Abstract class can contain abstract methods. Abstract methods do not have implementation. Abstract methods should be implemented in the subclasses which inherit them. So if an abstract class has an abstract method class inheriting the abstract class should implement the method or else java compiler will through an error. In this way, an abstract class can define a complete programming interface thereby providing its subclasses with the method declarations for all of the methods necessary to implement that programming interface. Abstract methods are defined using "abstract" keyword. Below is a sample code snippet.
abstract class pcdsGraphics
abstract void draw();
Any class inheriting from "pcdsGraphics" class should implement the "draw" method or else the java compiler will throw an error. so if we do not implement a abstract method the program will not compile.