This is one of the comprehensive Java slides meant to target those who already had knowledge of Core-Java Day-1

1. Arrays in Java
Arrays in Java are like arrays in C/C++ except in…
• Arrays are not only set of homogeneous data types stored in
contiguous memory locations, but they are set of reference types
(not necessarily homogeneous in direct hierarchy) also.
• Java arrays provide you with one final property length that helps
you find out the length of the array without any extra coding
• Arrays in java are treated as objects of type java.lang.Object class
and once you allocate memory to an array, the array elements get
filled up with the default value of the corresponding type ( 0 for
Numbers, ‘u0000’ for char, false for boolean and null for any
reference type.
• Arrays are reusable.
Now there are 4 ways to define an array which we shall see in the next
slide…. [Contd.]
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2. Four Ways To define an Array
int x[]; // declare an int array
x=new int[10]; // allocate
memeory
The first line declares an array of
type int. Not recommended by
experts
The second line allocates memory for
10 elements with all initialized to 0
int x[]=new int[10]; This is nothing but a merged version
of the above. Not recommended by
experts.
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int[] x;
x=new int[10];
The first line declares an array of
type int. This is recommended.
The second line allocates memory for
10 elements with all initialized to 0
int [] x=new int[10]; It’s a merged version of the above
and the this is recommended.

3. Inline declaration and initialization of array
• int[] x={719,38, 940, 8};
Is OK. It contains the
following elements and
size 4
• But the following is
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wrong…
int[] x;
x={719,38, 940, 8}; // Error
• But to make it OK, do
this…
int[] x;
x=new int[]{719,38, 940, 8};
• byte[] x=new byte[6];
for(int i=0;i<6;i++){
// codes go here
} BAD Practice
• byte[] x=new byte[6];
for(int i=0;i<x.length;i++){
// codes go here
} GOOD Practice

4. Anonymous & Reusable Arrays
• Look at the following code…
System.out.println(new int[]{0,1,2,3,4}.length);
This gives an output 5 as this is an array which has no
name.
• int[] x={1,2,3,4,5,6,7};
// codes go here
x=new int[90]; // the previous elements are lost
Thus arrays can be reused
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5. Arrays as return type of methods
• To return some data rather than just printing,
enhances the flexibility of the code.
• The following method returns an array containing first n natural numbers
• int[] getFirstNNumbers(int n){
int[] retVal=new int[n]; // allocating memory for n int numbers
for(int k=0;k<retVal.length;k++){ // iterate n times i.e = retVal.length
retVal[k]=k+1; // assigning the values, i.e. 1,2,3,4 so on
}
return retVal; // returns the array
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}

6. 2-D Arrays in Java
• Java deals multi dimensional arrays as Array of Arrays.
• We can declare a 2-D int array as…
1. int[][] x=new int[4][3]; // 4 rows & 3 cols per row
2. int[][] x=new int[4][]; // 4 rows and cols to be dynamically alocated
the size
x[0]=new int[5]; // means 5 cols in 0-th row
x[2]=new int[3]; // means 3 cols in 2-th row (3rd row)
3. int[][] x=new int[][5]; // Error; u can’t fix-up cols size without rows.
4. int[][] x={{1,2,3},{4,5,6,7},}; // first row has 3 cols where as
second row has 4 cols
5. In general, for a 2-D array myArray, myArray.length is the number
of rows and myArray[i].length is the number of cols in the i-th row
6. For any array x, x.length=90; is WRONG since length is final
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7. Miscellany
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• Code snippet:
int[][] x;
int[] t={1,2};
x=new int[][]{t, new int[]{1,2,3},{4,5,6,7},new int[6]};
System.out.println(x[0].length);
System.out.println(x[1].length);
• More on Arrays in the LAB Session
• On to LAB with a fresh mind