Arrays

An array is a fixed-size collection of values that are all the same data type, stored together in a single block of memory. Instead of creating a separate variable for every value you need, an array lets you group them all under one name.

int[] canIDs = { 1, 2, 3, 4 };

Here, canIDs holds four int values, one for each motor's CAN ID on a drivetrain.

Creating an Array

Array Formula

The general formula for creating an array is:

[data type][] [array name] = new [data type][size];

Arrays can be created empty, with a specific size, and filled in later:

int[] canIDs = new int[4]; // Creates an array that can hold 4 ints, all initialized to 0
canIDs[0] = 1;
canIDs[1] = 2;
canIDs[2] = 3;
canIDs[3] = 4;

Or they can be declared and filled all at once using an array literal:

int[] canIDs = { 1, 2, 3, 4 };

Once an array is created, its size is locked in. You cannot add a fifth element to canIDs later without creating a brand new array. For a collection that can grow and shrink, see the ArrayList section.

Indexing

Each value in an array is accessed using its index, which is its position in the array. Java arrays are zero-indexed, meaning the first element is at index 0, not 1.

int[] canIDs = { 1, 2, 3, 4 };

System.out.println(canIDs[0]); // Prints 1
System.out.println(canIDs[3]); // Prints 4

Out of Bounds

Trying to access an index that doesn't exist, either negative or beyond the array's last valid index, throws an ArrayIndexOutOfBoundsException at runtime.

int[] canIDs = { 1, 2, 3, 4 };

System.out.println(canIDs[4]); // Throws ArrayIndexOutOfBoundsException

Since canIDs has 4 elements, its valid indices are 0 through 3. Index 4 is one past the end of the array, this is a very common mistake known as an off-by-one error.

The length Field

Every array has a length field that tells you how many elements it can hold. Note that this is a field, not a method, so it's accessed without parentheses.

int[] canIDs = { 1, 2, 3, 4 };

System.out.println(canIDs.length); // Prints 4

length is especially useful for looping through every element of an array without hard-coding the number of elements. For more information about loops, see the Control Flow section.

int[] canIDs = { 1, 2, 3, 4 };

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

Multi-Dimensional Arrays

Arrays can also hold other arrays, creating a grid-like, multi-dimensional structure. A common use case is representing a 2D grid of positions on the field.

int[][] fieldGrid = {
  { 0, 0, 0 },
  { 0, 1, 0 },
  { 0, 0, 0 }
};

System.out.println(fieldGrid[1][1]); // Prints 1

Here, fieldGrid is an array of int[] arrays. The first index selects the row, and the second index selects the column within that row.