inherit from, separated by comma. Here is a simple interface inheritance example:
// TalkingInterface inherits all field and methods
// from GreetingsInterface and PartingsInterface .
public interface TalkingInterface extends GreetingsInterface, PartingsInterface {
        public void sayItAll();
}

// A class implementing an interface which inherits
// from multiple interfaces must implement all methods
// from the interface and its superinterfaces
public class HamletTalking implements TalkingInterface {

        public void sayGreetings() {
                System.out.println("To be or not to be");
        }

        public void sayPartings() {
                System.out.println("That is the question");
        }

        public void sayItAll() {
                sayGreetings();
                sayPartings();
        }

        // We create an instance of HamletTalking in main method
        public static void main(String[] args){
                TalkingInterface myTalking = new HamletTalking();
                myTalking.sayItAll();
        }
}
Question 2.3 :
What gets printed to the console after executing the method main?
Answer
In short:
    • An interface contains only abstract methods. An abstract method does not have an implementation.
    • The keyword implements is used to express the relationship between the class and the interface.
    • When a class implements an interface, this is a sort of contract that means that the class will have to implement all the methods of the interface.
    • You cannot create instances of an interface by itself. You must create an instance of a class that implements the interface, and reference this instance using a reference variable whose type is the name of the interface
    • A class can implement ("implements") several interfaces
    • An interface extend ("extends") several interfaces
3 - "Super" classes
The keyword "super" is used to call the constructor, methods, and properties of a parent class. If your method overrides one of its parent class’s methods, you can invoke the overridden method through the use of the keyword super. Here are some examples of how the keyword super is used:
    • super() calls the parent class constructor with no arguments.
    • It can also be used with arguments. i.e. super(argument1), and it will call the constructor that accepts 1 parameter of the type of argument1 (if it exists).
    • Also it can be used to call methods from the parent i.e. super.aMethod()
    • and variables from the parent i.e. super.aVariable
Important note: the constructor call super() must be the first statement in a constructor.
Consider the following example where Frog is derived ("extends") from the class Animal and makes two types of noises, “Ribbit” and “Bark”.
public class Animal {
        protected final String noise;

        protected Animal() {
                this.noise = "silent";
        }

        protected Animal(String noise) {
                this.noise = noise;
        }

        public void makeNoise() {
                System.out.println(noise);
        }
}

public class Frog extends Animal {
        private final String noise;
        public Frog() {

                // constructor call super() must be the
                // first statement in a constructor
                super("Ribbit"); // call parent constructor

                super.makeNoise(); // call parent method

                this.noise = "Bark";

                // call parent variable and compare to class variable
                if (!super.noise.equals(this.noise)) {
                        makeNoise(); // call class method
                }
        }

        public void makeNoise() {
                System.out.println(noise);
        }
}
Question 3 :
If you were to create a new object Frog with a constructor call, what would be printed to the console?
Answer