Java 简明教程

Java - Data Types

Data types 定义不同 types of variables 、常量、 method 参数、返回类型等数据类型和取值范围。数据类型告知编译器要存储的数据类型和所需的内存。为了存储和处理不同类型的数据，所有变量都必须具有指定的数据类型。

Data types define the type and value range of the data for the different types of variables, constants, method parameters, returns type, etc. The data type tells the compiler about the type of data to be stored and the required memory. To store and manipulate different types of data, all variables must have specified data types.

Java 数据类型分为两部分：

Java data types are categorized into two parts −

Primitive Data Types
Reference/Object Data Types

Java Primitive Data Types

基本数据类型由语言预定义并由关键字命名。Java 支持八种基本数据类型。以下是基本数据类型的列表：

Primitive data types are predefined by the language and named by a keyword. There are eight primitive data types supported by Java. Below is the list of the primitive data types:

byte
short
int
long
float
double
boolean

Java byte Data Type

Byte data type is an 8-bit signed two’s complement integer
Minimum value is -128 (-2^7)
Maximum value is 127 (inclusive)(2^7 -1)
Default value is 0
Byte data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an integer.
Example − byte a = 100, byte b = -50

Java short Data Type

Short data type is a 16-bit signed two’s complement integer
Minimum value is -32,768 (-2^15)
Maximum value is 32,767 (inclusive) (2^15 -1)
Short data type can also be used to save memory as byte data type. A short is 2 times smaller than an integer
Default value is 0.
Example − short s = 10000, short r = -20000

Java int Data Type

Int data type is a 32-bit signed two’s complement integer.
Minimum value is - 2,147,483,648 (-2^31)
Maximum value is 2,147,483,647(inclusive) (2^31 -1)
Integer is generally used as the default data type for integral values unless there is a concern about memory.
The default value is 0
Example − int a = 100000, int b = -200000

Java long Data Type

Long data type is a 64-bit signed two’s complement integer
Minimum value is -9,223,372,036,854,775,808(-2^63)
Maximum value is 9,223,372,036,854,775,807 (inclusive)(2^63 -1)
This type is used when a wider range than int is needed
Default value is 0L
Example − long a = 100000L, long b = -200000L

Java float Data Type

Float data type is a single-precision 32-bit IEEE 754 floating point
Float is mainly used to save memory in large arrays of floating point numbers
Default value is 0.0f
Float data type is never used for precise values such as currency
Example − float f1 = 234.5f

Java double Data Type

double data type is a double-precision 64-bit IEEE 754 floating point
This data type is generally used as the default data type for decimal values, generally the default choice
Double data type should never be used for precise values such as currency
Default value is 0.0d
Example − double d1 = 123.4

Java boolean Data Type

boolean data type represents one bit of information
There are only two possible values: true and false
This data type is used for simple flags that track true/false conditions
Default value is false
Example − boolean one = true

Java char Data Type

char data type is a single 16-bit Unicode character
Minimum value is '\u0000' (or 0)
Maximum value is '\uffff' (or 65,535 inclusive)
Char data type is used to store any character
Example − char letterA = 'A'

Example: Demonstrating Different Primitive Data Types

以下示例展示了上面讨论的各种原始数据类型的用法。我们在数字数据类型上使用了 add 运算，而布尔变量和 char 变量则按原样打印。

Following examples shows the usage of variour primitive data types we’ve discussed above. We’ve used add operations on numeric data types whereas boolean and char variables are printed as such.

public class JavaTester {
   public static void main(String args[]) {

      byte byteValue1 = 2;
      byte byteValue2 = 4;
      byte byteResult = (byte)(byteValue1 + byteValue2);

      System.out.println("Byte: " + byteResult);

      short shortValue1 = 2;
      short shortValue2 = 4;
      short shortResult = (short)(shortValue1 + shortValue2);

      System.out.println("Short: " + shortResult);

      int intValue1 = 2;
      int intValue2 = 4;
      int intResult = intValue1 + intValue2;

      System.out.println("Int: " + intResult);

      long longValue1 = 2L;
      long longValue2 = 4L;
      long longResult = longValue1 + longValue2;

      System.out.println("Long: " + longResult);

      float floatValue1 = 2.0f;
      float floatValue2 = 4.0f;
      float floatResult = floatValue1 + floatValue2;

      System.out.println("Float: " + floatResult);

      double doubleValue1 = 2.0;
      double doubleValue2 = 4.0;
      double doubleResult = doubleValue1 + doubleValue2;

      System.out.println("Double: " + doubleResult);

      boolean booleanValue = true;

      System.out.println("Boolean: " + booleanValue);

      char charValue = 'A';

      System.out.println("Char: " + charValue);
   }
}

Byte: 6
Short: 6
Int: 6
Long: 6
Float: 6.0
Double: 6.0
Boolean: true
Char: A

Java Reference/Object Data Type

使用 classes 的已定义 constructors 创建引用数据类型。它们用于访问对象。将这些变量声明为不能更改的特定类型。例如，Employee、Puppy 等。

The reference data types are created using defined constructors of the classes. They are used to access objects. These variables are declared to be of a specific type that cannot be changed. For example, Employee, Puppy, etc.

类对象和各种类型的数组变量属于引用数据类型。任何引用变量的默认值为 null。引用变量可用于引用所声明类型或任何兼容类型的任何对象。

Class objects and various types of array variables come under reference datatype. The default value of any reference variable is null. A reference variable can be used to refer to any object of the declared type or any compatible type.

Example

以下示例演示引用（或对象）数据类型。

The following example demonstrates the reference (or, object) data types.

// Creating an object of 'Animal' class
Animal animal = new Animal("giraffe");

// Creating an object of 'String' class
String myString = new String("Hello, World!");