Abstract classes vs. interfaces in Java

Posted on 03-01-2013 21:43 by graham
The difference between abstract classes and interfaces in Java is a question that often remains obscure for Java beginners. This article aims to list the similarities and differences between the two, as well as give hints as when to use which.


- They are declared with the interface keyword (obviously...)
- They cannot contain fields
- All methods in an interface must be abstract and public. Note, however, that methods in an interface cannot be marked with the abstract keyword - they are abstract by default and cannot have a body.
- Interfaces are implemented, not extended.

Declaring an interface:
public interface FileReader
public String getFileContents();

An interface declared above can be implemented by some class that wants to use its functionality:
public class TextFileReader implements FileReader
public String getFileContents()
// the actual reading of file contents takes place here

Abstract classes

- Must be marked with the abstract keyword.
- May contain both abstract and fully implemented methods.
- May contain fields and properties
- May contain private and protected methods
- They are extended (not implemented) by their child classes

Declaring an abstract class:
public abstract class CustomPageController
protected Date dateCreated;
private Integer viewCount;

// abstract method - to be implemented in a subclass
public abstract String getPageTitle();

// non-abstract method - implemented here
public Integer getViewCounter()
return ++viewCount;

// constructor
public CustomPageController()
viewCount = 0;

A class created this way can be extended the following way:
public class HomePageController extends CustomPageController
// abstract methods need to be implemented
public String getPageTitle()
return "My Home Page";


So what do abstract classes and interfaces have in common.
- Both abstract classes and interfaces cannot be instantiated.
- Both represent abstract (unfinished, if you prefer) entities rather than concrete objects.


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