Java 7 for Absolute Beginners

CHAPTER 3 ■ DATA TYPES

Java provides a convenience class for arrays. The Arrays class consists of many static methods to

do handy things such as copy and sort arrays. For example, Listing 3-9 shows one way to sort an array

of ints.

Listing 3-9. Using the Arrays convenience class

int[] a = {5, 4, 3, 2};

// at the top of the program, we had to import

// java.util.Arrays for this to work correctly

Arrays.sort(a);

for (int i = 0; i < a.length; i++) {

System.out.println(a[i]);

}

The result is 2, 3, 4, 5 (each on its own line) in the console.

The Non-Existent Type: null

Java includes a value that isn't anything: null. It refers to a memory address that has not been assigned.

In Java terms, that means it refers to an object or primitive that has not been created. As I mentioned in

the “Arrays” section, when you create an array without specifying its values, you are creating a collection

of null values. They have no memory address, no corresponding primitive or object exists for them, and

so they are null. That might sound like a problem, and the whole concept of null often causes novice

programmers some trouble. You can keep it straight by remembering that a null is a non-existent

reference.

The oft-maligned null has its uses (otherwise, it wouldn't exist—programmers are pragmatic

people, most of the time). For example, we often compare an object to null to be sure that something

exists before we try to use it. If the graphics library is supposed to give us an object of type Color and we

get null instead, we have a problem. So we might compare that Color value to null to ensure that we are

getting a Color object and do something useful (such as trying another way or at least logging an error)

if not.

Also, it's common practice to create a variable in one place and assign it in another place. In

between the creation and assignment, the value of that variable might be null (it also might not be null

because some primitives, such as int, have default values). This technique is handy because we might

want to assign different values to the variable based on some logic. For example, a series of if-else

statements or a switch statement might contain code to assign the value of a variable. Let's consider a

small example (from a minesweeper game), shown in Listing 3-10.

Listing 3-10. Using a null value

public getMineIcon(int x, int y) {

// x and y are the position within the game grid

int numberOfAdjacentMines = getNumberOfAdjacentMines(x, y);

MineIcon mineIcon = null; // here's our null