Arrays in Java

ARRAYS

-is a container object that holds a fixed number of values of a single type-Each item in an array is called an element-each element is accessed by its numerical index

Min Index: 0Max Index: Size-1

Creating an Array0 <data_type>[] <name_of_array> // declares an array of integers int[ ] anArray;

The square brackets -are special symbols indicating that this variable holds an array.

The size of the array is not part of its type (which is why the brackets are empty).

Example0 byte[] anArrayOfBytes; 0 short[] anArrayOfShorts; 0 long[] anArrayOfLongs; 0 float[] anArrayOfFloats; 0 double[] anArrayOfDoubles; 0 boolean[] anArrayOfBooleans; 0 char[] anArrayOfChars; 0 String[] anArrayOfStrings;

// this form is discouraged float anArrayOfFloats[ ];

Initializing an Array0 One way to create an array is with the new operator.

int[] anArray= new int[5];

0 0 0 0 0anArray

0 1 2 3 4

Variable anArray may not have been initialized

If missing

Accessing an Array

12 11 2 3 25anArray

0 1 2 3 4

anArray[0] anArray[2] anArray[4] anArray[1] anArray[3]

int[] anArray= new int[5];anArray[0]=12;anArray[1]=11;anArray[2]=2;anArray[3]=3;anArray[4]=25;

Each array element is accessed by its numerical index:

Shortcut Syntax

0 To create and initialize an array

int[] anArray= {12,11,2,3,25}

Here the length of the array is determined by the number of values provided between { and }.

Summary

0 Each element in an array is just a variable0 Anything you would put in a variable of that type can

be assigned to an array element of that type. 0 An array starts at 0.0 An array ends at size -1. (accessing indexes other than the declared index will result to “ArrayIndexOutOfBounds” error during runtime)

Example: Reference Types and Arrayspublic class Dog{ private String name; public void setName(String newName){ name=newName; } public String getName(){ return name; } public void bark(){ System.out.println(name+ " says Ruff! "); } }

public class B { public static void main(String [] args){ Dog dog1 = new Dog(); // make a Dog object and access it dog1.setName("Bart"); dog1.bark(); Dog[] myDogs = new Dog[3]; //now make a Dog array myDogs[0]=new Dog(); myDogs[1]=new Dog(); myDogs[2]=dog1; myDogs[0].setName("Fred"); //accessing the dogs using the array myDogs[1].setName("Marge"); //what is myDogs[2] name? System.out.println("last dog's name is: "+myDogs[2].getName()); int x=0; //call all your dogs and make them bark while(x<3){ //since there are 3 dogs myDogs[x].bark(); x=x+1; } }}

Output

Simple Tracing Sample int x = 0; int[] hq = new int[5]; while ( x < 3 ){ hq[x] = x; x = x + 1; } hq[3] = hq[1]; hq[4] =hq[1] ; hq[3] = 0; hq [4]= hq [0] ; hq[0] = hq[3]; hq[3] =hq[2]; hq[2] = hq [0] ; x=0; while ( x < 5 ){ System.out.print(hq[x]+" "); x = x + 1; }

Seatwork : Be the compiler.Correct any error, and predict the output

1. ) class Hobbit{ String name; public static void main(String[] a){

Hobbit[] h = new Hobbit[3];int z=0;while(z<=4){

h[z]=new Hobbit(); h[z].name="bilbo"; if(z==1){

h[z].name="frodo";} if(z==2){ h[z].name="sam";}System.out.println(h[z].name + "

is a "); z = z + 1;

} } }

2.) public class Books{ String title; String author; }

public class BooksTestDrive{ public static void main(String[] a){ Books[] myBooks = new Books[3]; myBooks[0]= new Books(); myBooks[1]= new Books(); myBooks[2]= new Books(); int x = 0; myBooks[0].title="The Java Cookbook"; myBooks[1].title="Programming Java"; myBooks[2].title="Java Language"; myBooks[0].author="Maria"; myBooks[1].author="Pedro"; myBooks[2].author="Juan"; while(x<3){ System.out.print(myBooks[x].title); System.out.print(" by “+" "); System.out.println(myBooks[x].author); x=x+1; } }}

2D and multidimensional arrays0 You can also declare an array of arrays (also known as a

multidimensional array) by using two or more sets of square brackets, such as int[][] nums=new int[4][5];

0 Each element, therefore, must be accessed by a corresponding number of index values.

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

0 1 2 3 4

0

1

2

3

Example

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

String[][] names = { {"Mr. ", "Mrs. ", "Ms. "}, {"Smith", "Jones"}

}; System.out.println(names[0][0] + names[1][0]); System.out.println(names[0][1] + names[1][1]); System.out.println(names[0][1] + names[2][0]); } }

Copying arrays

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

char[] copyFrom = { 'd', 'e', 'c', 'a', 'f ', 'f ', 'e', 'i', 'n', 'a', 't', 'e', 'd' }; char[] copyTo = new char[7]; System.arraycopy(copyFrom, 2, copyTo, 0, 7); for(int x=0;x<7;x++){ System.out.print(x+” “);System.out.println(new String(copyTo)); }

} }

public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)

Another Example: Copying Arrays

int [] myArray = { 1,2,3,4,5,6 };int [] hold = { 10,9,8,7,6,5,4,3,2,1}//copy all of the myArray array to the hold array,//starting with the 0th index

System.arraycopy(myArray, 0, hold, 0, myArray.length);

Note: the System.arraycopy method copies references, not objects, when dealing with arrays of objects. The objects themselves do not change.

