Arrays
An array is a collection of variables of the same type, referred to by a common name. In java they are implemented as objects so they are dynamically allocated using the new operator.
To declare a one-dimensional array : type[] array-name = new type[size];
typedeclares the element type (base type) that can be contained in the array.sizedefines the number of elements that the array will holds.
int[] sample = new int[10];
// or
int[] sample;
sample = new int[10];An individual element within an array is accessed by use of an index. Zero as the index of their first element.
class ArrayDemo
{
public static void main(String[] args)
{
int[] sample = new int[10];
int i;
for(i=0; i<10; i++)
sample[i] = i;
for(i=0; i<10; i++)
System.out.println("This is ["
+ i + "] :" + sample[i] );
}
}To find Min and max in an array
class MinMax
{
public static void main(String[] args)
{
int[] nums = new int[10];
int min, max;
nums[0] = 99;
nums[1] = -10;
nums[2] = 100123;
nums[3] = 18;
nums[4] = -978;
nums[5] = 5623;
nums[6] = 463;
nums[7] = -9;
nums[8] = 287;
nums[9] = 49;
min = max = nums[0] // assuming
for(int i=1; i<10; i++)
{
if( nums[i] < min )
min = nums[i];
if( nums[i] > max)
max = nums[i];
}
System.out.println("min and max: " + min + " " + max);
}
}min and max: -978 100123Initializing Arrays
Arrays can be initialized when they are created. type[ ] array-name = { val1, val2, ... , valN };
Java automatically allocates an array large enough to hold the initializers that you specify. There is no need to explicitly use the new operator.
int[] nums = { 99, -10, 100123, 18, -978, 5623, 463, -9, 287, 49 };Array boundaries are strictly enforced in Java; it is a run-time error to overrun or under-run the end of an array. causing ArrayIndexOutOfBoundsException
Bubble sort
Logic for Moving each element through comparison and moving smallest element to front
for(a=1; a<size; a++) // loop to traverse
for(b=size-1; b>= a; b--)
{
if(nums[b-1] > nums[b])
{
temp = nums[b-1];
nums[b-1] = nums[b];
nums[b] = temp;
}
}// Bubble sort
class BubbleSort
{
public static void main(String[] args)
{
int[] nums = {99, -10, 100123, 18, -978, 5623, 463, -9, 287, 49 };
int a, b, temp;
int size = 10;
System.out.print("Original array: ");
for(int i=0; i<size; i++)
System.out.print(" " + nums[i] + " ");
System.out.println();
for(a=1; a<size; a++)
{
for(b=size-1; b>=a; b--)
{
if(nums[b-1] > nums[b])
{
temp = nums[b-1];
nums[b-1] = nums[b];
nums[b] = temp;
}
}
// To print each Iteration
System.out.print("Sort Iteration " + a + " is : ");
for(int i=0; i<size; i++)
System.out.print(" " + nums[i] + " ");
System.out.println();
}
// Final array
System.out.print("\nSorted Array is : ");
for(int i=0; i<size; i++)
System.out.print(" " + nums[i] + " ");
System.out.println();
}
}$ javac BubbleSort.java
$ java BubbleSort
Original array: 99 -10 100123 18 -978 5623 463 -9 287 49
Sort Iteration 1 is : -978 99 -10 100123 18 -9 5623 463 49 287
Sort Iteration 2 is : -978 -10 99 -9 100123 18 49 5623 463 287
Sort Iteration 3 is : -978 -10 -9 99 18 100123 49 287 5623 463
Sort Iteration 4 is : -978 -10 -9 18 99 49 100123 287 463 5623
Sort Iteration 5 is : -978 -10 -9 18 49 99 287 100123 463 5623
Sort Iteration 6 is : -978 -10 -9 18 49 99 287 463 100123 5623
Sort Iteration 7 is : -978 -10 -9 18 49 99 287 463 5623 100123
Sort Iteration 8 is : -978 -10 -9 18 49 99 287 463 5623 100123
Sort Iteration 9 is : -978 -10 -9 18 49 99 287 463 5623 100123
Sorted Array is : -978 -10 -9 18 49 99 287 463 5623 100123Multidimensional Arrays
In Java, a multidimensional array is an array of arrays. A two-dimensional array is, in essence, a list of one-dimensional arrays.
Two-dimensional integer array table of size 10, 20 : int[][] table = new int[10][20];
// Demonstrate a two-dimensional array.
class TwoD
{
public static void main(String[] args)
{
int t, i;
int[][] table = new int[3][4];
for(t=0; t < 3; ++t)
{
for(i=0; i < 4; ++i)
{
table[t][i] = (t*4)+i+1;
System.out.print(table[t][i] + " ");
}
System.out.println();
}
}
}Assigns values from 1 to 12 to all indexes.
Irregular Arrays
Since multidimensional arrays are implemented as arrays of arrays, the length of each array is under your control.
When you allocate memory for a multidimensional array, you need to specify only the memory for the first (leftmost) dimension. You can allocate the remaining dimensions separately.
int[][] table = new int[3][];
table[0] = new int[4];
table[1] = new int[2];
table[2] = new int[6];if you need a very large two-dimensional array that is sparsely populated (that is, one in which not all of the elements will be used), an irregular array might be a perfect solution.
