switch可以进行跳转优化，java中对switch有两种处理方式，生成不同的jvm指令，一是tableswitch，一个是lookupswitch.
对于case的分支比较密集的情况，如
Java code
public class Test {    public static void main(String[] args) {        int i = 3;        switch (i) {            case 0:                System.out.println("0");                break;            case 1:                System.out.println("1");                break;            case 3:                System.out.println("3");                break;            case 5:                System.out.println("5");                break;            case 10:                System.out.println("10");                break;            case 13:                System.out.println("13");                break;            case 14:                System.out.println("14");                break;        default:        System.out.println("default");                break;        }    }}

使用tableswitch,得到
< dl class="code" style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; width: 718px; overflow-x: auto; overflow-y: auto; font-size: 12px; ">
Java code
public static void main(java.lang.String[]);  Code:   0:    iconst_3   1:    istore_1   2:    iload_1   3:    tableswitch{ //0 to 14        0: 76;        1: 87;        2: 153;        3: 98;        4: 153;        5: 109;        6: 153;        7: 153;        8: 153;        9: 153;        10: 120;        11: 153;        12: 153;        13: 131;        14: 142;        default: 153 }   76:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   79:    ldc    #3; //String 0   81:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   84:    goto    161   87:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   90:    ldc    #5; //String 1   92:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   95:    goto    161   98:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   101:    ldc    #6; //String 3   103:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   106:    goto    161   109:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   112:    ldc    #7; //String 5   114:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   117:    goto    161   120:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   123:    ldc    #8; //String 10   125:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   128:    goto    161   131:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   134:    ldc    #9; //String 13   136:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   139:    goto    161   142:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   145:    ldc    #10; //String 14   147:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   150:    goto    161   153:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   156:    ldc    #11; //String default   158:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   161:    return}

从中可以看到tableswitch使用的跳转表。它这样查找，如果case值不在//0 to 14之间，直接执行default,如果在此范围之内，则取目标值-0这一项作为目标，比如switch(i),i为３，则跳转到3-0=3,使用数组中的第三项作为目标，也就是3: 98;直接去执行９８行。

如果case中的值比较稀疏，则使用lookupswitch:
Java code
public class Test2 {    public static void main(String[] args) {        int i = 3;        switch (i) {            case 3:                System.out.println("3");                break;            case 20:                System.out.println("20");                break;            case 50:                System.out.println("50");                break;            case 100:                System.out.println("100");                break;        }    }}

编译为
Java code
public static void main(java.lang.String[]);  Code:   0:    iconst_3   1:    istore_1   2:    iload_1   3:    lookupswitch{ //4        3: 44;        20: 55;        50: 66;        100: 77;        default: 85 }   44:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   47:

    ldc    #3; //String 3   49:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   52:    goto    85   55:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   58:    ldc    #5; //String 20   60:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   63:    goto    85   66:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   69:    ldc    #6; //String 50   71:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   74:    goto    85   77:    getstatic    #2; //Field java/lang/System.out:Ljava/io/PrintStream;   80:    ldc    #7; //String 100   82:    invokevirtual    #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V   85:    return

可以看到其中的
 3: lookupswitch{ //4
3: 44;
20: 55;
50: 66;
100: 77;
default: 85 }
这个就要挨着查表确定跳转位置了。