Arrays (Java Tutorial)

8/20/2019 Arrays (Java Tutorial)

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Java ArrayJava Array


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Agenda

● What is an array

● Declaration of an array

● Instantiation of an array

● Accessing array element

● Array length

● Multi-dimensional array


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What is an Array?What is an Array?


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Introduction to Arrays

● Suppose we have here three variables of type int withdifferent identifiers for each variable.

int number1;

int number2;int number3;

number1 = 1;

number2 = 2;number3 = 3;

As you can see, it seems lie a tedious tas in order to !ustinitiali"e and use the variables especially if they are used forthe same purpose.


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Introduction to Arrays

● In #ava and other programming languages, there is onecapability wherein we can use one variable to store a list ofdata and manipulate them more efficiently. $his type ofvariable is called an array.

● An array stores multiple data items of the same data type, ina contiguous bloc of memory, divided into a number ofslots.


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Declaration of Declaration of an Arrayan Array


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Declaring Arrays

● $o declare an array, write the data type, followed by a set ofs%uare bracets&', followed by the identifier name.

●

(or e)ample,int []ages;or

int ages[];


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Instantiation ofInstantiation ofan Arrayan Array


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Array Instantiation

● After declaring, we must create the array and specify itslength with a constructor statement.

● Definitions*

– Instantiation

● In #ava, this means creation

– +onstructor

●

In order to instantiate an ob!ect, we need to use a constructor for this. Aconstructor is a method that is called to create a certain ob!ect.

● We will cover more about instantiating ob!ects and constructors later.


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Array Instantiation

● $o instantiate or create an array, write the new eyword,followed by the s%uare bracets containing the number ofelements you want the array to have.

● (or e)ample,//declaration

int ages[];

//instantiate objectages = new int[100];

or, can also be written as,//declare and instantiate object

int ages[] = new int[100];


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Array Instantiation


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Array Instantiation

● ou can also instantiate an array by directly initiali"ing it withdata.

● (or e)ample,int arr[] = {1, 2, 3, 4, 5};

$his statement declares and instantiates an array of integerswith five elements initiali"ed to the values /, 0, 1, 2, and 3.


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Sample Program1 //creates an array of boolean variables with identifier2 //results. This array contains 4 elements that are3 //initialized to values {true, false, true, false}45 boolean results[] = { true, false, true, false }; 67 //creates an array of 4 double variables initialized

8 //to the values {100, 90, 80, 75};910 double []grades = {100, 90, 80, 75};1112 //creates an array of Strings with identifier days and13 //initialized. This array contains 7 elements14

15 String days[] = { “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”,“Sun”};


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Accessing ArrayAccessing ArrayElement Element


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Accessing an Array Element

● $o access an array element, or a part of the array, you usea number called an inde) or a subscript.

● inde) number or subscript

– assigned to each member of the array, to allow the program toaccess an individual member of the array.

– begins with "ero and progress se%uentially by whole numbers to theend of the array.

–

45$6* 6lements inside your array are from 7 to si"e5fArray-/.


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● (or e)ample, given the array we declared a while ago, wehave

//assigns 10 to the first element in the arrayages[0] = 10;

//prints the last element in the arraySystem.out.print(ages[99]);


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● 45$6*

– once an array is declared and constructed, the stored value of eachmember of the array will be initiali"ed to "ero for number data.

– for reference data types such as Strings, they are 45$ initiali"ed to

blans or an empty string 89. $herefore, you must populate the Stringarrays e)plicitly.


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● $he following is a sample code on how to print all theelements in the array. $his uses a for loop, so our code isshorter.

1 public class ArraySample{

2 public static void main( String[] args ){3 int[] ages = new int[100];

4 for( int i=0; i


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Coding Guidelines

1. It is usually better to initialize or instantiate thearray right away after you declare it. For example,the declaration,

int []arr = new int[100];

is preferred over,

int []arr;

arr = new int[100];


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Coding Guidelines

2. The elements of an nelement array have indexesfrom ! to n1. "ote that there is no array elementarr#n$% This will result in an arrayindexoutofbounds exception.

&. 'emember( )ou cannot resize an array.


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Array LengthArray Length


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Array Length

● In order to get the number of elements in an array, you canuse the length field of an array.

● $he length field of an array returns the si"e of the array. Itcan be used by writing,

arrayName.length


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Array Length1 public class ArraySample {2 public static void main( String[] args ){3 int[] ages = new int[100];45 for( int i=0; i


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Coding Guidelines

/. When creating for loops to process the elements of anarray, use the array ob!ect:s length field in the conditionstatement of the for loop. $his will allow the loop to ad!ustautomatically for different-si"ed arrays.

0. Declare the si"es of arrays in a #ava program using namedconstants to mae them easy to change. (or e)ample,

final int ARRAY_SIZE = 1000; //declare a constant

. . .int[] ages = new int[ARRAY_SIZE];


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Multi-DimensionalMulti-DimensionalArrayArray


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Multidimensional Arrays

● Multidimensional arrays are implemented as arrays ofarrays.

● Multidimensional arrays are declared by appending the

appropriate number of bracet pairs after the array name.


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● (or e)ample,

// integer array 512 x 128 elements

int[][] twoD = new int[512][128];

// character array 8 x 16 x 24char[][][] threeD = new char[8][16][24];

// String array 4 rows x 2 columnsString[][] dogs = {{ "terry", "brown" },

{ "Kristin", "white" },{ "toby", "gray"},{ "fido", "black"}};


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● $o access an element in a multidimensional array is !ust thesame as accessing the elements in a one dimensional array.

● (or e)ample, to access the first element in the first row of

the array dogs, we write,

System.out.print( dogs[0][0] );

$his will print the String ;terry; on the screen.


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Summary

● Arrays

– Definition

– Declaration

–

Instantiation and constructors brief overview < to be discussedmore later

– Accessing an element

– $he length field

– Multidimensional Arrays

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Arrays (Java Tutorial)