Abstract Classes and Abstract Methods

Every employee gets a payslip with the same layout: name, employee id, pay for the month. Only the pay calculation differs: full-time staff get a fixed salary, contractors are paid by the hour. An abstract class lets you write the shared part once (the payslip layout) and leave a blank (the pay calculation) that every subclass must fill in. You can't create an object of the abstract class itself, because it isn't finished.

A Class with a Blank

public class Payslips {
    abstract static class Employee {
        private final String name;
        private final String id;
 
        Employee(String name, String id) {
            this.name = name;
            this.id = id;
        }
 
        abstract long monthlyPay();
 
        String payslip() {
            return id + " " + name + ": Rs. " + monthlyPay();
        }
    }
 
    static class FullTimeEmployee extends Employee {
        private final long salary;
 
        FullTimeEmployee(String name, String id, long salary) {
            super(name, id);
            this.salary = salary;
        }
 
        @Override
        long monthlyPay() { return salary; }
    }
 
    static class Contractor extends Employee {
        private final int hours;
        private final long ratePerHour;
 
        Contractor(String name, String id, int hours, long ratePerHour) {
            super(name, id);
            this.hours = hours;
            this.ratePerHour = ratePerHour;
        }
 
        @Override
        long monthlyPay() { return hours * ratePerHour; }
    }
 
    public static void main(String[] args) {
        Employee[] staff = {
            new FullTimeEmployee("Navika", "E101", 85000),
            new Contractor("Advitha", "C207", 120, 600)
        };
        for (Employee e : staff) {
            System.out.println(e.payslip());
        }
    }
}

E101 Navika: Rs. 85000
C207 Advitha: Rs. 72000

How it works

1. abstract class Employee can have fields, a constructor and finished methods, just like a normal class.
2. abstract long monthlyPay(); has no body, only a semicolon. It's a blank that subclasses must fill.
3. payslip() is finished code that calls the blank. At runtime, the subclass's monthlyPay() runs: 120 × 600 = 72,000 for Advitha.
4. The Employee constructor still runs, through super(name, id), to set up the shared fields.

Try it yourself: add an Intern subclass with a fixed stipend of Rs. 15,000, and add Srihitha as an intern.

The Rules

Breaking the rules looks like this:

abstract class Shape { abstract double area(); }
class Circle extends Shape { }
class Use { Shape s = new Shape(); }
abstract final class X {}
class Y { abstract void m(); }

E1.java:4: error: illegal combination of modifiers: abstract and final
abstract final class X {}
               ^
E1.java:2: error: Circle is not abstract and does not override abstract method area() in Shape
class Circle extends Shape { }
^
E1.java:3: error: Shape is abstract; cannot be instantiated
class Use { Shape s = new Shape(); }
                      ^
E1.java:5: error: Y is not abstract and does not override abstract method m() in Y
class Y { abstract void m(); }
^
4 errors

The Template Method Idea

payslip() above is a template method: the abstract class fixes the steps and the order, and subclasses supply one or more steps. The JDK uses it widely. java.util.AbstractList implements dozens of list methods on top of two abstract ones, get(int) and size(). Write those two and you have a working read-only list.

Common Mistakes

Trying to create the abstract class with new. Create a concrete subclass instead, or an anonymous class (see "Anonymous Inner Class in Java").

Forgetting one abstract method in a subclass. The error names the missing method; implement it or make the subclass abstract.

Calling an abstract method from the constructor. The subclass's fields aren't set yet when it runs, so it can see zeros and nulls.

Interview Corner

Practice

1. Easy: Create an abstract class Shape with an abstract area() and a finished describe() that prints the area. Implement Circle and Square.
2. Harder: Create an abstract class Report with a final print() that calls header(), body() and footer(). Make header() and footer() finished and body() abstract, then write SalesReport and StockReport.

Key Takeaways