为什么Java中的switchcase语句这样工作？

但是Java在C中有一些起源,这样做也是一样的,在80年代,这个决定被解释给我,因为那时编译器可以把开关转成一个跳转表：基本上每个代码块的地址都放在表和交换机成为范围检查,后跟表查找(通常使用您传入的值来索引到数组或至少链接数组列表)以获取地址,然后跳转到该地址.在这种情况下,只有整数才有意义.记住,电脑并不总是像现在那样快. C是在70年代初基于60年代末期的电脑设计的,电脑慢得多.

与Java和C相同的句法传统中的某些语言(如JavaScript)使得切换只是另一种写入方式,如果… else / if … else,并且不限制要检查的类型为整数类型,也许是因为在90年代被设计,成为一个现实的选择.或者也许只是因为JavaScript的设计者(Brendan Eich)喜欢这样.

下面,Baadshah问：

Out of curiosity : Then now how its supporting Strings ??? can you please give some sort of idea ?

首先,让我们回头看一下int的情况：

num = Integer.parseInt(args[0]);
  switch (num) {
      case 1:
          System.out.println("You used the special value one");
          break;
      case 42:
          System.out.println("You used the special value forty-two");
          break;
      case 67:
          System.out.println("You used the special value sixty-seven");
          break;
      default:
          System.out.println("You used the a non-special value " + num);
          break;
  }

这样生成字节码：

 19: iload_2       
 20: lookupswitch  { // 3
                1: 56
               42: 67
               67: 78
          default: 89
     }
 56: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
 59: ldc           #9   // String You used the special value one
 61: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
 64: goto          114
 67: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
 70: ldc           #11  // String You used the special value forty-two
 72: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
 75: goto          114
 78: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
 81: ldc           #12  // String You used the special value sixty-seven
 83: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
 86: goto          114
 89: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
 92: new           #13  // class java/lang/StringBuilder
 95: dup           
 96: invokespecial #14  // Method java/lang/StringBuilder."":()V
 99: ldc           #15  // String You used the a non-special value 
101: invokevirtual #16  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
104: iload_2       
105: invokevirtual #17  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
108: invokevirtual #18  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
111: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

我们可以看到在行动中的表查找.

那么你怎么用字符串呢？那么一个答案就是将开关转换成if … else if … else结构.但是他们做的比这更聪明：他们使用哈希码进行优化,然后使用equals来防止冲突：

switch (str) {
      case "abc":
          System.out.println("You used the special value 'abc'");
          break;
      case "def":
          System.out.println("You used the special value 'def'");
          break;
      case "ghi":
          System.out.println("You used the special value 'ghi'");
          break;
      default:
          System.out.println("You used the a non-special value '" + str + "'");
          break;
  }

变为：

124: aload         4
126: invokevirtual #19  // Method java/lang/String.hashCode:()I
129: lookupswitch  { // 3
            96354: 164
            99333: 180
           102312: 196
          default: 209
     }
164: aload         4
166: ldc           #20  // String abc
168: invokevirtual #21  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
171: ifeq          209
174: iconst_0      
175: istore        5
177: goto          209
180: aload         4
182: ldc           #22  // String def
184: invokevirtual #21  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
187: ifeq          209
190: iconst_1      
191: istore        5
193: goto          209
196: aload         4
198: ldc           #23  // String ghi
200: invokevirtual #21  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
203: ifeq          209
206: iconst_2      
207: istore        5
209: iload         5
211: tableswitch   { // 0 to 2
                0: 236
                1: 247
                2: 258
          default: 269
     }
236: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
239: ldc           #24  // String You used the special value 'abc'
241: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
244: goto          299
247: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
250: ldc           #25  // String You used the special value 'def'
252: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
255: goto          299
258: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
261: ldc           #26  // String You used the special value 'ghi'
263: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
266: goto          299
269: getstatic     #8   // Field java/lang/System.out:Ljava/io/PrintStream;
272: new           #13  // class java/lang/StringBuilder
275: dup           
276: invokespecial #14  // Method java/lang/StringBuilder."":()V
279: ldc           #27  // String You used the a non-special value '
281: invokevirtual #16  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
284: aload_3       
285: invokevirtual #16  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
288: ldc           #28  // String '
290: invokevirtual #16  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
293: invokevirtual #18  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
296: invokevirtual #10  // Method java/io/PrintStream.println:(Ljava/lang/String;)V

看看他们在那里做了什么现在基本上是两个开关：一个可以根据哈希码获得一个唯一的数字,但是用等号进行双重检查,然后是第二个发送.