Arrays and Indexes

An array stores an ordered group of values of the same type. Every element is accessed by index. A Java array has a fixed length after creation and does not provide append, insert, or pop.

Creating arrays

Use an array initializer when the values are known. new int[n] creates n slots first. The element type comes before the variable name, and brackets mean “a group of this type.”

int[] scores = {88, 91, 76, 95, 83};
int[] counts = new int[4];
counts[0] = 12;
counts[1] = 9;

New arrays receive default values: numeric values are 0, boolean is false, char is the null character, and object references such as String are null. The slot count is decided when the array is created.

Accessing array elements

Array indexes start at 0, so the first element is scores[0]. An array of length 5 has indexes 0 through 4. Accessing scores[5] throws ArrayIndexOutOfBoundsException.

int[] scores = {88, 91, 76, 95, 83};
System.out.println(scores[0]); // 88
System.out.println(scores[4]); // 83
scores[2] = 80;
System.out.println(scores[2]); // 80

Array length and loops

Arrays use the length field without parentheses. The last valid index is array.length - 1. Use an index loop when positions matter and an enhanced for loop when only values matter.

for (int i = 0; i < scores.length; i++) {
    System.out.printf("scores[%d] = %d%n", i, scores[i]);
}

for (int score : scores) {
    System.out.println(score);
}

Fixed length and common operations

You can rewrite existing elements and search with a loop. Neither action adds or removes slots.

int[] values = {4, 0, 7, 0, 2, 9};
values[2] = 5;
for (int value : values) {
    System.out.print(value + " ");
}