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

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7.4 Polymorphism — an array of a superclass type

Lesson

A mixed collection, one loop

Because a superclass variable can hold any subclass object, an array of the superclass type can hold a mix of subclasses: Animal[] zoo = { new Dog(...), new Cat(...) };. Loop over it and call the same method — each element responds with its own version. One loop, many behaviors.

Worked example

Animal[] animals = { new Dog("Rex"), new Cat("Felix"), new Animal("Thing") };
for (int i = 0; i < animals.length; i++) {
    System.out.println(animals[i].describe());
}
// Rex is a dog
// Felix is a cat
// Thing is an animal

The same describe() call gives a different answer for each object.

▶ Try it: run this — one loop makes each animal describe itself in its own way.

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 {
    public static void main(String[] args) {
        Animal[] animals = { new Dog("Rex"), new Cat("Felix"), new Animal("Thing") };
        for (int i = 0; i < animals.length; i++) {
            System.out.println(animals[i].describe());
        }
    }
}

An Animal[] array can hold...

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Looping over an Animal[] of a Dog and a Cat and calling describe() gives...

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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 { public static void main(String[] a){
    Animal[] arr = { new Dog("Rex"), new Cat("Mia") };
    for (int i = 0; i < arr.length; i++) System.out.println(arr[i].describe());
}}

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Storing different subclasses in one superclass array lets you...

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True or false?

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