Chapter 13 - java.util.ArrayList. In this chapter, the learning objectives are: Learn about java.util.List interface; learn about the java.util.ArrayList class, its constructors and methods; review some of the ArrayList pitfalls; practice with Part 4 of GridWorld ― “Critters”.
Java Methods Object-Oriented Programming and Data Structures 2nd AP edition with GridWorld Maria Litvin ● Gary Litvin capacity size Chapter13 java.util.ArrayList Copyright © 2011 by Maria Litvin, Gary Litvin, and Skylight Publishing All rights reserved Objectives: • Learn about java.util.List interface • Learn about the java.util.ArrayList class, its constructors and methods • Review some of the ArrayList pitfalls • Practice with Part of GridWorld ― “Critters” 132 java.util.ArrayList • Implements a list using an array • Implements java.util.List interface ôinterfaceằ java.util.List java.util.ArrayList java.util.LinkedList 13ư3 java.util.ArrayList contd ã Implements a list using an array • Can only hold objects (of a specified type), not elements of primitive data types • Keeps track of the list capacity (the length of the allocated array) and list size (the number of elements currently in the list) capacity size "Cat" "Hat" "Bat" 134 ArrayList Generics • Starting with Java 5, ArrayList and other collection classes hold objects of a specified data type • The elements’ data type is shown in angle brackets and becomes part of the List and ArrayList type For example: ArrayList words = new ArrayList(); List nums = new ArrayList(); 135 ArrayList Constructors Java docs use the letter E as the type parameter for elements in generic collections ArrayList ( ) Creates an empty ArrayList of default capacity (ten) ArrayList (int capacity) Creates an empty ArrayList of the specified capacity 136 ArrayList Methods (a Subset) int size() boolean isEmpty () boolean add (E obj) returns true void add (int i, E obj) inserts obj as the i-th value; i must be from to size() E set(int i, E obj) E get(int i) E remove(int i) boolean contains(E obj) int indexOf(E obj) i must be from to size() -1 both use equals to compare objects 137 ArrayList Example ArrayList names = new ArrayList( ); names.add("Ben"); names.add("Cat"); names.add(0, "Amy"); System.out.println(names); Output [Amy, Ben, Cat] ArrayList’s toString method returns a string of all the elements, separated by commas, within [ ] 138 ArrayList Details • Automatically increases (doubles) the capacity when the list runs out of space (allocates a bigger array and copies all the values into it) • get(i) and set(i, obj) are efficient because an array provides random access to its elements • Throws IndexOutOfBoundsException when i < or i size() (or i > size() in add (i, obj) ) 139 ArrayList Autoboxing • If you need to put ints or doubles into a list, use a standard Java array or convert them into Integer or Double objects • In Java 5, conversion from int to Integer and from double to Double is, in most cases, automatic (a feature known as autoboxing or autowrapping); the reverse conversion (called autounboxing) is also automatic 1310 ArrayList Autoboxing Example ArrayList counts = new ArrayList( ); counts.add(17); int count = counts.get(0); Autoboxing: compiled as counts.add(new Integer(17)); Autounboxing: count gets the value 17 1311 ArrayList Pitfalls // Remove all occurences // of "like" from words: int i = 0; Caution: when you remove elements, a simple for loop doesn’t work: while (i < words.size()) for (int i = 0; i < words.size(); i++) { { if ("like".equals(words.get(i)) if ("like".equals(words.get(i)) words.remove(i); words.remove(i); else } i++; } Shifts all the elements after the i-th to the left and decrements the size 1312 “For Each” Loop • Works with List / ArrayList: ArrayList words = new ArrayList ( ); for (String word : words) { // process word } Basically the same as: for (int i = 0; i < words.size (); i++) { String word = words.get (i); // process word } 1313 Lab: Index Maker fish.txt One fish Two fish Red fish Blue fish Black fish Blue fish Old fish New fish This one has a little star This one has a little car Say! What a lot of fish there are fishIndex.txt A 12, 14, 15 ARE 16 BLACK BLUE 4, CAR 14 FISH 1, 2, 3, 4, 6, 7, 8, 9, 16 HAS 11, 14 LITTLE 12, 14 LOT 15 NEW OF 16 OLD ONE 1, 11, 14 RED SAY 15 STAR 12 THERE 16 THIS 11, 14 TWO WHAT 15 1314 Index Maker (cont’d) java.util ArrayList IndexMaker extends DocumentIndex has IndexEntry Your job 1315 GridWorld’s Critters • Described in Part of the GridWorld case study • Critter is subclass of Actor • You will create subclasses of Critter • Study the examples provided with GridWorld: ChameleonCritter and CrabCritter 1316 GridWorld’s Critters (cont’d) public void act() { if (getGrid() == null) return; ArrayList actors = getActors( ); processActors(actors); ArrayList moveLocs = getMoveLocations( ); Location loc = selectMoveLocation(moveLocs); makeMove(loc); } • Do not override the act method in Critter’s subclasses; override other methods to achieve the desired functionality 1317 GridWorld’s Critters (cont’d) • Pay attention to postconditions when you override Critter’s methods! ArrayList getActors( ) The state of all actors in the grid remains unchanged void processActors(ArrayList actors) The state of this actor can change (except its location) The states of actors in the actors list can change Some of the actors from the list can be removed New actors can be added in empty grid locations All other actors in the grid remain unchanged 1318 GridWorld’s Critters (cont’d) ArrayList getMoveLocations( ) The state of all actors in the grid remains unchanged Location selectMoveLocation (ArrayList moveLocs) The state of all actors in the grid remains unchanged void makeMove(Location loc) If loc is null, this critter is removed from the grid; otherwise this critter moves to loc This critter’s state can change A new actor can be added in this critter’s old location The state of all other actors in the grid remains unchanged 1319 Review: • When is an ArrayList more convenient than an array? • Explain the difference between the capacity and size in an ArrayList? • What method returns the number of elements currently stored in an ArrayList? • What method is used to insert an element into an ArrayList? 1320 Review (cont’d): • Can a double value be stored in an ArrayList? • What is autoboxing? • Can a “for each” loop be used with ArrayLists? • Can a class extend ArrayList? • Can an object change after it has been added to an ArrayList? 1321 Review (cont’d): • Name the three Critter’s methods that cannot change the state of any actor in the grid • Which Critter’s methods are allowed to add new actors to the grid? • Which Critter’s methods are allowed to change the location of this critter? 1322 ... “Critters” 13? ?2 java. util.ArrayList • Implements a list using an array ã Implements java. util.List interface ôinterfaceằ java. util.List java. util.ArrayList java. util.LinkedList 13? ?3 java. util.ArrayList... Starting with Java 5, ArrayList and other collection classes hold objects of a specified data type • The elements’ data type is shown in angle brackets and becomes part of the List and ArrayList... primitive data types • Keeps track of the list capacity (the length of the allocated array) and list size (the number of elements currently in the list) capacity size "Cat" "Hat" "Bat" 13? ?4 ArrayList