Accessing and Mutating Arrays

Now that you know how to create an array, what can you do with it? Let's start by printing the array:

int[] primes = new int[] { 2, 3, 5, 7, 11, 13 };
println(primes);

Was the output what you expected? Probably not. Java's default method for converting objects into strings doesn't produce a very useful string for an array. We can convert an array to a string with a loop, but first we need to learn how to access the individual values in an array.

Indexing

We mentioned indexing in the String API section, but it's worth going into more detail now that we're dealing with arrays. To access a single value stored in an array (we call these values the array's elements), we need to specify its index. The index of an element is that element's distance from the beginning of the array. This means the first element has an index of 0 because it is at the beginning of the array. The second element has an index of 1 because it's one space away from the start of the array.

If we want to refer to a specific element of an array, we put that element's index in square brackets after the name of the array:

int[] primes = new int[] { 2, 3, 5, 7, 11, 13 };
println(primes[0]); // prints 2
println(primes[1]); // prints 3
println(primes[2]); // prints 5
println(primes[3]); // prints 7
println(primes[4]); // prints 11
println(primes[5]); // prints 13

We can reassign one element of the array as long as we specify that element's index:

int[] primes = new int[] { 2, 3, 5, 7, 11, 13 };
primes[2] = 10;
println(primes[0]); // prints 2
println(primes[1]); // prints 3
println(primes[2]); // prints 10!

If you don't specify an index, you'll be attempting to reassign the variable that holds the array. This will usually result in a syntax error:

int[] primes = new int[] { 2, 3, 5, 7, 11, 13 };
primes = 10; // primes is an array, not an int, so we can't reassign it to 10

Getting Used to Zero-Indexing

Many people find zero-indexing counterintuitive. If you think of the position of an element, then you need to subtract one from the position to get the element's index. This isn't intuitive, and it forces you to do extra work each time you think about the index as a position.

The best analogy I've heard for zero-indexing is a ruler: the first mark on a ruler represents a distance of zero, not one. This is why I recommend thinking about indexes as distances rather than positions.

Array Bounds

You can get the length of an array by typing .length after the name:

int[] primes = new int[] { 2, 3, 5, 7, 11, 13 };
println(primes.length); // prints 6

The only valid indexes for an array are the integers from zero to its length minus 1. The length is never a valid index. For example, if we had an array of length 10 then the tenth and last element would be at index 9. An index of 10 would be the eleventh element, and the array only has ten elements.

If we try to index an array with a negative number, or with a number greater than or equal to its length, then this will cause an ArrayIndexOutOfBoundsException, which is a type of error that will cause the program to crash.