Practice›Java Fundamentals›Module 7. Inheritance & Polymorphism›7.4 Polymorphism

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7.4 Polymorphism — one method, many types

Lesson

Why overriding pays off

The payoff: a method that takes a superclass parameter works with any subclass, automatically using each one's overridden behavior. You write the method once and it handles every current — and future — subclass. This is polymorphism: "many forms" through one interface.

Worked example

static void announce(Animal a) {
    System.out.println("Meet: " + a.describe());
}

announce(new Dog("Rex"));    // Meet: Rex is a dog
announce(new Cat("Felix"));  // Meet: Felix is a cat

announce takes an Animal, so it accepts a Dog, a Cat, or any other Animal — each described in its own way.

▶ Try it: run this — the same announce method handles a Dog and a Cat.

class Animal {
    private String name;
    public Animal(String name) { this.name = name; }
    public String getName() { return name; }
    public String describe() { return name + " is an animal"; }
}
class Dog extends Animal {
    public Dog(String name) { super(name); }
    @Override public String describe() { return getName() + " is a dog"; }
}
class Cat extends Animal {
    public Cat(String name) { super(name); }
    @Override public String describe() { return getName() + " is a cat"; }
}
public class Main {
    static void announce(Animal a) { System.out.println("Meet: " + a.describe()); }
    public static void main(String[] args) {
        announce(new Dog("Rex"));
        announce(new Cat("Felix"));
    }
}

A method with an Animal parameter can be called with...

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Inside announce(Animal a), calling a.describe() uses...

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Type exactly what this program prints.

class Animal { String n; Animal(String x){n=x;} String describe(){return n+" is an animal";} }
class Dog extends Animal { Dog(String x){super(x);} String describe(){return n+" is a dog";} }
class Cat extends Animal { Cat(String x){super(x);} String describe(){return n+" is a cat";} }
public class Main {
    static void show(Animal a){ System.out.println(a.describe()); }
    public static void main(String[] x){
        show(new Dog("Rex"));
        show(new Cat("Mia"));
    }
}

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Polymorphism means...

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True or false?

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