ArrayList Collections

How Do I Use an ArrayList? First, import it so it is available for use: import java.util.ArrayList; -or- import java.util.*;

How Do I Use an ArrayList? Next declare and initialize! ArrayList<Point> myPoints; myPoints = new ArrayList<Point>(); //note: The initial capacity of an ArrayList is 10 Recall the Java array style Point[] myPoints; myPoints = new Points[10];

How do I add items? Call the add method (adds the item to the back of the ArrayList). Point p = new Point(1,2); myPoints.add(p); myPoints.add(new Point(2,3));

How many items are in the ArrayList? For an ArrayList, we use myPoints.size() For an array, we use myPointsArr.length Note: size() gives number of items, length gives the capacity

How do I retrieve items? For an ArrayList For an array, Point p = myPoints.get(0); For an array, Point p = myPointArr[0];

Example (Traversal and set) Given an ArrayList called myNames filled with names, replace all occurrences of “marc” with “mark”. for(int i = 0; i < myNames.size(); i++) { if (myNames.get(i).equals(“marc”)) myNames.set(i, “mark”); } Note: The set method replaces what is at i with the new element and returns the old element (which is ignored in this case).

Traversal with “for each” loop for(String s: myNames) { if (myNames.get(i).equals(“marc”)) myNames.set(i, “mark”); }

Can elements only be added at the back of the ArrayList? No! You can add an element at any valid index. Consider the ArrayList named myAlphabet: [“apple”, “bear”, “cake”, “dog”] The call myAlphabet.add(2, “zebra”); results in [“apple”,“bear”,“zebra”,“cake”,“dog”] Note: No need to shift things over by hand! ArrayList does this for you!!!

Special note on add Given the following ArrayList named myAlphabet: [“apple”, “bear”, “cake”, “dog”] The following two calls would produce identical results: myAlphabet.add(“zebra”) myAlphabet.add(4, “zebra”); Note: myAlphabet.add(5, “zebra”) will cause an IndexOutOfBoundsException.

Easy to remove as well! [“apple”, “bear”, “cake”, “dog”] Remember the pain of removing with arrays (having to shift elements to the left). ArrayList does this for you! Given [“apple”,“bear”,“zebra”,“cake”,“dog”] myAlphabet.remove(2) results in [“apple”, “bear”, “cake”, “dog”]

Summary of ArrayList methods class java.util.ArrayList<E> int size() boolean add(E e) appends e to end of list, returns true void add(int i, E e) inserts e at index of i (0 <= i <= size) moving elements right as necessary E get(int i) returns element at index of i E set(int i,E e) replaces element at i with e, returns former element at i E remove(int i) removes element at I moving elements left as necessary.

Arrays vs. ArrayList Arrays ArrayList Declaration/Initialization Point[] p; p = new Point[5]; ArrayList<Point> p; p=new ArrayList<Point>(); Assignment (replace) p[3]=aPoint; p.set(3, aPoint); Insertion Lots of code to shift element right p.add(2, aPoint); Deletion Lots of code to shift elements left p.remove(2); Number of items p.length p.size(); Access p[4].getX(); p.get(4).getX();