Arrays in Java

What is an array

An array holds many values of the same type in a single variable, each reached by an index number. Instead of 5 separate variables for 5 marks, one array stores them all. Arrays have a fixed size once created.

Declaring and initialising

int[] marks = {90, 85, 78, 92};      // with values
int[] scores = new int[5];           // size 5, all 0s
String[] names = {"Aman", "Diya"};   // string array

You can create an array directly with values in { }, or make an empty one of a fixed size with new int[5]. The type has square brackets: int[].

Accessing elements

int[] marks = {90, 85, 78, 92};
System.out.println(marks[0]);   // first  -> 90
System.out.println(marks[3]);   // fourth -> 92
System.out.println(marks.length); // size -> 4
marks[1] = 88;                  // change second

Use the index in square brackets to read or change an element. marks.length gives the size (a field, no brackets). Remember the first element is index 0.

Looping over an array

int[] marks = {90, 85, 78, 92};
for (int i = 0; i < marks.length; i++) {
    System.out.println(marks[i]);
}

A for loop from 0 to length - 1 visits every element. Using marks.length as the limit keeps it correct even if the size changes.

The for-each loop

int[] marks = {90, 85, 78, 92};
for (int m : marks) {
    System.out.println(m);
}

The for-each loop reads each value directly into m — no index needed. It is cleaner when you just want to go through all values in order.

2D arrays

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6}
};
System.out.println(grid[1][2]);   // row 1, col 2 -> 6

A 2D array is an array of arrays — like a table with rows and columns. You access an element with two indexes: grid[row][col].

Common mistakes

• Index out of bounds. Valid indexes are 0 to length - 1; anything else throws an error.
• Using length() on an array. Arrays use length (a field), Strings use length().
• Forgetting arrays are fixed size. To grow, use an ArrayList instead.
• Starting the loop at 1. Start at index 0, or you miss the first element.

Summary

• An array holds many same-type values, accessed by index.
• Indexes start at 0; the last is length - 1.
• Use arr.length (a field) for the size.
• Loop with a for (index) or a for-each (values).
• 2D arrays are arrays of arrays, accessed as arr[row][col].

Next, use a flexible list with ArrayList vs LinkedList, or loop over arrays in Java loops.