CodeMagnets:

Seatwork : ½ lengthwise paperA working java program is all scrambled up.Reconstruct the code snippets to make a working java program that produces the output below. Some of the curly braces fell on the floor and they are small to Pick up, so feel free to add as many of those as you need.

Answerclass SeatworkArray{ public static void main(String[] args) { int [] index = new int[4]; index[0] =1; index[1]= 3; index[2]= 0; index[3]=2; String [] islands = new String[4]; islands[0] ="Bermuda"; islands[1] = "Fiji"; islands[2]= "Azores"; islands[3] ="Cozumel"; int y = 0; int ref; while (y < 4) { ref = index[y]; System.out.print("island ="); System.out.println(islands[ref]); y = y + 1; } }}

Array example: Initializing the elements of an array to default values of zero.

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

int[] array; // declare array named array array = new int[ 10 ]; // create the array object System.out.println( "Index" + " " + "Value" ); // column headings for ( int counter = 0; counter < array.length; counter++ ) System.out.println( " "+ counter + " " + array[counter] ); } // end main } // end class InitArray

Array example: Initializing the elements of an array with an array initializer

public class InitArray2 { public static void main( String[] args ) { // initializer list specifies the value for each element int[] array = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 }; System.out.println( "Index"+" " + "Value" ); // column headings // output each array element's value for ( int counter = 0; counter < array.length; counter++ ) System.out.println( " " + counter + " " + array[ counter ] ); } // end main } // end class

Array example: Calculating the values to be placed into the elements of an array

public class InitArray3 { public static void main( String[] args ) { final int ARRAY_LENGTH = 10; // declare constant int[] array = new int[ ARRAY_LENGTH ]; // create array

// calculate value for each array element for ( int counter = 0; counter < array.length; counter++ ) array[ counter ] = 2 + 2 * counter; System.out.println( "Index"+ " " + "Value" ); // column headings // output each array element's value for ( int counter = 0; counter < array.length; counter++ ) System.out.println( " " + counter+ " " + array[ counter ] ); } // end main } // end class

Seatwork: 1 whole yellow paper. (by pair)

1. Create a program that ask to enter 10 integers as elements of an array and display in column 1 all even integers and in column 2 all odd integers with column headings even and odd.

2. Create a program that accept a five characters as elements of an array and display the reverse.

Example: characters: hallo display: ollah

Answer no.1 & 2import java.util.Scanner;

class OddEven {

public static void main(String[] args) { Scanner input = new Scanner(System.in); final int a =10; int[] num = new int[a];

for (int x=0; x<num.length; x++){ System.out.println("enter an integer: "); num[x]=input.nextInt();}

System.out.println("ODD"+" "+"EVEN"); for (int y=0; y<num.length; y++){ if((num[y]%2) == 0){ System.out.println(" "+num[y]);} else {System.out.println(num[y]);}}

} }

import java.util.Scanner;

public class RevChar {

public static void main(String[] args) { String var; char[] a = new char[5]; Scanner x= new Scanner(System.in); System.out.println("Enter a string"); var=x.next(); a[0]=var.charAt(0); a[1]=var.charAt(1); a[2]=var.charAt(2); a[3]=var.charAt(3); a[4]=var.charAt(4);

for (int y=a.length-1;y>=0; y--) System.out.print(a[y]); } }

Array example: Computing the sum of the elements of an array

public class SumArray{ public static void main( String[] args ){ int[] array = { 87, 68, 94, 100, 83, 78, 85, 91, 76, 87 }; int total = 0; // add each element's value to total for ( int counter = 0; counter < array.length; counter++ ) total += array[ counter ]; System.out.println( "Total of array elements: “ + total ); } // end main} // end class SumArray

2D Array example:Initializing two-dimensional arrayspublic class TwoDArray { // create and output two-dimensional arrays public static void main( String[] args ) { int[][] array1 = { { 1, 2, 3 }, { 4, 5, 6 } }; int[][] array2 = { { 1, 2 }, { 3 }, { 4, 5, 6 } }; System.out.println( "Values in array1 by row are" ); outputArray( array1 ); // displays array1 by row System.out.println( "\nValues in array2 by row are" ); outputArray( array2 ); // displays array2 by row } // end main // output rows and columns of a two-dimensional array

public static void outputArray(int[][] array ) { // loop through array's rows for ( int row = 0; row < array.length; row++ ){ // loop through columns of current row for ( int column = 0; column < array[ row ].length; column++ ) System.out.print( array[ row ][ column ] ); System.out.println(); // start new line of output } // end outer for } // end method outputArray} // end class