Core Java Volume I — 3.4. Variables

3.4. Variables
In Java, every variable has a type. You declare a variable by placing the type first, followed by the name of the variable. Here are some examples:

double salary;
int vacationDays;
long earthPopulation;
boolean done;

Notice the semicolon（分号）at the end of each declaration. The semicolon is necessary because a declaration is a complete Java statement.
A variable name must begin with a letter and must be a sequence of letters or digits. Note that the terms "letter" and "digit" are much broader in Java than in most languages. A letter is defined as 'A'–'Z', 'a'–'z', '_', '$', or any Unicode character that denotes a letter in a language. For example, German users can use umlauts such as 'ä' in variable names; Greek speakers could use a π. Similarly, digits are '0'–'9' and any Unicode characters that denote a digit in a language. Symbols like '+' or '©' cannot be used inside variable names, nor can spaces. All characters in the name of a variable are significant and case is also significant. The length of a variable name is essentially unlimited.

Tip
If you are really curious as to what Unicode characters are "letters" as far as Java is concerned, you can use the isJavaIdentifierStart and isJavaIdentifierPart methods in the Character class to check.
Tip
Even though $ is a valid Java letter, you should not use it in your own code. It is intended for names that are generated by the Java compiler and other tools.

You also cannot use a Java reserved word for a variable name. (See the Appendix for a list of reserved words.)

You can have multiple declarations on a single line:

int i, j; // both are integers

However, we don’t recommend this style. If you declare each variable separately, your programs are easier to read.

Note

As you saw, names are case sensitive, for example, hireday and hireDay are two separate names. In general, you should not have two names that only differ in their letter case. However, sometimes it is difficult to come up with a good name for a variable. Many programmers then give the variable the same name as the type, for example
Box box; // "Box" is the type and "box" is the variable name
Other programmers prefer to use an “a” prefix for the variable:
Box aBox;

3.4.1. Initializing Variables
After you declare a variable, you must explicitly initialize it by means of an assignment statement—you can never use the value of an uninitialized variable（声明一个变量后必须要初始化）. For example, the Java compiler flags the following sequence of statements as an error:

int vacationDays;
System.out.println(vacationDays); // ERROR--variable not initialized

You assign to a previously declared variable by using the variable name on the left, an equal sign (=), and then some Java expression with an appropriate value on the right.

int vacationDays;
vacationDays = 12;

You can both declare and initialize a variable on the same line. For example:

int vacationDays = 12;

Finally, in Java you can put declarations anywhere in your code（可以在任意地方声明变量）. For example, the following is valid code in Java:

double salary = 65000.0;
System.out.println(salary);
int vacationDays = 12; // OK to declare a variable here

In Java, it is considered good style to declare variables as closely as possible to the point where they are first used.

C++ Note
C and C++ distinguish between the declaration and definition of a variable. For example,
int i = 10;
is a definition, whereas
extern int i;
is a declaration. In Java, no declarations are separate from definitions.

3.4.2. Constants
In Java, you use the keyword final to denote a constant（使用final关键字来声明产量）. For example:

public class Constants
{
    public static void main(String[] args)
    { 
        final double CM_PER_INCH = 2.54;
        double paperWidth = 8.5;
        double paperHeight = 11;
        System.out.println("Paper size in centimeters: "+ paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH);
    }
}

The keyword final indicates that you can assign to the variable once, and then its value is set once and for all. It is customary to name constants in all uppercase.
It is probably more common in Java to create a constant so it's available to multiple methods inside a single class. These are usually called class constants（类常量）. Set up a class constant with the keywords static final. Here is an example of using a class constant:

public class Constants2
{
    public static final double CM_PER_INCH = 2.54;
    public static void main(String[] args)
    {
        double paperWidth = 8.5;
        double paperHeight = 11;
        System.out.println("Paper size in centimeters: "+ paperWidth * CM_PER_INCH + " by " + paperHeight * CM_PER_INCH);
    }
}

Note that the definition of the class constant appears outside the main method. Thus, the constant can also be used in other methods of the same class. Furthermore, if the constant is declared, as in our example, public, methods of other classes can also use it—in our example, as Constants2.CM_PER_INCH.

C++ Note
const is a reserved Java keyword, but it is not currently used for anything（const是Java中的一个关键字，但是没有使用）. You must use final for a constant.

Core Java Volume I — 3.4. Variables

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