Practice›Java Fundamentals›Module 7. Inheritance & Polymorphism›7.1 Inheritance

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7.1 Inheritance — extends and inherited methods

Lesson

Building one class on another

A class can build on an existing class with the keyword extends. The new class — the subclass — automatically inherits the fields and methods of the original — the superclass. This models an is-a relationship: since Dog extends Animal, a Dog is an Animal, so it already has everything Animal can do.

Worked example

class Animal {
    public String describe() { return name + " is an animal"; }
}
class Dog extends Animal {   // Dog is-an Animal
    public Dog(String name) { super(name); }
}

Dog d = new Dog("Rex");
System.out.println(d.describe());   // "Rex is an animal" — inherited!

Dog never defines describe(), yet d.describe() works — it is inherited from Animal.

▶ Try it: run this — the Dog object can call describe() and getName(), both defined in Animal.

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); }
    public String fetch() { return getName() + " fetches the ball"; }
}
public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Rex");
        System.out.println(d.describe());
        System.out.println(d.getName());
    }
}

class Dog extends Animal means Dog...

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'Dog extends Animal' expresses that...

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

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Match each term to its meaning.

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True or false?

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