Core Java 6: Interfaces & Abstract Classes

Question 1

Interface

Answer 1

• abstract type used to declare behavior classes must implement
• can inherit from as many interfaces as you want (how Java handles multiple inheritance)
• all fields are public, static, final (constants)
• all methods are implicitly public
• interface cannot have constructor
• any non-abstract methods must use default or static keyword
• meant for inheritance; meant to define function or group of related functions for child to implement

Question 2

Abstract classes

Answer 2

classes used solely for inheritance, cannot be instantiated directly

Question 3

Abstract method

Answer 3

method w/o implementation in class / interface where defined

Question 3

When to use interface vs. abstract class

Answer 3

if you need instance variables to inherit, you should use an abstract class; interfaces define something your abstract class can do (behaviors/methods), abstract classes define what classes are; interfaces are intentionally restrictive, so you use them to only inherit method or group of related methods