General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus: Abstraction

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

public abstract class Dog {
  abstract void Bark();
}

Summary

Abstraction is an important aspect of your software design and Java implements a robust set of abstract features. To learn more about how abstract classes are similar concepts to interfaces, how they relate to Plato's Forms theory, and more, read our Abstract Members / Class definition article next.