Java语言提供了八种基本类型WW

变量就是申请内存来存储值。也就是说，当创建变量的时候，需要在内存中申请空间。

内存管理系统根据变量的类型为变量分配存储空间，分配的空间只能用来储存该类型数据。

因此，通过定义不同类型的变量，可以在内存中储存整数、小数或者字符。

Java 的两大数据类型:

• 内置数据类型
• 引用数据类型

内置数据类型

Java语言提供了八种基本类型。六种数字类型（四个整数型，两个浮点型），一种字符类型，还有一种布尔型。

byte：

• byte 数据类型是8位、有符号的，以二进制补码表示的整数；
• 最小值是 -128（-2^7）；
• 最大值是 127（2^7-1）；
• 默认值是 0；
• byte 类型用在大型数组中节约空间，主要代替整数，因为 byte 变量占用的空间只有 int 类型的四分之一；
• 例子：byte a = 100，byte b = -50。

short：

• short 数据类型是 16 位、有符号的以二进制补码表示的整数
• 最小值是 -32768（-2^15）；
• 最大值是 32767（2^15 – 1）；
• Short 数据类型也可以像 byte 那样节省空间。一个short变量是int型变量所占空间的二分之一；
• 默认值是 0；
• 例子：short s = 1000，short r = -20000。

int：

• int 数据类型是32位、有符号的以二进制补码表示的整数；
• 最小值是 -2,147,483,648（-2^31）；
• 最大值是 2,147,483,647（2^31 – 1）；
• 一般地整型变量默认为 int 类型；
• 默认值是 0 ；
• 例子：int a = 100000, int b = -200000。

long：

• long 数据类型是 64 位、有符号的以二进制补码表示的整数；
• 最小值是 -9,223,372,036,854,775,808（-2^63）；
• 最大值是 9,223,372,036,854,775,807（2^63 -1）；
• 这种类型主要使用在需要比较大整数的系统上；
• 默认值是 0L；
• 例子： long a = 100000L，long b = -200000L。 "L"理论上不分大小写，但是若写成"l"容易与数字"1"混淆，不容易分辩。所以最好大写。

float：

• float 数据类型是单精度、32位、符合IEEE 754标准的浮点数；
• float 在储存大型浮点数组的时候可节省内存空间；
• 默认值是 0.0f；
• 浮点数不能用来表示精确的值，如货币；
• 例子：float f1 = 234.5f。

double：

• double 数据类型是双精度、64 位、符合 IEEE 754 标准的浮点数；
• 浮点数的默认类型为 double 类型；
• double类型同样不能表示精确的值，如货币；
• 默认值是 0.0d；

• 例子：

  double d1 = 7D ;
  double d2 = 7.;
  double d3 = 8.0;
  double d4 = 8.D;
  double d5 = 12.9867;

  7 是一个 int 字面量，而 7D，7. 和 8.0 是 double 字面量。

boolean：

• boolean数据类型表示一位的信息；
• 只有两个取值：true 和 false；
• 这种类型只作为一种标志来记录 true/false 情况；
• 默认值是 false；
• 例子：boolean one = true。

char：

• char 类型是一个单一的 16 位 Unicode 字符；
• 最小值是 \\u0000（十进制等效值为 0）；
• 最大值是 \\uffff（即为 65535）；
• char 数据类型可以储存任何字符；
• 例子：char letter = 'A';。

