Collections & Definite Loops CSE 115 Spring 2006 April 10, 12, & 14 2006

Discussion of PacMan Make note of the requirements listed in the lab description. Attend recitations for additional advice and assistance. In class, we will build a game (Tic Tac Toe).

Collections Storage for many objects. Two main types: Bags Associations We will discuss use of collections, you will see how to write your own collection classes in CSE 116 & 250.

Java Collections Framework Java provides implementations for a number of “standard” collections classes in the java.util package. The root interface of the collections hierarchy is Collection.

The Collection<E> interface Note the <E> after the word collection. The <E> indicates that this class can use a generic type (parameterized type). When you create an instance of a class with a generic type, you specify in the <> the actual type for the generic.

Using Generic Types For collections, what you are specifying with the generic type is the type of objects you will be storing in a collection. Eg) I want a bag of cats. When you do this, Java ensures that only objects of the type specified go in and you can be assured that only objects of that type come out.

A usable bag java.util.ArrayList<E> Note the operations that you can perform on this collection. The most important will be creating an instance of the collection, inserting elements, removing elements, and finding if elements are in the collection.

Another important collection operation Iterating over all the elements of a collection and performing some operation with/on each element of the collection. This process can be accomplished by using a special object provided by Java called an iterator. In Java 5, the use of the iterator has been replaced with the for-each loop.

For-each loop Allows access to each element of a collection. Syntax: for(TypeOfElementInCollection giveNameToElement: NameOfCollection) { //write code for what to do with each //element. }

ArrayLists and PacMan Note that in the CSE115.Pacman.BoardPositions class there are ArrayLists for each of the type of cells on the PacMan board. Further explanation will be provided in recitation.

A useful association java.util.HashMap<K, V> Associates a key with a value. Both the key and value are objects. User specifies what type of key and value is used when HashMap is created.

Useful operations with HashMaps put (put a key/value pair in the HashMap) Remove Look up a value using its key Iterating using the for-each loop

Using for-each with HashMaps (note .values()) java.util.HashMap<Position, Cell> _board = new java.util.HashMap <Position, Cell>(); //magic happens to put things into board. for(Cell c: _board.values() { c.draw(); }

The keyword for The for-each is a specialized loop designed to work with collections. for is a keyword in Java that tells us there is a loop. You can create a regular “for-loop” for use in your programs.

Loops (Iteration/Repetition) The ability to do a task repeatedly. The functionality of repetition is most often implemented in programming languages using loops.

Definite Loop The “for-loop” is characterized as a definite loop and is normally used when you know how many times you want a specific task to be performed. It is sometimes referred to as a counting loop.

Entry Test Loop A “for-loop” is also characterized as an entry-test loop. That is, a condition about whether the loop should continue is tested before actually doing the work of the loop.

Syntax of for-loop { //loop body } for (initialization; condition; increment) { //loop body } Usually, the initialization is of a loop counter variable that is checked against a bounds in the condition and is incremented in the increment step.