Initializing multi dimensional arrays
type[ ] [ ]...[ ] name = new type[size1][size2]...[sizeN];
int[][][] multidim = new int[4][10][3]; Creates a 4 × 10 × 3 three-dimensional integer array.
A multidimensional array can be initialized by enclosing each dimension’s initializer list within its own set of curly braces.
type-specifier[ ] [ ] array_name = {
{ val, val, val, ..., val },
{ val, val, val, ..., val },
.
.
.
{ val, val, val, ..., val }
};Each inner block designates a row. commas separate the initializer blocks and that a semicolon follows the closing }.
// Initialize a two-dimensional array.
class Squares
{
public static void main(String[] args)
{
int[][] sqrs = {
{ 1, 1 },
{ 2, 4 },
{ 3, 9 },
{ 4, 16 },
{ 5, 25 },
{ 6, 36 },
{ 7, 49 },
{ 8, 64 },
{ 9, 81 },
{ 10, 100 }
};
int i, j;
for(i=0; i < 10; i++)
{
for(j=0; j < 2; j++)
System.out.print(sqrs[i][j] + " ");
System.out.println();
}
}
}Alternate Array Declaration
The square brackets follow the name of the array variable, not the type specifier.
Two declarations are equivalent:
int counter[] = new int[3];
int[] counter = new int[3];char table[][] = new char[3][4];
char[][] table = new char[3][4];This alternative declaration form offers convenience when converting code from C/C++ to Java. In C/C++, arrays are declared in a fashion similar to Java’s alternative form.
Assigning Array Reference
When assigning one array reference variable to another, it changes what object that variable refers to. Copy of the array is not made, nor the contents of one array copies to the other.
Using length Member
Java, arrays are implemented as objects. One benefit of this approach is that each array has associated with it a length instance variable that contains the number of elements that the array can hold. (In other words, length contains the size of the array.)
A two-dimensional array is an array of arrays. Thus, when the expression table.length is used, it obtains the number of arrays stored in table, which is 3 in this case.
To obtain the length of any individual array in table, you will use an expression such as this, table[0].length
class LengthDemo
{
public static void main(String[] args)
{
int[] list = new int[10];
int[] nums = {1,2,3,4};
int[][] table = {
{1,2,3},
{4,5},
{6,7,8,9}
};
System.out.println("length of list is " + list.length);
System.out.println("length of nums is " + nums.length);
System.out.println("length of table is " + table.length);
System.out.println("length of table[0] is " + table[0].length);
System.out.println("length of table[1] is " + table[1].length);
System.out.println("length of table[2] is " + table[2].length);
System.out.println();
// Using length to initialize for loop
for(int i=0; i<list.length; i++)
list[i] = i*i;
System.out.print("Here is list: ");
for(int i=0; i<list.length; i++)
System.out.print(list[i] + " ");
System.out.println();
}
}length of list is 10
length of nums is 3
length of table is 3
length of table[0] is 3
length of table[1] is 2
length of table[2] is 4
Here is list: 0 1 4 9 16 25 36 49 64 81Copying array using length
class ACopy
{
public static void main(String[] args)
{
int i;
int[] nums1 = new int[10];
int[] nums2 = new int[10];
// Initializing first array
for(i=0; i < nums1.length; i++)
nums1[i] = i;
// copy nums1 to nums2
if(nums2.length >= nums1.length)
for(i = 0; i < nums1.length; i++)
nums2[i] = nums1[i];
// printig copied array
for(i=0; i < nums2.length; i++)
System.out.print(nums2[i] + " ");
}
}Implementing Queue using arrays
class Queue
{
char[] q; // this array holds the queue
int head, tail; // the put and get indices
Queue(int size)
{
q = new char[size]; // allocate memory for queue
head = tail = 0;
}
// put a character into the queue
void put(char ch)
{
if(tail==q.length)
{
System.out.println(" – Queue is full.");
return;
}
q[tail++] = ch;
}
// get a character from the queue
char get()
{
if(getloc == putloc)
{
System.out.println(" – Queue is empty.");
return (char) 0;
}
return q[head--];
}
}Operations using the Queue class
class QDemo
{
public static void main(String[] args)
{
Queue bigQ = new Queue(100);
Queue smallQ = new Queue(4);
char ch;
int i;
System.out.println("Using bigQ to store the alphabet.");
// put some numbers into bigQ
for(i=0; i < 26; i++)
bigQ.put((char) ('A' + i));
// retrieve and display elements from bigQ
System.out.print("Contents of bigQ: ");
for(i=0; i < 26; i++)
{
ch = bigQ.get();
if(ch != (char) 0) System.out.print(ch);
}
System.out.println("\n");
System.out.println("Using smallQ to generate errors.");
for(i=0; i < 5; i++)
{
System.out.print("Attempting to store " + (char) ('Z' - i));
smallQ.put((char) ('Z' - i));
System.out.println();
}
System.out.println();
// more errors on smallQ
System.out.print("Contents of smallQ: ");
for(i=0; i < 5; i++)
{
ch = smallQ.get();
if(ch != (char) 0)
System.out.print(ch);
}
}
}Using bigQ to store the alphabet.
Contents of bigQ: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Using smallQ to generate errors.
Attempting to store Z
Attempting to store Y
Attempting to store X
Attempting to store W
Attempting to store V – Queue is full.
Contents of smallQ: ZYXW – Queue is empty.