1. Abstract methods and classes

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

3. 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 );

4. 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 ); }

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

6. 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

7. 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 ) { }

8. public abstract class Employee{
private String name;
private Date hireDate;
public abstract double getPay( );
public Employee( )
name = "No name";
hireDate = new Date("Jan", 1, 1000); //Just a place holder. }
/**
Precondition: Neither theName nor theDate are null. */
public Employee(String theName, Date theDate)
if (theName == null || theDate == null)
System.out.println("Fatal Error creating employee.");
System.exit(0);
name = theName;
hireDate = new Date(theDate);

9. public Employee(Employee originalObject){
name = originalObject.name;
hireDate = new Date(originalObject.hireDate); }
public boolean samePay(Employee other)
if (other == null)
System.out.println("Error: null Employee object.");
System.exit(0);
//else
return (this.getPay( ) == other.getPay( ));
public String getName( )
return name;
public Date getHireDate( )
return new Date(hireDate);
...

10. 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…

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

12. Example
Employee
SalariedEmployee
HourlyEmployee