1.2. Variables

In Java, a variable is a storage location that has a type and holds one value at a time. Variables declared in code also have names, which let programmers store and retrieve values without knowing where in memory those values are kept.

When we declare a variable in Java, we must provide both a name and a type. Here are two examples of variable declarations:

// variable declarations
int x;
double[] myArray;

Notice that you don't see an equals sign (=) in the code above. The equals sign is used to assign a value to a variable. The first time you assign a value to the variable, it is called initializing the variable.

// variable initializations
x = 7;
myArray = new double[10];

Of course, we could combine declarations and initializations in the following way:

// declaring and initializing variables in a single line
int x = 7;
double[] myArray = new double[5];

You should be able to identify a variable's:

• value, the data that the variable currently holds;

• type, the attribute that tells the computer how to interpret the value and how much space is needed to store the value; and

• name, the identifier that code uses to refer to the variable.

Test Yourself

Take a moment to identify the value, type, and name of the variables declared and initialized below. Before looking ahead, write your answers in your notes.

int x = 17;
String s = "Hello!";

Test Yourself Solution (Don't open until completing the question above)

Did you come up with the answers below?

Declaration/Initialization	Value	Type	Name
int x = 17	17	int	x
String s = "Hello!"	a reference to a String object	String	s

Notice the types of the two variables above. The first, int, is a primitive type, and the second, String, is a reference type. You can tell that String is a reference type because it is not one of Java's eight primitive types; its capital letter is only a naming convention. Both variables hold values, but they behave differently in memory:

Final captured memory state. [download] [open]

As shown in the diagram above, x and s are both local variables of main, so both appear in the frame for that method call. The value of x is the int value 17, whereas the value of s is a reference that refers to a String object on the heap. The number shown in the box for s and in the object's label identifies that object; it is not, strictly speaking, a memory address. In this chapter, we will examine the important differences between these two kinds of variables.