The document discusses arrays in Java. It defines an array as an ordered collection of similar items that have a single name and are accessed by their index position. Arrays can be one-dimensional or multi-dimensional. The key points covered include declaring and initializing array variables, accessing and modifying array elements, passing arrays to methods, and returning arrays from methods. Examples are provided for common array operations like summing elements, finding the largest/smallest value, and more.

1. Java - Arrays
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

2. Introduction
 An array is a data structure that consists of an ordered
collection of similar items.
 An array has a single name.
 The items in an array are referred / accessed in terms of
their position / index in the array.
 The items in an array are called elements.
 All of the elements need to be of the same type.
 The type can be any primitive or reference type.
 The length of an array is measured by the number of
elements.
 The first element is element[0], the second is
element[1], etc.
 An item’s position with an array is its index or subscript.
2Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

3. Introduction
3Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

4. Introduction
 Declaring Array Variables (One Dimensional Arrays)
 Creating Arrays
 Declaring and Creating in One Step
 Array Initialization
 Default Values
 The Length of an Array
 Array Size through Input
 Accessing Array Elements
 Validating Indexes
 Two Dimensional Arrays
4Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

5. Declaring Array Variables
 Syntax: datatype[] arrayRefVar;
 Example: double[] myList;
 Alternative syntax: datatype arrayRefVar[];
// This style is allowed, but not preferred
 Example:
 double myDouble[];
 Int[] myInt;
5Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

6. Creating Arrays
 Syntax:
arrayRefVar = new datatype[arraySize];
 Example:
myDouble = new double[10];
 myDouble[0] references the first element in the array.
 myDouble[9] references the last element in the array.
myInt = new int[5];
6Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

7. Declaring and Creating in One Step
 Syntax:
datatype[] arrayRefVar = new datatype[arraySize];
 Example: double[] myDouble = new double[10];
 Alternative Syntax:
datatype arrayRefVar[] = new datatype[arraySize];
 Example: int myInt[] = new int[10];
String[] myString = new String[5];
MyClass[] myObject= new MyClass[2]; //object
7Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

8. Array Initializers
 Java has a shorthand notation to declare, create, and
initialize an array in one statement:
double[] myList = {1.9, 2.9, 3.4, 3.5};
 This shorthand notation is equivalent to the following
statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
8Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

9. CAUTION
 This shorthand syntax must be in one statement
 The following is incorrect:
double[] myList = new double[4];
myList= {1.9, 2.9, 3.4, 3.5};
9Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

10. Default Initialization
 When array is created, array elements are initialized
 Numeric values (int, double, etc.) to 0
 Boolean values to false
 Char values to ‘u0000’ (unicode for blank character)
 Class types to null
10Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

11. The Length of an Array
 Once an array is created, its size is fixed
 Array size cannot be changed
 We can find its size using the following syntax:
arrayRefVar.length
 Note that length is a constant, not a method
 There is no parentheses following length
 For example, myList.length returns 10
 myInt.length
Sample Code:
long[] primes = new long[20];
System.out.println(primes.length);
Output: 20
11Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

12. Accessing Array Elements
 Index of an array is defined as
 Positive int, byte or short values
 Expression that results into these types
 Any other types used for index will give error
 long, double, etc.
 Incase Expression results in long, then type cast to int
 Indexing starts from 0 and ends at N-1
primes[2]=0;
int k = primes[2];
…
12Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

13. Array Size through Input
13Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

14. What happens if …
long[] primes = new long[20];
primes[25]=33;
….
Runtime Error:
Exception in thread “main” java.lang.ArrayIndexOutOfBoundsException: 25
at MorePrimes.main(MorePrimes.java:6)
14Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

15. 15
Copying Arrays (The Wrong Way)
 Often, in a program, you need to duplicate an array or a part of an array.
In such cases you could attempt to use the assignment statement (=), as
follows:
myList1= new int[10];
myList2 = new int[10];
myList2 = myList1;
Contents
of myList1
list1
Contents
of myList2
list2
Before the assignment
Contents
of myList1
list1
Contents
of myList2
list2
After the assignment
Garbage
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

16. 16
Copying Arrays (The Correct Way)
 Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

17. 17
Passing Arrays to Methods
 Defining a method that takes an int array as a formal
parameter
public static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
 Invoking the method with an int array as an argument
(actual parameter)
int[] list = {3, 1, 2, 6, 4, 2};
printArray(list);
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

18. 18
Returning an Array from a Method
 Define the method that returns an array
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length - 1;
i < list.length; i++, j--) {
result[j] = list[i];
}
return result;
}
 Call the method
int[] list1 = {1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

19. Arrays of Arrays
 Two-Dimensional arrays
 float[][] temperature=new float[10][365];
 10 arrays each having 365 elements
 First index: specifies array (row)
 Second Index: specifies element in that array (column)
 In JAVA float is 4 bytes, total Size=4*10*365=14,600 bytes
19Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

20. Graphical Representation
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
Sample[0]
Sample[1]
Sample[2]
20Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

21. Initializing Array of Arrays
int[][] array2D = {
{99, 42, 74, 83, 100},
{90, 91, 72, 88, 95},
{88, 61, 74, 89, 96},
{61, 89, 82, 98, 93},
{93, 73, 75, 78, 99},
};
//5 arrays with 5 elements each
21Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

22. Sample Program
class unevenExample3
{
public static void main( String[] arg )
{ // declare and construct a 2D array
int[][] uneven = { { 1, 9, 4 }, { 0, 2}, { 0, 1, 2, 3, 4 } };
// print out the array
for ( int row=0; row < uneven.length; row++ ) //changes row
{
System.out.print("Row " + row + ": ");
for ( int col=0; col < uneven[row].length; col++ ) //changes column
System.out.print( uneven[row][col] + " ");
System.out.println();
}
}
}
22Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

23. Output
Row 0: 1 9 4
Row 1: 0 2
Row 2: 0 1 2 3 4
23Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

24. Multidimensional Arrays
 A farmer has 10 farms of beans each in 5 countries, and
each farm has 30 fields!
 Three-dimensional array
long[][][] beans=new long[5][10][30];
//beans[country][farm][fields]
24Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

25. 25
Examples of Arrays Processing
 Initializing an array with random values between 0.0
and 99.0
double[] myList = new double[4];
for (int i = 0; i < myList.length; i++) {
myList[i] = Math.random() * 100;
}
 Printing an array
for (int i = 0; i < myList.length; i++) {
System.out.print(myList[i] + " ");
}
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

26. 26
 Summing all elements
double total = 0;
for (int i = 0; i < myList.length; i++) {
total += myList[i];
}
 Finding the largest element
double max = myList[0];
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max)
max = myList[i];
}
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

27. 27
 Finding the smallest index of the largest element
double max = myList[0];
int indexOfMax = 0;
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max) {
max = myList[i];
indexOfMax = i;
}
}
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam

28. 28
The End…
Dr. P. Victer Paul, Indian Institute of Information Technology Kottayam