实例

对于数值类型的基本类型的取值范围，我们无需强制去记忆，因为它们的值都已经以常量的形式定义在对应的包装类中了。请看下面的例子：

实例

public class PrimitiveTypeTest { public static void main(String[] args) { // byte System.out.println("基本类型：byte 二进制位数：" + Byte.SIZE); System.out.println("包装类：java.lang.Byte"); System.out.println("最小值：Byte.MIN_VALUE=" + Byte.MIN_VALUE); System.out.println("最大值：Byte.MAX_VALUE=" + Byte.MAX_VALUE); System.out.println(); // short System.out.println("基本类型：short 二进制位数：" + Short.SIZE); System.out.println("包装类：java.lang.Short"); System.out.println("最小值：Short.MIN_VALUE=" + Short.MIN_VALUE); System.out.println("最大值：Short.MAX_VALUE=" + Short.MAX_VALUE); System.out.println(); // int System.out.println("基本类型：int 二进制位数：" + Integer.SIZE); System.out.println("包装类：java.lang.Integer"); System.out.println("最小值：Integer.MIN_VALUE=" + Integer.MIN_VALUE); System.out.println("最大值：Integer.MAX_VALUE=" + Integer.MAX_VALUE); System.out.println(); // long System.out.println("基本类型：long 二进制位数：" + Long.SIZE); System.out.println("包装类：java.lang.Long"); System.out.println("最小值：Long.MIN_VALUE=" + Long.MIN_VALUE); System.out.println("最大值：Long.MAX_VALUE=" + Long.MAX_VALUE); System.out.println(); // float System.out.println("基本类型：float 二进制位数：" + Float.SIZE); System.out.println("包装类：java.lang.Float"); System.out.println("最小值：Float.MIN_VALUE=" + Float.MIN_VALUE); System.out.println("最大值：Float.MAX_VALUE=" + Float.MAX_VALUE); System.out.println(); // double System.out.println("基本类型：double 二进制位数：" + Double.SIZE); System.out.println("包装类：java.lang.Double"); System.out.println("最小值：Double.MIN_VALUE=" + Double.MIN_VALUE); System.out.println("最大值：Double.MAX_VALUE=" + Double.MAX_VALUE); System.out.println(); // char System.out.println("基本类型：char 二进制位数：" + Character.SIZE); System.out.println("包装类：java.lang.Character"); // 以数值形式而不是字符形式将Character.MIN_VALUE输出到控制台 System.out.println("最小值：Character.MIN_VALUE=" + (int) Character.MIN_VALUE); // 以数值形式而不是字符形式将Character.MAX_VALUE输出到控制台 System.out.println("最大值：Character.MAX_VALUE=" + (int) Character.MAX_VALUE); } }

运行实例 »

编译以上代码输出结果如下所示：

基本类型：byte 二进制位数：8
包装类：java.lang.Byte
最小值：Byte.MIN_VALUE=-128
最大值：Byte.MAX_VALUE=127

基本类型：short 二进制位数：16
包装类：java.lang.Short
最小值：Short.MIN_VALUE=-32768
最大值：Short.MAX_VALUE=32767

基本类型：int 二进制位数：32
包装类：java.lang.Integer
最小值：Integer.MIN_VALUE=-2147483648
最大值：Integer.MAX_VALUE=2147483647

基本类型：long 二进制位数：64
包装类：java.lang.Long
最小值：Long.MIN_VALUE=-9223372036854775808
最大值：Long.MAX_VALUE=9223372036854775807

基本类型：float 二进制位数：32
包装类：java.lang.Float
最小值：Float.MIN_VALUE=1.4E-45
最大值：Float.MAX_VALUE=3.4028235E38

基本类型：double 二进制位数：64
包装类：java.lang.Double
最小值：Double.MIN_VALUE=4.9E-324
最大值：Double.MAX_VALUE=1.7976931348623157E308

基本类型：char 二进制位数：16
包装类：java.lang.Character
最小值：Character.MIN_VALUE=0
最大值：Character.MAX_VALUE=65535

Float和Double的最小值和最大值都是以科学记数法的形式输出的，结尾的"E+数字"表示E之前的数字要乘以10的多少次方。比如3.14E3就是3.14 × 103 =3140，3.14E-3 就是 3.14 x 10-3 =0.00314。

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实际上，JAVA中还存在另外一种基本类型 void，它也有对应的包装类 java.lang.Void，不过我们无法直接对它们进行操作。

类型默认值

下表列出了 Java 各个类型的默认值：

说明：

• int, short, long, byte 的默认值是0。
• char 的默认值是 \\u0000（空字符）。
• float 的默认值是 0.0f。
• double 的默认值是 0.0d。
• boolean 的默认值是 false。
• 引用类型（类、接口、数组）的默认值是 null。

实例

public class Test { static boolean bool; static byte by; static char ch; static double d; static float f; static int i; static long l; static short sh; static String str; public static void main(String[] args) { System.out.println("Bool :" + bool); System.out.println("Byte :" + by); System.out.println("Character:" + ch); System.out.println("Double :" + d); System.out.println("Float :" + f); System.out.println("Integer :" + i); System.out.println("Long :" + l); System.out.println("Short :" + sh); System.out.println("String :" + str); } }

