Java Abstract Classes

An abstract class is a class which you can not create objects of. It is made to be a base for other classes. It can have abstract methods (no body, only the name) which the subclasses must write, and also normal methods with code.

Example

abstract class Employee {
    String name;

    Employee(String name) {
        this.name = name;
    }

    abstract double salary();            // no body: every subclass must write it

    void show() {                        // a normal method
        System.out.println(name + " earns " + salary());
    }
}

class FullTime extends Employee {
    double monthly;
    FullTime(String name, double monthly) { super(name); this.monthly = monthly; }

    @Override
    double salary() { return monthly; }
}

class Freelancer extends Employee {
    double hourly; int hours;
    Freelancer(String name, double hourly, int hours) { super(name); this.hourly = hourly; this.hours = hours; }

    @Override
    double salary() { return hourly * hours; }
}

public class Main {
    public static void main(String[] args) {
        Employee[] team = { new FullTime("Riya", 60000), new Freelancer("Aman", 500, 40) };
        for (Employee e : team) {
            e.show();
        }
    }
}

You can not create an object

abstract class Shape {
    abstract double area();
}

public class NoObject {
    public static void main(String[] args) {
        Shape s = new Shape();
    }
}

A subclass must implement all abstract methods

abstract class Shape {
    abstract double area();
}

class Square extends Shape {
    // forgot to write area()
}

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

Template method pattern

A very useful use of abstract classes: the parent defines the steps, and the children fill the details.

abstract class Report {
    final void print() {                 // the fixed steps
        System.out.println("--- " + title() + " ---");
        System.out.println(body());
        System.out.println("--- end ---");
    }

    abstract String title();
    abstract String body();
}

class SalesReport extends Report {
    String title() { return "Sales"; }
    String body() { return "Total: 120 items"; }
}

public class Template {
    public static void main(String[] args) {
        new SalesReport().print();
    }
}

Facts

• An abstract class can have constructors, fields and normal methods.
• A class with even one abstract method must be declared abstract.
• An abstract method can not be private, static or final.

Abstract class or interface?

Point	Abstract class	Interface
Inherit from	Only one class	Many interfaces
State (fields)	Can have instance fields	Only constants
Use when	Related classes share code and state	Different classes share a behavior

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