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Example:

abstract class Account 
{
	public void deposit() {} 
	public void withdraw() {} 
	public void checkBalance() {} 
	public abstract float rateOfInterest(); 
	public abstract String[] services(); 
 }
class SavingAccount extends Account
{
    public float rateOfInterest(){}
    public String[] services(){}
}
class CurrentAccount extends Account
{
	public float rateOfInterest(){}
    /* Now this class become abstract , if you not override all the  */ 
    /* abstract methods of abstract class, your class become abstract too. */
}

Interface vs Abstract class

Interface	Abstract Class
Main difference is methods of a Java interface are implicitly abstract and cannot have implementations.	A Java abstract class can have instance methods that implements a default behavior.
Variables declared in a Java interface is by default final.	An abstract class may contain non-final variables.
Members of a Java interface are public by default.	A Java abstract class can have the usual flavors of class members like private, protected, etc..
Java interface should be implemented using keyword "implements";	A Java abstract class should be extended using keyword "extends".
An interface can extend another Java interface only	an abstract class can extend another Java class and implement multiple Java interfaces.
A Java class can implement multiple interfaces but it can extend only one abstract class.	A Java class can extend only one abstract class.
Interface is absolutely abstract and cannot be instantiated;	A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.
Java interfaces are slow as it requires extra indirection.	Java abstract classes are faster than the Interface.

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