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CS 335 Lecture 04

Java Programming Arrays

Fall 2003

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Arrays in Java

Indexes start at 0

All data types (primitive and user-defined

objects) can be put into arrays Any expression which evaluates to an integer

can be used as the array subscript

Arrays can be re-allocated to change the

size; they are static once created

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Declaring Arrays

Arrays must be both declaredand allocated(using the new command):

int myarray[] = new int [100]

int myarray[] = new int [100]

int myarray[];myarray = new int[100];

int myarray[];myarray = new int[100];

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Example Declarations

char chararray[] = new char [255]byte bytearray[] = new byte [255]

MyObj ObjArray[];ObjArray = new MyObj[10]

byte [] array1, array2;array1 = new byte[100];

array2 = new byte[200];

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Allocating Arrays

Must allocate with the new command

null pointer exception!!

Fixed size after allocation The allocated size can be dynamically

determined with an expression:

char chararray[];...chararray = new char [size*6];

char chararray[];...

chararray = new char [size*6];

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Dynamic Arrays

Re-allocation possible:

char myarray[] = new char [255];char tmparray[] = new char [255];...

for (int i=0; i

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// Fig. 7.4: InitArray.java// initializing an array with a declarationimport javax.swing.*;

public class InitArray {public static void main( String args[] ) {

String output = "";// Initializer list specifies number of elements and// value for each element.int n[] = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 };

output += "Subscript\tValue\n";

for ( int i = 0; i < n.length; i++ )output += i + "\t" + n[ i ] + "\n";

JTextArea outputArea = new JTextArea( 11, 10 );

outputArea.setText( output );

JOptionPane.showMessageDialog( null, outputArea,"Initializing an Array with a Declaration",JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 ); }}

// Fig. 7.4: InitArray.java// initializing an array with a declarationimport javax.swing.*;

public class InitArray {public static void main( String args[] ) {

String output = "";// Initializer list specifies number of elements and

// value for each element.int n[] = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 };

output += "Subscript\tValue\n";

for ( int i = 0; i < n.length; i++ )output += i + "\t" + n[ i ] + "\n";

JTextArea outputArea = new JTextArea( 11, 10 );outputArea.setText( output );

JOptionPane.showMessageDialog( null, outputArea,"Initializing an Array with a Declaration",JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 ); }}

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// Fig. 7.5: InitArray.java// initialize array n to the even integers from 2 to 20import javax.swing.*;

public class InitArray {public static void main( String args[] ) {

final int ARRAY_SIZE = 10;int n[]; // reference to int arrayString output = "";

n = new int[ ARRAY_SIZE ]; // allocate array// Set the values in the arrayfor ( int i = 0; i < n.length; i++ )

n[ i ] = 2 + 2 * i;

output += "Subscript\tValue\n";

for ( int i = 0; i < n.length; i++ )output += i + "\t" + n[ i ] + "\n";

JTextArea outputArea = new JTextArea( 11, 10 );outputArea.setText( output );

JOptionPane.showMessageDialog( null, outputArea,"Initializing to Even Numbers from 2 to 20",

JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 ); }}

// Fig. 7.5: InitArray.java// initialize array n to the even integers from 2 to 20import javax.swing.*;

public class InitArray {public static void main( String args[] ) {

final int ARRAY_SIZE = 10;int n[]; // reference to int arrayString output = "";

n = new int[ ARRAY_SIZE ]; // allocate array// Set the values in the arrayfor ( int i = 0; i < n.length; i++ )

n[ i ] = 2 + 2 * i;

output += "Subscript\tValue\n";

for ( int i = 0; i < n.length; i++ )output += i + "\t" + n[ i ] + "\n";

JTextArea outputArea = new JTextArea( 11, 10 );outputArea.setText( output );

JOptionPane.showMessageDialog( null, outputArea,"Initializing to Even Numbers from 2 to 20",JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 ); }}

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// Fig. 7.8: Histogram.java// Histogram printing programimport javax.swing.*;

public class Histogram {public static void main( String args[] ){

int n[] = { 19, 3, 15, 7, 11, 9, 13, 5, 17, 1 };String output = "";

output += "Element\tValue\tHistogram";

for ( int i = 0; i < n.length; i++ ) {output += "\n" + i + "\t" + n[ i ] + "\t";

for ( int j = 1; j

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Arrays as Parameters

Java treats arrays as objects. When the

actual parameter is an array, it is passed to

the method by reference:int a[] = { 1, 2, 3, 4, 5};...

modifyArray ( a );...public void modifyArray (int b[]) {...}

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Arrays as Parameters

When elements in an array are primitive data

types, the elements can be passed by value:

int a[] = { 1, 2, 3, 4, 5};...

modifyElement ( a[4] );...

public void modifyElement (int elem) {

...}

int a[] = { 1, 2, 3, 4, 5};...

modifyElement ( a[4] );...

public void modifyElement (int elem) {

...}

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Example: Binary Search// Binary searchpublic int binarySearch( int array[], int key ){

int low = 0; // low subscriptint high = array.length - 1; // high subscriptint middle; // middle subscript

while ( low

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Multi-Dimensional Arrays

Java supports single arrays whose elements

are also single arrays, thus achieving the

effect of double-subscripted arrays:

int my2darray[][] = { {1, 2}, {3, 4}};

exprResult = my2darray[0][0]*my2darray[1][0];

int my2darray[][] = { {1, 2}, {3, 4}};

exprResult = my2darray[0][0]*my2darray[1][0];

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Example Game: Memory

Basic constructs: arrays, buttons, objects

Simple GUI http://www.lizardpoint.com/fun/java/conc/Memory.html

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