Notes on core java

1. Core Java
Debasish Pratihari
Array :
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In Java arrays are objects that represent a
group of contiguous memory locations of the
same type and referred to by the same name.
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The element type for an array can be any
primitive data type or object.
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All Java arrays are technically one-dimensional.
Two-dimensional arrays are arrays of arrays.
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Declaring an array does not create an array
object or allocate space in memory; it creates a
variable with a reference to an array
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Array variable declarations must indicate a
dimension by using []
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All arrays are zero-based
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Arrays must be indexed by int values or byte,
short or char values (as these can be promoted
to int)
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Using a long index value to access an array
causes a compile error
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Attempting to access an array with an index less
than 0 or greater than the length of the array
causes an ArrayIndexOutOfBoundsException
to be thrown at runtime
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Since arrays are Objects they can be initialized
using the new operator
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When created, arrays are automatically
initialized with the default value of their type
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Array references declared as members are
initialized to null BUT array references declared
in methods are not initialized
Lecture/core/array/06
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2. Core Java
Debasish Pratihari
Examples of valid array declarations :
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String[]s;
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String s[];
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String[][]s;
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String s[][];
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String[] s[];
Initializing Array :
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String[ ] s = new String[100]; // default values: null
boolean[] b = new boolean[4]; // default values: false
int i[ ]= new int[5];
int[][] i = new int[10][10];
// default values: 0
String[] oneDimArray = { "abc","def","xyz" };
int[] array = null;
int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} };
int[] a = { 1, primes[2], c, (int)Math.pow(2,2) };
int[] arr = new int[] {1,2,3};
int[][] myArray = new int[5][];
int arrSize = 100;
String[] myArray = new String[arrSize];
Illegal Initialization of Array :
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String[5] s;
int[] array = new int[];
int[][] myArray = new int[][5];
String[] s;
s = { "Lakshya", "debasish", "java"};
Lecture/core/array/06
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3. Core Java
Debasish Pratihari
Finding length of an Array :
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Arrays have a length data field (read only),
specifying the number of elements in the array.
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The length data field can be accessed through
the dot operator.
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The bounds of an array are integers between 0
and length – 1
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Example
for ( int k=0; k < a.length ; k++ )
System.out.println( a[ k ] );
Multidimensional Arrays:
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Multidimensional arrays are implemented as
arrays of arrays. Multidimensional arrays are
declared by appending the appropriate number
of bracket pairs after the array name.
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The number of elements in each direction need
not be specified in the declaration.
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The number of elements in each direction need
not be equal.
Example of irregular Array :
int[][] irregular = { { 1 },
{ 2, 3 },
{ 4, 5, 6, 7 },
{0}
};
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4. Core Java
Debasish Pratihari
An Example -Triangle Array :
Output
int[][] triangle = new int[10][];
for ( int j = 0; j<triangle.length ; j++ )
triangle[ j ] = new int[ j + 1 ];
for ( int i = 0 ; i < triangle.length ; i++ )
{
for ( int j = 0 ; j < triangle[i].length ; j++ )
System.out.print( triangle[i][j] + " " );
System.out.println();
}
0
00
000
0000
00000
000000
0000000
00000000
000000000
0000000000
Coping Array :
25% 
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Arrays can be copied using the Java System
method arraycopy():
Signature:public static native void
arraycopy(Object src, int src_position, Object
dst, int dst_position, int length)
Parameters:
src - the source array.
src_position - start position (first cell to copy) in the
source array.
dst - the destination array.
dst_position - start position in the destination array.
length - the number of array elements to be copied.
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By default, all Java arrays support the clone
method.
Things to Remember :
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An array is a container object that holds a
fixed number of values of a single type.
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The length of an array is established when
the array is created. After creation, its
length is fixed.
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Each item in an array is called an element
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each element is accessed by its numerical
index.
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