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

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7.4 Polymorphism — a superclass reference

Lesson

One variable, many object types

A variable of a superclass type can hold a subclass object: Animal a = new Dog("Rex");. The declared type is Animal, but the actual object is a Dog. When you call an overridden method, Java runs the actual object's version — this is dynamic dispatch (polymorphism). So a.describe() runs Dog's version even though a is typed as Animal.

Worked example

Animal a = new Dog("Rex");   // declared Animal, actual Dog
System.out.println(a.describe());   // "Rex is a dog" — Dog's version runs

The variable's type does not decide which overridden method runs — the real object does.

▶ Try it: run this — through Animal variables, each object still uses its own describe().

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 an = new Dog("Rex");
        Animal b = new Cat("Felix");
        System.out.println(an.describe());
        System.out.println(b.describe());
    }
}

Can a variable of type Animal hold a Dog object?

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For Animal a = new Dog("Rex"), a.describe() runs...

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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";} }
public class Main { public static void main(String[] a){
    Animal x = new Dog("Sam");
    System.out.println(x.describe());
}}

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Running the actual object's version of a method is called...

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True or false?

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