实例输出结果为：

Bool :false
Byte :0
Character:
Double :0.0
Float :0.0
Integer :0
Long :0
Short :0
String :null

引用类型

• 在Java中，引用类型的变量非常类似于C/C++的指针。引用类型指向一个对象，指向对象的变量是引用变量。这些变量在声明时被指定为一个特定的类型，比如 Employee、Puppy 等。变量一旦声明后，类型就不能被改变了。
• 对象、数组都是引用数据类型。
• 所有引用类型的默认值都是null。
• 一个引用变量可以用来引用任何与之兼容的类型。
• 例子：Site site = new Site("Runoob")。

Java 常量

常量在程序运行时是不能被修改的。

在 Java 中使用 final 关键字来修饰常量，声明方式和变量类似：

final double PI = 3.1415927;

虽然常量名也可以用小写，但为了便于识别，通常使用大写字母表示常量。

字面量可以赋给任何内置类型的变量。例如：

byte a = 68;
char a = 'A'

byte、int、long、和short都可以用十进制、16进制以及8进制的方式来表示。

当使用字面量的时候，前缀 0 表示 8 进制，而前缀 0x 代表 16 进制, 例如：

int decimal = 100;
int octal = 0144;
int hexa = 0x64;

和其他语言一样，Java的字符串常量也是包含在两个引号之间的字符序列。下面是字符串型字面量的例子：

"Hello World"
"two\\nlines"
"\\"This is in quotes\\""

字符串常量和字符变量都可以包含任何 Unicode 字符。例如：

char a = '\\u0001';
String a = "\\u0001";

Java语言支持一些特殊的转义字符序列。

自动类型转换

整型、实型（常量）、字符型数据可以混合运算。运算中，不同类型的数据先转化为同一类型，然后进行运算。

转换从低级到高级。

低 ————————————> 高

byte,short,char—> int —> long—> float —> double

数据类型转换必须满足如下规则：

• 1. 不能对boolean类型进行类型转换。

• 2. 不能把对象类型转换成不相关类的对象。

• 3. 在把容量大的类型转换为容量小的类型时必须使用强制类型转换。

• 4. 转换过程中可能导致溢出或损失精度，例如：

  int i =128;
  byte b = (byte)i;

  因为 byte 类型是 8 位，最大值为127，所以当 int 强制转换为 byte 类型时，值 128 时候就会导致溢出。

• 5. 浮点数到整数的转换是通过舍弃小数得到，而不是四舍五入，例如：

  (int)23.7 == 23;        
  (int)-45.89f == -45

自动类型转换

必须满足转换前的数据类型的位数要低于转换后的数据类型，例如: short数据类型的位数为16位，就可以自动转换位数为32的int类型，同样float数据类型的位数为32，可以自动转换为64位的double类型。

实例

public class ZiDongLeiZhuan{ public static void main(String[] args){ char c1='a';//定义一个char类型 int i1 = c1;//char自动类型转换为int System.out.println("char自动类型转换为int后的值等于"+i1); char c2 = 'A';//定义一个char类型 int i2 = c2+1;//char 类型和 int 类型计算 System.out.println("char类型和int计算后的值等于"+i2); } }

运行结果为:

char自动类型转换为int后的值等于97
char类型和int计算后的值等于66

解析：c1 的值为字符 a ,查 ASCII 码表可知对应的 int 类型值为 97， A 对应值为 65，所以 i2=65+1=66。

强制类型转换

• 1. 条件是转换的数据类型必须是兼容的。

• 2. 格式：(type)value type是要强制类型转换后的数据类型 实例：

  实例

  public class ForceTransform { public static void main(String[] args){ int i1 = 123; byte b = (byte)i1;//强制类型转换为byte System.out.println("int强制类型转换为byte后的值等于"+b); } }

  运行结果：

  int强制类型转换为byte后的值等于123

隐含强制类型转换

• 1、 整数的默认类型是 int。

• 2、 小数默认是 double 类型浮点型，在定义 float 类型时必须在数字后面跟上 F 或者 f。

这一节讲解了 Java 的基本数据类型。下一节将探讨不同的变量类型以及它们的用法。