Chapter 8: Collections: Arrays

2 Objectives One-Dimensional Arrays Array Initialization The Arrays Class: Searching and Sorting Arrays as Arguments The Collections Framework: ArrayLists Two-Dimensional Arrays Common Programming Errors

3 One-Dimensional Arrays Lists of related values with the same data type –Stored using a single group name Array declaration example: double prices[]; prices = new double[6];

4 One-Dimensional Arrays (continued) Using the new operator: –Array elements are automatically initialized to: Zero for numerical built-in types False for Boolean built-in types Null for reference types

5

6 One-Dimensional Arrays (continued) Common programming practice: –Define the number of array items as a symbolic constant Element –Item in an array Index –Position of an item in an array –Also called a subscript

7 Accessing Elements in One- Dimensional Arrays grade[0]: –Refers to the first value stored in grade array –Read as “grade sub zero” –Can be used anywhere that scalar variables are valid The subscript contained within brackets need not be an integer constant –Any expression that evaluates to an integer may be used

8 Accessing Elements in One- Dimensional Arrays (continued) An important advantage of using integer expressions as subscripts: –Allows sequencing through an array using a loop Example of looping through an array: sum = 0; // initialize the sum to zero for (i = 0; i < NUMELS; i++) sum = sum + grade[i]; // add in a grade –i is used both as a counter in the for loop and as a subscript

9 Accessing Elements in One- Dimensional Arrays (continued) When accessing an array element: –Java checks the value of the index being used at run time –If the index exceeds the length of the array, Java will notify you of an ArrayIndexOutOfBounds exception

10

11 One-Dimensional Array Length The size of an array is automatically stored in a variable named length Looping using for loop and array length: sum = 0; // initialize the sum to zero for (i = 0; i < grade.length; i++) sum = sum + grade[i]; // add in a grade

12 Input and Output of Array Values Input: –Individual array elements can be assigned values interactively using: readLine() showInputDialog() Output: –Array elements can be printed using: print() println()

13 Aggregate Data Types Any type whose: –Individual elements are other data types –Elements are related by some defined structure Also called: –Structured type –Data structure Arrays are aggregate data types

14 String Arrays Arrays of reference data types may also be constructed Declaring String array: –String names[] = new String[4]; Arrays of reference types are stored differently from arrays of built-in data types

15

16

17

18 Run-Time Dimensioning The size of an array can also be entered interactively at run time An entered value can be used to allocate space for an array using the new operator

19 Array Initialization Arrays can be initialized within declaration statements: –May continue across multiple lines –No method of indicating repetition of initialization value –No way to initialize later array elements without first specifying values for earlier elements Example: –int grade[] = {98, 87, 92, 79, 85};

20 import javax.swing.*; public class ElementInputAndDisplay { public static void main(String[] args) { final int NUMELS= 5; String s1; int i; int grade[]; // declare the array grade = new int[NUMELS]; // allocate the array for (i = 0; i < NUMELS; i++) // enter the grades { s1 = JOptionPane.showInputDialog("Enter a grade: "); grade[i] = Integer.parseInt(s1); } for (i = 0; i < NUMELS; i++) // print the grades System.out.println("grade[" +i +"] is " + grade[i]); System.exit(0); } }

21 import javax.swing.*; public class AccumulateElements { public static void main(String[] args) { final int NUMELS= 5; String s1; int i; int total = 0; // declare and allocate the array int grade[] = new int[NUMELS]; // allocate the array for (i = 0; i < NUMELS; i++) // enter the grades { s1 = JOptionPane.showInputDialog("Enter a grade: "); grade[i] = Integer.parseInt(s1); } System.out.print("The total of the grades"); for (i = 0; i < NUMELS; i++) // display and total the grades { System.out.print(" " + grade[i]); total = total + grade[i]; } System.out.print(" is " + total); System.exit(0); } }

22 public class StringArray { public static void main(String[] args) { int i; String names[]; // declare the array names = new String[4]; // allocate the array // assign values to each array element names[0] = "Joe"; names[1] = "Harriet"; names[2] = "Allyn"; names[3] = "Roberta"; // display the names for (i = 0; i < names.length; i++) System.out.println("names[" + i + "] is " + names[i]); } }