TCSS 143, Autumn 2004 Lecture Notes Introduction to Collections: Array List

Collections collection: an object that stores a group of objects inside it; a.k.a. a "data structure" the objects stored are called elements some collections allow duplicates, some don't some maintain an ordering, some don't an array is like a very crude "collection" typical operations: add element, remove element, clear all elements, contains or find element, get size examples: java.util.ArrayList, java.util.HashMap, java.util.TreeSet

Collection interface Java provides an interface java.util.Collection with the following methods: public boolean add(Object o) Appends the specified element to this collection. public void clear() Removes all of the elements of this collection. public boolean contains(Object o) Returns true if this collection contains the specified element. public boolean containsAll(Collection coll) Returns true if this collection contains all of the elements in the specified collection.

Collection interface, cont'd. public boolean isEmpty() Returns true if this list contains no elements. public Iterator iterator() Returns a special object for examining the elements of the list in order (seen later). public boolean remove(Object o) Removes the first occurrence in this list of the specified element. public int size() Returns the number of elements in this list. public Object[] toArray() Returns an array containing all of the elements from this list.

Lists list: an ordered sequence of elements, each accessible by a 0-based index one of the most basic collections

List features maintains ordering in order elements were added (new elements are added to the end by default) duplicates and null elements allowed list manages its own size; user of the list does not need to worry about overfilling it operations: add element to end of list, insert element at given index, clear all elements, search for element, get element at given index, remove element at given index, get size

List interface Java interface java.util.List adds the following methods to those in Collection: public void add(int index, Object o) Inserts the specified element at the specified position in this list. public Object get(int index) Returns the element at the specified position in this list. public int indexOf(Object o) Returns the index in this list of the first occurrence of the specified element, or -1 if the list does not contain it.

List interface, cont'd. More java.util.List methods: public int lastIndexOf(Object o) Returns the index in this list of the last occurrence of the specified element, or -1 if the list does not contain it. public Object remove(int index) Removes the object at the specified position in this list. public Object set(int index, Object o) Replaces the element at the specified position in this list with the specified element.

Some list questions all of the list operations on the previous slide could be performed using an array instead! open question: What are some reasons why we might want to use a special List class, rather than an array, to store our data? thought question: How might a List be implemented, under the hood? why do all the List methods use type Object?

ArrayList Class java.util.ArrayList implements List using an array as the internal implementation

ArrayList, cont'd. ArrayList features think of it as an auto-resizing array, that can hold any type of object, with many convenient methods maintains most of the benefits of arrays, such as fast random access can call toString on an ArrayList to print it remember to import java.util.*;

ArrayList vs. array construction storage retrieval String[] names = new String[5]; ArrayList namesList = new ArrayList(); storage names[0] = "Jennifer"; namesList.add("Jennifer"); retrieval String name = names[0]; String name = (String)namesList.get(0); Why the type-cast? What happens without it?

ArrayList vs. array, cont'd. removal (of element #2) for (int i = 2; i < names.length - 1; i++) names[i] = names[i+1]; namesList.remove(2); search to see if "Marty" is there for (int i = 0; i < names.length; i++) if (names[i].equals("Marty")) { ... } if (namesList.contains("Marty")) { ... } erase all names from the list for (int i = 0; i < names.length; i++) names[i] = null; namesList.clear();

ArrayList and references a collection such as ArrayList stores references to its elements if two collections refer to the same object, side effects from one can be seen in the other BankAccount acct = new BankAccount("Ed", 0); ArrayList list1 = new ArrayList(); list1.add(acct); ArrayList list2 = new ArrayList(); list2.add(acct); acct.deposit(100.00); System.out.println(list1); System.out.println(list2);

Storing primitives in collection collections like ArrayList store Objects, not primitive values like int, double, char, boolean to store these primitives, we need special object wrappers Integer / public int intValue Double / public double doubleValue Character / public char charValue Boolean / public boolean booleanValue ArrayList list = new ArrayList(); list.add(new Integer(42)); int value = ((Integer)list.get(0)).intValue();

Collections class Class java.util.Collections has many utility methods that do useful things to collections public static void copy(List dest, List src) public static void fill(List list, Object value) public static Object max(Collection coll) public static Object min(Collection coll) public static void reverse(List list) public static void shuffle(List list) public static void sort(List list) public static void swap(List list, int i, int j)

Abstract data types (ADTs) abstract data type: a specification for a collection of data and the operations that can be performed on it does not specify how the collection must be implemented internally does specify what ways the data can be viewed and manipulated (what methods the ADT has) many collections may implement the same ADT (example: linked list and array list implement List) in Java, interfaces specify ADTs, and classes implement the specific collections

Practice problems Using Scanner, read names from the keyboard until an empty line is entered. Store the names in an ArrayList, and print the names separated by commas after all have been entered. Use an ArrayList to store and print the numbers 1-50 in random order, one per line.

References Koffman, Chapter 4, pp. 193-199