Abstract classes and Abstract Methods in Java

Abstract Methods:

• They are present in abstract classes
• The keyword "abstract" is prepended to the methods
• Abstract methods are implemented only in sub-class / inherited class and not in "Abstract" class
• Since abstract methods are implemented in sub-class, the return type of abtract methods is either "protected" or "public"

Abstract Classes:

• A class that contains "abstract" methods is an abstract class
• Abstract classed can contain "non static" and "non final" member variables
• They can NEVER be instantiated. They can be ONLY INHERITED
• One abstract class can inherit another abstract class
• Abstract classes can contain ordinary methods as well as abstract methods

Sample Implementation:


/**
 * ABSTRACT CLASS
 * /
public abstract class AbstractTest {
    public abstract void testPrint1();
    protected abstract void testPrint2();
}

/**
 * ABSTRACT CLASS IMPLEMENTED
 * NOTE: IMPL CLASS MUST IMPLEMENT ALL ABSTRACT METHODS IN ABSTRACT CLASS
 */
public class AbstractTestImpl extends AbstractTest {
    public void testPrint1() {
        System.out.println("Test Print One!");
    }

    public void testPrint2() {
        System.out.println("Test Print Two!");
    }

    public static void main(String args[]) {
        AbstractTestImpl obj = new AbstractTestImpl();
        obj.testPrint1();
    }
}