Collections & Definite Loops

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

2. 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).

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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. }

10. 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.

11. 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.

12. 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

13. 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(); }

14. 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.

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

16. 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.

17. 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.

18. Syntax of for-loop 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.