Control Flow›Core· 30 min read

Arrays

An array stores many values of the same type in one variable, numbered from 0.

What you will learn

Create and fill an array
Read values by index
Loop over an array

Many values, one name

A single variable holds one value. An array holds many values of the same type, all under one name. Each value sits at a numbered position called an index.

Important: indexes start at 0, not 1. So the first item is at index 0.

An array of three strings

String[] fruits = {"apple", "banana", "cherry"};

System.out.println(fruits[0]);          // first item
System.out.println(fruits[2]);          // third item
System.out.println("How many: " + fruits.length);

Note: Output: apple cherry How many: 3 fruits[0] is apple (index 0), fruits[2] is cherry (index 2). fruits.length tells you how many items there are — note it is .length with no brackets.

Looping over an array

Arrays and loops are best friends. A for-each loop visits every item without you tracking the index:

A for-each loop adding up scores

int[] scores = {80, 95, 60, 100};
int total = 0;

for (int s : scores) {
    total = total + s;
}

System.out.println("Sum of scores: " + total);
System.out.println("Average: " + (total / scores.length));

Note: Output: Sum of scores: 335 Average: 83 The for-each loop reads for (int s : scores) as for each score s in scores. We added them to 335, then divided by 4 items to get the average (integer division, so 83).

Looping with the index

The for-each loop is tidy, but sometimes you need to know which position you are at — to number items, or to change a value. For that, use a normal for loop that counts from 0 up to length - 1:

An indexed for loop that prints each position

String[] fruits = {"apple", "banana", "cherry"};

for (int i = 0; i < fruits.length; i++) {
    System.out.println(i + ": " + fruits[i]);
}

Note: Output: 0: apple 1: banana 2: cherry Here i is the index, going 0, 1, 2. We stop at i < fruits.length (which is 3) so the last round uses index 2 — the final valid position. Use this style when the position matters; use for-each when it does not.

Watch out: Going out of bounds crashes the program. An array of length 3 has indexes 0, 1, 2 only. Asking for fruits[3] throws an ArrayIndexOutOfBoundsException. The last valid index is always length - 1. This is why the loop condition is i < length (not i <= length).

Tip: To make an empty array of a fixed size, write int[] nums = new int[5];. It starts filled with zeros (or empty text for a String array), ready for you to fill with a loop.

Q. In the array {"red", "green", "blue"}, what is at index 1?

Answer: Indexes start at 0, so red is index 0, green is index 1, and blue is index 2.

✍️ Practice

Create an int array of five numbers and print the first and last using [0] and [length - 1].
Use a for-each loop to print every item of a String array on its own line.

🏠 Homework

Make an array of test scores, then loop through it to find and print the highest score.