Array, Arraylist, and Vector

Array, ArrayList, and Vector 1 Agenda • Review – – – – Abstract classes and abstract methods Overriding and overloading Static method sand static variables Final variables, final methods and final classes • Array • ArrayList • Vector 2 Array • A program language construct used to group g data and organized • In Java, array indexes always begin at zero • In Java, an array is an object that must be instantiated 3 1 Array (cont’d) • Two ways to declare an array reference in Java – int[] grades; – int grades[]; • Arrays must be instantiated – int[] grades = new int[10]; – int grades[] = new int[10]; 4 Array (cont’d) • The length of the array is the number of elements it can hold, thus the maximum index of an array is length – 1 • int[] number = new int[10]; • Off-by-one error 5 Initializer Lists • Instantiate an array and provide the initial values for the elements of the arrayy • The size of the array is determined by the number of items in the initializer list • int[] numbers = {1, 2, 3, 4, 5} 2 Command-Line Arguments • The formal parameter to the main method of a Java application is always an array of String objects • The String[] parameter, which we call args, represents command-line arguments that are provided when the interpreter is invoked 7 Command-Line Arguments(cont’d) • Any extra information on the command line when the interpreter is invoked is stored in the args array for use by the program • This technique is another way to provide input to a program 8 Command-Line Arguments(cont’d) public class NameTag{ public static void main(String[] args){ System.out.println(); y p (); System.out.println("\t" + args[0]); System.out.println("My name is " + args[1]); } } 9 3 Two-Dimensional Arrays • Use two indexes to refer to a value in a twodimensional array, one specifying the row, and another the column • int[][] table = new int[5][10]; 10 Exercise 1 • Find and correct the error of the following g program segment: final int ARRAY_SIZE = 5; ARRAY_SIZE = 10; 11 Exercise 2 • Find and correct the error of the following program segment segment: int b[] = new int[10]; for (int i = 0; I <= b.length; i++) b[i] = 1; 12 4 Exercise 3 • Find and correct the error of the following g program segment: int a[][] = {{1, 2}, {3, 4}}; a[1,1] = 5; 13 The ArrayList Class • An ArrayList object is similar to an array, but it dynamically changes size as needed, and element can be inserted and removed • The ArrayList class is part of the java.util pacakge 14 The ArrayList Class (cont’d) • The ArrayList class is part of the Collection API • Java Co C llecti tion API – Set, List, Map, SortedSet, SortedMap, HashSet, TreeSet, ArrayList, LinkedList, Vector, Collections, Arrays, AbstractCollection 15 5 Some Methods of ArrayList ArrayList () Constructor: creates an initially empty list boolean add (Object obj) Inserts the specified object to the end of this list void add (int index, Object obj) p object j into this list at the specified p index inserts the specified void clear () removes all elements from this list Object remove (int index) removes the element at the specified index in this list and returns it. Object get (int index) returns the object at the specified index in this list without removing it Int indexOf (Object obj) returns the index of the first occurrence of the specified object int size() returns the number of elements in this list boolean isEmpty() returns true if this list contains no elements 16 import java.util.ArrayList; public class Presidents{ public static void main (String[] args){ ArrayList presidents = new ArrayList(); presidents.add("Jimmy Carter"); presidents.add("Ronald Reagan"); presidents.add("G.H. Bush"); presidents.add("Bill Clinton"); presidents.add("George Bush"); System.out.println(presidents); int location = presidents.indexOf("Ronald Reagan"); System.out.println(presidents.get(location)); presidents.remove(location); System.out.println(presidents); System.out.println("At index 1: " + presidents.get(1)); presidents.add(3, "Ronald Regan"); System.out.println(presidents); System.out.println("Size of the presidents: " + presidents.size()); } 17 } The Vector Class • Vector is similar to ArrayList • Th The Vector V t class l iis partt off th the java.util j til pacakge • The Vector class is part of the Collection API 18 6 Some Methods of Vector ArrayList () Constructor: creates an initially empty list boolean add (Object obj) Inserts the specified object to the end of this list void add (int index, Object obj) p object j into this list at the specified p index inserts the specified void clear () removes all elements from this list Object remove (int index) removes the element at the specified index in this list and returns it. Object get (int index) returns the object at the specified index in this list without removing it Int indexOf (Object obj) returns the index of the first occurrence of the specified object int size() returns the number of elements in this list boolean isEmpty() returns true if this list contains no elements 19 import java.util.Vector; public class PresidentsVector{ public static void main (String[] args){ Vector presidents = new Vector(); presidents.add("Jimmy Carter"); presidents.add("Ronald Reagan"); presidents.add("G.H. Bush"); presidents.add("Bill Clinton"); presidents.add("George Bush"); System.out.println(presidents); int location = presidents.indexOf("Ronald Reagan"); System.out.println(presidents.get(location)); presidents.remove(location); System.out.println(presidents); System.out.println("At index 1: " + presidents.get(1)); presidents.add(3, "Ronald Regan"); System.out.println(presidents); System.out.println("Size of the presidents: " + presidents.size()); } 20 } ArrayList vs. Vector • ArrayLists y behave like unsynchronized y Vectors and therefore execute faster than Vectors, because ArrayLists do not have the overhead thread synchronization. 21 7