转自:http://www.hollischuang.com/archives/61
Java7中switch中支持的数据类型有: byte short int char String类型
其实switch是只支持一种数据类型,那就是整型:
1. byte short int 本身就是整型,可以直接按照整型进行比较
2. char的字符会依照ascii表,转换为对应的整型,然后进行switch条件选择
3. String的字符串会 先将字符串转换为hash值, 然后再对字符串进行equals方法比较, 以此来完成switch的条件选择
因为有ascii表存在,所以char类型和整数型可以进行转换, 也就是char类型可以直接赋值为整数型,整数型也可以直接赋值为字符型(无法转换的情况还没去归纳)
都说代码知识都是敲出来的,于是每种情况我都对比了下,如下图:
字节类型代码
这是字节类型的java源码,因为有字码表的存在,所以整数类型的变量也可以直接赋值字符
package compiler.demobyte;public class Demo01 { public static void main(String[] args) { byte byt1 = 'a'; switch (byt1) { case 'a': System.out.println("hello"); break; case 'b': System.out.println("world"); break; default: break; } byte byt2 = 97; switch (byt2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); break; default: break; } }} 这是.class文件
//// Source code recreated from a .class file by IntelliJ IDEA// (powered by Fernflower decompiler)//package compiler.demobyte;public class Demo01 { public Demo01() { } public static void main(String[] args) { byte byt1 = 97; switch(byt1) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } byte byt2 = 97; switch(byt2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } }}
short类型代码
short类型与byte类型是一样的
package compiler.demoshort;public class Demo01 { public static void main(String[] args) { short sht = 'a'; switch (sht) { case 'a': System.out.println("hello"); break; case 'b': System.out.println("world"); break; default: break; } short sht2 = 97; switch (sht2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); break; default: break; } }} 这是.clas文件
//// Source code recreated from a .class file by IntelliJ IDEA// (powered by Fernflower decompiler)//package compiler.demoshort;public class Demo01 { public Demo01() { } public static void main(String[] args) { short sht = 97; switch(sht) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } short sht2 = 97; switch(sht2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } }}
int类型也是一样的,没有进行测试
char类型代码
package compiler.demochar;public class Demo01 { public static void main(String[] args) { char ch = 'a'; switch (ch) { case 'a': System.out.println("hello"); break; case 'b': System.out.println("world"); break; default: break; } char ch2 = 97; switch (ch2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); break; default: break; } }} 生成.class文件
//// Source code recreated from a .class file by IntelliJ IDEA// (powered by Fernflower decompiler)//package compiler.demochar;public class Demo01 { public Demo01() { } public static void main(String[] args) { char ch = 97; switch(ch) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } char ch2 = 97; switch(ch2) { case 97: System.out.println("hello"); break; case 98: System.out.println("world"); } }}
String 类型代码
String会稍微特殊一些,分两步进行转换:
第一步: 先将字符串转换为哈希值,进行条件选择. 然后因为存在哈希值碰撞的问题, 所以在条件选择成功后, 又使用了equals方法进行内容的比较.
哈希值的表现形式是 int整型
所以实质上: String类型也是视为是int整型的比较.
java代码如下:
package compiler.demostring;public class Demo01 { public static void main(String[] args) { String str = "wor"; switch (str) { case "hello": System.out.println("hello"); break; case "world": System.out.println("world"); break; default: break; } }} .clas文件
//// Source code recreated from a .class file by IntelliJ IDEA// (powered by Fernflower decompiler)//package compiler.demostring;public class Demo01 { public Demo01() { } public static void main(String[] args) { String str = "wor"; byte var3 = -1; switch(str.hashCode()) { case 99162322: if (str.equals("hello")) { var3 = 0; } break; case 113318802: if (str.equals("world")) { var3 = 1; } } switch(var3) { case 0: System.out.println("hello"); break; case 1: System.out.println("world"); } }}
综上所述: 又回归到我们的结论: 其实switch是只支持一种数据类型,那就是整型.