super and Overriding Methods

Calling the parent's constructor: super

When the parent has a constructor, the child calls it with super(...) as its very first line:

class Employee {
    String name;
    int salary;

    Employee(String name, int salary) {
        this.name = name;
        this.salary = salary;
    }
}

class Manager extends Employee {
    int teamSize;

    Manager(String name, int salary, int teamSize) {
        super(name, salary);
        this.teamSize = teamSize;
    }
}

public class Main {
    public static void main(String[] args) {
        Manager m = new Manager("Asha", 90000, 6);
        System.out.println(m.name + " manages " + m.teamSize + " people");
    }
}

Output

Asha manages 6 people

Overriding: doing it differently

A child can override a parent's method – write its own version with the same name and parameters:

class Animal {
    void sound() {
        System.out.println("Some sound");
    }
}

class Cat extends Animal {
    @Override
    void sound() {
        System.out.println("Meow");
    }
}

class Cow extends Animal {
    @Override
    void sound() {
        super.sound();
        System.out.println("Moo");
    }
}

public class Main {
    public static void main(String[] args) {
        new Animal().sound();
        new Cat().sound();
        new Cow().sound();
    }
}

Output

Some sound
Meow
Some sound
Moo

• @Override is optional but very useful: if you misspell the method name, the compiler warns you.
• super.sound() runs the parent's version, so the child can add to it instead of replacing it.

Stopping inheritance: final

A final method can't be overridden, and a final class can't be extended. String is a final class.

final class Secret {
}

class Spy extends Secret {
}

public class Main {
    public static void main(String[] args) {
    }
}

Output (the program does not compile)

Main.java:4: error: cannot inherit from final Secret
class Spy extends Secret {
                  ^