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11.13. Inheritance and Interfaces

An interface in Java is a special type of abstract class that can only contain public abstract methods (every method is assumed to be public and abstract even if these keywords are not specified) and public class constants. List is an interface in Java. Interfaces are declared using the interface keyword. One interface can inherit from another interface.

public interface Checker
      boolean check (Object obj);

The code above declares an interface called Checker that contains a public abstract method called check that returns true or false. Classes that implement this interface must provide the body for the check method.

Another example of an interface in Java is the Iterator interface. It is used to loop through collection classes (classes that hold groups of objects like ArrayList).

11.13.1. What is the purpose of an interface?

The purpose of an interface is to separate what you want a type to be able to do (defined by the method signatures) from how it does that. This makes it easy to substitute one class for another if they both implement the same interface and you have declared the variable using the interface type. The List interface defines what a class needs to be able to do in order to be considered a List. You have to be able to add an item to it, get the item at an index, remove the item from an index, get the number of elements in the list, and so on. There are several classes that implement the List interface. You only have to know about ArrayList for the exam, which is a class that implements the List interface using an array.

See for the Java documentation for the full definition of the List interface.

The following are the List methods that you need to know for the AP CS A exam. These are included on the quick reference that you will receive during the exam. You can get it at

  • int size() returns the number of elements in the list

  • boolean add(E obj) appends obj to the end of the list and returns true

  • void add(int index, E obj) moves any current objects at index or beyond to the right (to a higher index) and inserts obj at the index

  • E get(int index) returns the item in the list at the index

  • E set(int index, E obj) replaces the item at index with obj

  • E remove(int index) removes the item at the index and shifts remaining items to the left (to a lower index)

Interfaces make it easy to write general methods that use the methods defined in the interface.

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