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Abstract Classes in Java
Introduction to Abstract Classes in Java
The Java programming language supports a rich variety of OOP (Object Oriented Programming) constructs, including abstract classes. In this tutorial we introduce the concept of an abstract class and demonstrate the use of such classes in Java.
The process of instantiation describes the process by which a software class is used as a template to allocate memory space for an object during the execution of a program, including a Java program. An abstract class cannot be instantiated to create an object. Generally, an OOP programmer creates a class in anticipation of using it to create an object. The abstract class serves as a base class, or superclass, for non-abstract classes that can be instantiated.
Why create an Abstract Class?
The purpose of an abstract class is to provide a common container for subclasses of the abstract class. For example, a class that models a phone might contain methods such as Dial(), and Reset() that are common to all phones. Certainly a non-abstract class could provide this functionality, but implementing this superclass as an abstract class prevents our Phone class from being instantiated.
A Brief Example
In the following code, a simple abstract class is defined. The class is called Phone. The class declaration contains the decoration abstract, which is the required syntax for declaring this type of class. Including this keyword prevents the class from being used as to instantiate objects.
Zero or more methods in the abstract class may be declared as abstract. Methods declared as abstract, such as the Dial method below, appear in the abstract class as a method signature with no body. The method signature is also decorated with the abstract keyword. This type of method should be completed in a subclass.
Note that the abstract class defined below also contains a non-abstract method, DEBUG(). This is one capability that sets abstract classes apart from interfaces in Java. An interface cannot contain method logic, only signatures.
/**
* An abstract class that models everything common to all phone objects
* This class cannot be instantiated because it is an abstract class.
* @author nicomp
* (c) 2012 nicomp
*
*/
/* The abstract keyword is required because there's an abstract method in the class. */
public abstract class Phone {
/**
* This is an abstract method that must be fleshed out in the derived class.
* @param Number The telephone number to be dialed
* @return 0 if the dial operation succeeded, !0 otherwise.
*/
public abstract int Dial(String Number);
/**
* Print a debug message
* @param msg
*/
public void DEBUG(String msg) {System.out.println(msg);}
}
The Subclass is Introduced
Below is an example of a subclass that inherits from our phone abstract class. We use the standard extends keyword to indicate inheritance. Our subclass provides a (structurally) complete definition for the Dial( ) method that is declared in the base class.
Our subclass models a specific make/model of phone. It can be instantiated.
/**
* A class that models a Motorola Milestone cell phone
* @author nicomp
* (c) nicomp
*
*/
// Inherit from the Phone class.
public class MotorolaMilestone extends Phone {
private Boolean activationStatus;
/**
* Attempt to place a call
* @param number The telephone number to be called
* @return 0 on success, !0 on failure
*/
public int Dial(String number) {
int status = 0;
return status;
}
/**
* Constructor
*
*/
public MotorolaMilestone() {
activationStatus = false; // default to not activated
DEBUG("A Motorola Milestone object has been created."); // Where is this method? Use Eclipse to find it.
}
/**
* Get the Activation Status
* @return The current value of the Activation Status
*/
public Boolean GetActivationStatus(){return activationStatus;}
/**
* Set the Activation Status
* @param activationStatus The new value of the Activation Status
* @return the new value of the Activation Status
*/
public Boolean SetActivationStatus(Boolean activationStatus){
this.activationStatus = activationStatus;
return activationStatus;
}
/**
* Overrides the base class toString() method.
* @return A string representation of the object.
*/
public String toString(){
return("Motorola Milestone");
}
}
The Main Method
Finally, we introduce the Main method that exercises our class. In the Main we demonstrate that our subclass can be instantiated and that the non-abstract class in the subclass can be invoked.
/** Entry Point Class
*
* @author nicomp
* (c) nicomop
*
*/
public class Main {
/**
* @param args
*/
public static void main(String[] args) {
MotorolaMilestone myPhone = new MotorolaMilestone();
// Invoke the method defined in the abstract superclass
myPhone.DEBUG("Hello World");
}
}
Conclusion and Future Work
In this tutorial we introduced the concept of an abstract class and demonstrated the use of such classes in Java.
Future work might include additional subclasses based on the Phone class.
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