Libraries Study Guide
Author: Kartik Kapur

Lecture Code

Code from this lecture available at


Abstract Data Types Previously, we went over interfaces which, in a traditional sense (disregarding default methods which will be defined a bit lower), requires certain methods to be implemented in a class if it is said a type of that interface. Abstract Data Types follow this philosophy, and are defined to be some sort of Object that is defined by some set of operations rather than the implementation of these operations.

Interfaces There are 2 types of inheritance that we have gone over in previous lectures:

Default Methods The way we have dealt with interfaces, there is no content in them. We only define a certain set of operations that need to be fulfilled by anything that implements the interface. However, we can create default methods that take the following form:

default void methodName(){...}

Normal interface methods have no body and only state what needs to be defined. Default methods on the other hand provide how a method by providing a method body.

Here are some bullet points about interfaces

Abstract Classes Abstract classes can be thought of as a hybrid of a normal class and an interface. Abstract classes are like interfaces in that they cannot be instantiated. All methods in an Abstract class are like normal methods in classes unless they have word abstract in front of them. If that is the case then they are treated like normal methods in interfaces and do not need to have a body and can instead have an implementation in whatever class extends them. A very important difference between abstract classes and interfaces is that a class can only extend one abstract class but can implement more than one interface.

Packages A namespace is a region that can be used to organize code. Packages are a specific type of namespace that is used to organize classes and interfaces. To use a class from a different package use the following syntax:

package_name.classname.subclassname a = new package_name.classname.subclassname();

To make your life easier while typing out code, you can simply import the class following the syntax below:

import package_name.classname.subclassname;

Replace the subclassname with a * if you want to important everything from the class.


C level

  1. If an abstract class extends an abstract class, would it need to have function definitions for the abstract methods in the parent class? Similarly would an interface that implements another interface have to implement the non default methods in the parent interface.
  2. Can an abstract class be the subclass of a normal class?
  3. If you don’t specify the package a class is in, is it part of a package? If so, which package?

B level

  1. Would the following compile

     public class Tree(){
       public int bark(){...}
     public interface Dog{
       default int bark(){...}
     public class Mutant extends Tree implements Dog{...}

    If so, which bark method would be called?