Incremental Java
Arrays: Size vs. Index

Length vs. Index

When you construct an array 
  int [] arr = new int[ 10 ] ;
10 refers to the length, which is the number of elements in the array.

Construction of arrays specify the length of the array

Another Other Time It's Index

If you're not constructing an array, then you are accessing an element in the array using its index. 
  arr[ 2 ] = 10 ;
2 refers to an index. In this case, the brackets access a single element from an array. In the example, we access element 2 (recall that indexing begins at 0, and so even though it's index 2, it's the third element of the array).

Determining the Length of an Array

Arrays are objects. They don't look like objects because you can use the index operator (i.e., using brackets), but they are objects.

Because arrays are objects, there are methods and instance variables. In particular, arrays have a public instance variable called length, which gives you the length of the array.

Generally, you should avoid using public instance variables to improve encapsulation. However, Java arrays use them, so we must use them as well.

Here's how to print a Java array using the length instance variable. 

  int [] arr = { 1, 3, 5, 7, 9 } ;
  for ( int i = 0 ; i < arr.length ; i++ )
  {
     System.out.println( "The value at index " + i + " is " + arr[ i ] ) ;
  }
In the example above, the length of arr is 5.

The output is: 

The output at index 0 is 1
The output at index 1 is 3
The output at index 2 is 5
The output at index 3 is 7
The output at index 4 is 9
This is one very convenient feature of Java arrays.

Arrays know their own length.

In C and C++, arrays don't know their own length, so you must often pass an additional variable to keep track of the array's length.