javaarray

Agenda
● What is an array
● Declaration of an array
● Instantiation of an array
● Accessing array element
● Array length
● Multi-dimensional array

2

What is an Array?

3

Introduction to Arrays
● Suppose we have here three variables of type int with
different identifiers for each variable.
int number1;
int number2;
int number3;

number1 = 1;
number2 = 2;
number3 = 3;

As you can see, it seems like a tedious task in order to just
initialize and use the variables especially if they are used for
the same purpose.

4

Introduction to Arrays
● In Java and other programming languages, there is one
capability wherein we can use one variable to store a list of
data and manipulate them more efficiently. This type of
variable is called an array.

● An array stores multiple data items of the same data type, in
a contiguous block of memory, divided into a number of
slots.

5

Declaration of
an Array
6

Declaring Arrays
● To declare an array, write the data type, followed by a set of
square brackets[], followed by the identifier name.

● For example,
int []ages;
or
int ages[];

7

Instantiation of
an Array
8

Array Instantiation
● After declaring, we must create the array and specify its
length with a constructor statement.

● Definitions:
– Instantiation
● In Java, this means creation

– Constructor
● In order to instantiate an object, we need to use a constructor for this. A
constructor is a method that is called to create a certain object.
● We will cover more about instantiating objects and constructors later.

9

Array Instantiation
● To instantiate (or create) an array, write the new keyword,
followed by the square brackets containing the number of
elements you want the array to have.
● For example,
//declaration
int ages[];

//instantiate object
ages = new int[100];

or, can also be written as,
//declare and instantiate object
int ages[] = new int[100];

10

Array Instantiation

11

Array Instantiation
● You can also instantiate an array by directly initializing it with
data.

● For example,
int arr[] = {1, 2, 3, 4, 5};

This statement declares and instantiates an array of integers
with five elements (initialized to the values 1, 2, 3, 4, and 5).

12

Sample Program
1 //creates an array of boolean variables with identifier
2 //results. This array contains 4 elements that are
3 //initialized to values {true, false, true, false}
4
5 boolean results[] = { true, false, true, false };
6
7 //creates an array of 4 double variables initialized
8 //to the values {100, 90, 80, 75};
9
10 double []grades = {100, 90, 80, 75};
11
12 //creates an array of Strings with identifier days and
13 //initialized. This array contains 7 elements
14
15 String days[] = { “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”,
“Sun”};

13

Accessing Array
Element
14

Accessing an Array Element
● To access an array element, or a part of the array, you use
a number called an index or a subscript.

● index number or subscript
– assigned to each member of the array, to allow the program to
access an individual member of the array.
– begins with zero and progress sequentially by whole numbers to the
end of the array.
– NOTE: Elements inside your array are from 0 to (sizeOfArray-1).

15

● For example, given the array we declared a while ago, we
have
//assigns 10 to the first element in the array
ages[0] = 10;

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

16

● NOTE:
– once an array is declared and constructed, the stored value of each
member of the array will be initialized to zero for number data.
– for reference data types such as Strings, they are NOT initialized to
blanks or an empty string “”. Therefore, you must populate the String
arrays explicitly.

17

● The following is a sample code on how to print all the
elements in the array. This uses a for loop, so our code is
shorter.
1 public class ArraySample{
2 public static void main( String[] args ){
3 int[] ages = new int[100];
4 for( int i=0; i<100; i++ ){
5 System.out.print( ages[i] );
6 }
7 }
8 }

18

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

int []arr = new int[100];

is preferred over,

int []arr;
arr = new int[100];

19

Coding Guidelines
2. The elements of an n-element array have indexes
from 0 to n-1. Note that there is no array element
arr[n]! This will result in an array-index-out-of-
bounds exception.

3. Remember: You cannot resize an array.

20

Array Length
● In order to get the number of elements in an array, you can
use the length field of an array.

● The length field of an array returns the size of the array. It
can be used by writing,
arrayName.length

22

Array Length
1 public class ArraySample {
2 public static void main( String[] args ){
3 int[] ages = new int[100];
4
5 for( int i=0; i<ages.length; i++ ){
6 System.out.print( ages[i] );
7 }
8 }
9 }

23

Coding Guidelines
1. When creating for loops to process the elements of an
array, use the array object's length field in the condition
statement of the for loop. This will allow the loop to adjust
automatically for different-sized arrays.

2. Declare the sizes of arrays in a Java program using named
constants to make them easy to change. For example,

final int ARRAY_SIZE = 1000; //declare a constant
. . .
int[] ages = new int[ARRAY_SIZE];

24

Multi-Dimensional
Array
25

Multidimensional Arrays
● Multidimensional arrays are implemented as arrays of
arrays.

● Multidimensional arrays are declared by appending the
appropriate number of bracket pairs after the array name.

26

● For example,
// integer array 512 x 128 elements
int[][] twoD = new int[512][128];

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

// String array 4 rows x 2 columns
String[][] dogs = {{ "terry", "brown" },
{ "Kristin", "white" },
{ "toby", "gray"},
{ "fido", "black"}
};

27

● To access an element in a multidimensional array is just the
same as accessing the elements in a one dimensional array.

● For example, to access the first element in the first row of
the array dogs, we write,

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

This will print the String "terry" on the screen.

28

Summary
● Arrays
– Definition
– Declaration
– Instantiation and constructors (brief overview – to be discussed
more later)
– Accessing an element
– The length field
– Multidimensional Arrays

29

javaarray

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