What does abstract modifier in Java do?

An abstract keyword is used to declare methods abstract methods and abstract classes. Once a method is declared abstract we should not specify body for those.

Example

 public abstract class Sample{ public abstract demo(){ } } 

Error

 Sample.java:2: error: invalid method declaration; return type required public abstract demo(){ ^ 1 error 

And once a class is declared abstract it cannot be instantiated.

Example

 public abstract class Sample{ public abstract void demo(); public static void main(String args[]){ new Sample(); } } 

Error

 C:\Sample>javac Sample.java Sample.java:4: error: Sample is abstract; cannot be instantiated new Sample(); ^ 1 error