In this section, you will learn how to declare variables.
You must declare a variable before using it. The compiler will display errors otherwise.
The basic form of a variable declaration statement is shown below.
type name;
A variable name is an identifier. You can refer the value stored in the variable with its name.
The type indicates the values that can be stored in a variable.
Notice that a variable declaration ends with a semicolon. That is because the variable declaration statement is a simple statement.
Here’s an example where two variables are declared.
String name;
int age;
In this example, you declared variables named name and age. The variable
name can hold string values. The variable age can hold integer values.
You will learn more about data types in the next section. Until then remember
that int variables can hold integer values. In other words, int variables can
hold whole numbers like 1, 20000, and 100. String variables can hold
text values such as "Hello", "Samuel Rowe", and "Shazam".
You can declare two or more variables in a single statement. You must separate the variable names with commas. All the variables will have the same type.
Here’s an example where four variables are declared.
int a, b, c, d;
Although this is easier, we suggest you to avoid this. Because it makes your code less easier to read.
A local variable is a variable that you declare within the body of a method. You can use a local variable only within that method. You can’t use it outside the method.
For example, the variable k exists only with the braces of the main method.
public class Example {
public static void main(String[] arguments) {
int k;
/* You can refer 'k' only within the braces of this method. */
}
}
The region in which your variable exists is known as scope. You will learn more about scopes later in the book.
The place where you declare a local variable is important. You must declare a variable before you use it.
For example, the compiler generates an error if you try to compile this example. Because it declares the variable after using it.
public class Example {
public static void main(String[] arguments) {
i = 10;
int i;
}
}
Don’t worry if you don’t understand what i = 10 means. You’ll learn about it
in the next section.
When you refer the value stored in a local variable, the compiler checks if you assigned a value previously.
For example, the compiler generates an error if you try to compile this example. Because it uses a variable without assigning a value.
public class Example {
public static void main(String[] arguments) {
String name;
System.out.println(name);
}
}
To avoid such errors, you must initialize local variables before you use them. You can assign a variable with an expression statement. In particular, you use the assignment expression to assign a variable. You will learn more about expressions later in the book.
The basic form of an assignment expression is shown here.
variable = expression
Here, expression can be any expression that evaluates to a value.
For example, this is the correct version of the previous example.
public class Example {
public static void main(String[] arguments) {
String name;
name = "Samuel Rowe";
System.out.println(name);
}
}
In this example, the variable is assigned a value of “Samuel Rowe” before printing it.
You can initialize a variable when you declare it. You use an initializer to do this. The general form of an initializer is shown here.
type name = expression;
The initializer allows you to combine declaration and initialization.
Take a look at some examples given below.
String name = "Samuel Rowe";
int age = 19;
float gravity = 9.8f;
double pi = 3.142857142857143;
In each of these statements, a variable is declared and initialized.