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©Rick MercerChapter 7The Java Array Object

wSome variables store precisely one value:•a double stores one floating-point number•an intstores one integer •a reference variable stores a "reference" to an object that may store many other variableswJava Arrays•store a collection of "elements"•May be references to objects or may be primitive values•programmers access individual objects with subscript notation [ ] (square brackets)The Java Array Object

Array Constructiontype []array-reference= new type[capacity]; •type specifies the type of element stored in the array •array-nameis any valid Java identifier that refers to all elements in the array•capacityis an integer expression representing the maximum number of elements that can be stored into the array

Example Array DeclarationswAn array named x that stores up to 8 numbersdouble[] x =newdouble[8]; wThis array stores references to 500 strings String[] name = newString[500];wAn array named test to store 2,000 integersintcapacity = 1000;int[] test = newint[2*capacity];wAn array named customer to stores references to up to 100 unique BankAccountobjects BankAccount[] customer = newBankAccount[100];

Accessing Individual Array ElementswIndividual array elements are referenced through subscripts of this form:array-name[int-expression]•int-expressionis an integer that should be in the range of 0..capacity-1wExamples:x[0] // Pronouncedx sub 0name[5] // Pronounced name sub 5test[99] // Pronounced test sub 99customer[12] // Pronounced customer sub 12

Assignment to individual array elements// All intelements are // set to 0 initiallyint[] x = newint[5];x[0] = 1;x[1] = 2x[2] = 5;x[3] = x[0] + x[2];x[4] = x[3] -1;for(intj = 0; j < x.length; j++) { // What is the Output?System.out.println(x[j]);}

Out of Range Subscripts wSubscripts can be "out of range"String[] name = new String[1000];name[-1] = "Subscript too low";name[ 0] = "This should be the first name";name[999] = "This is the last good subscript";name[1000] = "Subscript too high";wTwo of the above references will cause ArrayIndexOutOfBoundsexceptions and "ungracefully" terminate the program with:java.lang.ArrayIndexOutOfBoundsexception: -1

Array initializer (a shortcut) and the length variablewArrays can be initialized with an array initializerint[] arrayOfInts= { 55, 33, 77, 22, -99 };assertEquals(55, arrayOfInts[0]);assertEquals(33, arrayOfInts[1]);assertEquals(77, arrayOfInts[2]);assertEquals(22, arrayOfInts[3]);assertEquals(-99, arrayOfInts[4]);// arrayReference.lengthreturns capacityassertEquals(5, arrayOfInts.length);

Passing Array ArgumentswImagine a method that returns true if the first and last values are equaldouble[] a1 = { 5.0, 4.0, 3.0, 2.0 };assertFalse(endsSame(a1));double[] a2 = { 5.0, 4.0, 3.0, 2.0, -1.0, 5.0 };assertTrue(endsSame(a2));wThe method heading has an array parameterpublic booleanendsSame(double[] x) {}Let's complete this method in a JUnit test

Process all elements with a for loopwImagine a method that returns the average of the array elementsdouble[] x1 = { 5.0, 4.0, 3.0, 2.0 };assertEquals(3.5, average(x1), 0.001);double[] x2 = { 5.0, 4.0, 3.0, 2.0, -1.0, 5.0 };assertEquals(3.0, average(x2), 0.001);wThis method will use an array parameterpublic double average(double[] x) {}Let's complete this method in a JUnit test

AnswerspublicbooleanendsSame(double[] x) {returnx[0] == x[x.length-1];}publicdoubleaverage(double[] x) {intn = x.length;doubletotal = 0;// get the sum of all elementsfor(inti = 0; i < n; i++) {total+= x[i];}// Computeandreturn the averagereturntotal/ n;}

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