Abstract methods and classes

Abstract method A placeholder for a method that will be fully defined in a subclass.

Abstract method An abstract method has a complete method heading with the addition of the keyword abstract. Cannot be private or final. Ex. abstract public double getPay ( ); abstract public void doSomething ( int count );

Abstract class A class that has at least one abstract method is called an abstract class. The class definition must have the keyword abstract. Ex. abstract public class Feet { … abstract void doSomething ( int count ); …}

Abstract vs. concrete class A class without any abstract methods is called a concrete class.

Points to remember You cannot create an instance of an abstract class. –Ex. Employee joe = new Employee(); //the above is illegal if Employee is abstract

Points to remember An abstract class is a type. Therefore you can use abstract classes as parameters to functions. Ex. (perfectly “legal”) public abstract class Employee { … public boolean samePay ( Employee other ) { …}…}

Points to remember An abstract class is a type. Therefore you can use abstract classes as variable types for concrete classes derived from the abstract class. Example follows…

Points to remember Ex. (perfectly “legal”) public abstract class Employee { …} public class HourlyEmployee extends Employee { …} public class SalaryEmployee extends Employee { …}… Employee e = new HourlyEmployee();