Data Types - Java Tutorial 5.0 英文版

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Data Types

Every variable must have a data type. A variable's data type determines the values the variable can contain and the operations that can be performed on it. For example, in the MaxVariablesDemo program, the declaration int largestInteger declares that largestInteger has an integer data type (int). Integers can contain only integral values (both positive and negative). You can perform arithmetic operations, such as addition, on integer variables.

The Java programming language has two categories of data types — primitive and reference. A variable of primitive type contains a single value of the appropriate size and format for its type: a number, a character, or a boolean value (see the following figure). For example, an integer value is 32 bits of data in a format known as two's complement, the value of a char is 16 bits of data formatted as a Unicode character, and so on.

A variable of primitive type contains a value of a particular size and format.

The following table lists, by keyword, all the primitive data types supported by the Java platform, their sizes and formats, and a brief description of each. The MaxVariablesDemo program declares one variable of each primitive type.

Primitive Data Types

Keyword	Description	Size/Format
Integers
byte	Byte-length integer	8-bit two's complement
short	Short integer	16-bit two's complement
int	Integer	32-bit two's complement
long	Long integer	64-bit two's complement
Real numbers
float	Single-precision floating point	32-bit IEEE 754
double	Double-precision floating point	64-bit IEEE 754
Other types
char	A single character	16-bit Unicode character
boolean	A boolean value (true or false)	true or false

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Purity Tip: In other programming languages, the format and the size of primitive data types can depend on the system on which the program is running. In contrast, the Java programming language specifies the size and the format of its primitive data types. Hence, you don't have to worry about system dependencies.

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You can put a literal primitive value directly in your code. For example, if you need to assign the value 4 to an integer variable you can write this:

int anInt = 4;

The digit 4 is a literal integer value. The following table gives some examples of literal values for various primitive types.

Examples of Literal Values

Literal Value	Data Type
178	int
8864L	long
37.266	double
37.266D	double
87.363F	float
26.77e3	double
'c'	char
true	boolean
false	boolean

Generally speaking, a series of digits with no decimal point is typed as an integer. You can specify a long integer by putting an 'L' or 'l' after the number. 'L' is preferred because it cannot be confused with the digit '1'. A series of digits with a decimal point is of type double. You can specify a float by putting an 'f' or 'F' after the number. A literal character value is any single Unicode character between single quote marks. The two boolean literals are simply true and false.

Arrays, classes, and interfaces are reference types. The value of a reference type variable, in contrast to that of a primitive type, is a reference to (an address of) the value or set of values represented by the variable (see the following figure). A reference is called a pointer, or a memory address in other languages. The Java programming language does not support the explicit use of addresses like other languages do; you need to use the variable's name instead.

A variable of reference type contains a reference to (an address of) an object or an array.

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