Java switch to achieve a thorough understanding of the principles of

Java official document describes:

A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types, the String class, and a few special classes that wrap certain primitive types: Character, Byte, Short, and Integer

In the basic data types byte / char

public class SwitchBasicTest {
    
    private int switchByte(byte i) {
        int ret;
        switch (i) {
            case 0:
                ret = 10;
                break;
            case 1:
                ret = 11;
                break;
            default:
                ret = 0;
        }

        return ret;
    }
    private int switchChar(char i) {
        int ret;
        switch (i) {
            case 0:
                ret = 10;
                break;
            case 1:
                ret = 11;
                break;
            case 'a':
                ret = 20;
                break;
            case '写':
                ret = 21;
                break;
            default:
                ret = 0;
        }

        return ret;
    }
}
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Javap -v or use other tools to decompile SwitchBasicTest.class, as used herein, IDEA plug-injclasslib Bytecode viewer

Decompile obtain switchBytebytecodes method:

 0 iload_1
 1 lookupswitch 2
        0:  28 (+27)
        1:  34 (+33)
        default:  40 (+39)
28 bipush 10
30 istore_2
31 goto 42 (+11)
34 bipush 11
36 istore_2
37 goto 42 (+5)
40 iconst_0
41 istore_2
42 iload_2
43 ireturn

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switchChar The method bytecode:

 0 iload_1
 1 lookupswitch 4
        0:  44 (+43)
        1:  50 (+49)
        97:  56 (+55)
        20889:  62 (+61)
        default:  68 (+67)
44 bipush 10
46 istore_2
47 goto 70 (+23)
50 bipush 11
52 istore_2
53 goto 70 (+17)
56 bipush 20
58 istore_2
59 goto 70 (+11)
62 bipush 21
64 istore_2
65 goto 70 (+5)
68 iconst_0
69 istore_2
70 iload_2
71 ireturn

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It is known for byteother basic data types, switchthe direct use integer comparison

byte, short value can be transferred directly whole
char takes unicodeinteger code value
Integer and other packaging types, calls the intValuemethod of obtaining the value of the integer variable

In the role of String (version JDK1.7 and later)

public class SwitchBasicTest {
    private int switchString(String i) {
            int ret;
            switch (i) {
                case "0":
                    ret = 10;
                    break;
                case "1":
                    ret = 11;
                    break;
                case "a":
                    ret = 20;
                    break;
                default:
                    ret = 0;
            }
    
            return ret;
        }
}
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Use IDEA editor, open the file .class see decompile the code:

public class SwitchBasicTest {
    private int switchString(String i) {
        byte var4 = -1;
        switch(i.hashCode()) {
        case 48:
            if (i.equals("0")) {
                var4 = 0;
            }
            break;
        case 49:
            if (i.equals("1")) {
                var4 = 1;
            }
            break;
        case 97:
            if (i.equals("a")) {
                var4 = 2;
            }
        }

        byte ret;
        switch(var4) {
        case 0:
            ret = 10;
            break;
        case 1:
            ret = 11;
            break;
        case 2:
            ret = 20;
            break;
        default:
            ret = 0;
        }

        return ret;
    }
}
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Decompile obtained switchStringbytecodes:

  0 aload_1
  1 astore_3
  2 iconst_m1
  3 istore 4
  5 aload_3
  6 invokevirtual #8 <java/lang/String.hashCode>
  9 lookupswitch 3
        48:  44 (+35)
        49:  59 (+50)
        97:  74 (+65)
        default:  86 (+77)
 44 aload_3
 45 ldc #9 <0>
 47 invokevirtual #10 <java/lang/String.equals>
 50 ifeq 86 (+36)
 53 iconst_0
 54 istore 4
 56 goto 86 (+30)
 59 aload_3
 60 ldc #11 <1>
 62 invokevirtual #10 <java/lang/String.equals>
 65 ifeq 86 (+21)
 68 iconst_1
 69 istore 4
 71 goto 86 (+15)
 74 aload_3
 75 ldc #12 <a>
 77 invokevirtual #10 <java/lang/String.equals>
 80 ifeq 86 (+6)
 83 iconst_2
 84 istore 4
 86 iload 4
 88 tableswitch 0 to 2	
        0:  116 (+28)
        1:  122 (+34)
        2:  128 (+40)
        default:  134 (+46)
116 bipush 10
118 istore_2
119 goto 136 (+17)
122 bipush 11
124 istore_2
125 goto 136 (+11)
128 bipush 20
130 istore_2
131 goto 136 (+5)
134 iconst_0
135 istore_2
136 iload_2
137 ireturn
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We can see the switcheffect in Stringthe final split into two for integer switchstatements, specific procedures:

Calculated and compared hashcode
If the same hashcode, it is further used equalsto determine the content of the same string, treated in the same case where two strings hashcode

Acting under enumeration

public enum SeasonEnum {
    SPRING, SUMMER, AUTUMN, WINTER
}

public class SwitchEnumTest {
   
    private static int testSpring(SeasonEnum seasonEnum) {
        int ret;
        switch (seasonEnum) {
            case AUTUMN:
                ret = 1;
                break;
            case SPRING:
                ret = 2;
                break;
            case SUMMER:
                ret = 3;
                break;
            case WINTER:
                ret = 4;
                break;
            default:
                ret = 5;
        }
        return ret;
    }
}
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Decompile obtain testSpringbytecodes method:

 0 getstatic #6 <xx/SwitchEnumTest$1.$SwitchMap$xx$enumt$SeasonEnum>
 3 aload_0
 4 invokevirtual #7 <xx/enumt/SeasonEnum.ordinal>
 7 iaload
 8 tableswitch 1 to 4	1:  40 (+32)
        2:  45 (+37)
        3:  50 (+42)
        4:  55 (+47)
        default:  60 (+52)
40 iconst_1
41 istore_1
42 goto 62 (+20)
45 iconst_2
46 istore_1
47 goto 62 (+15)
50 iconst_3
51 istore_1
52 goto 62 (+10)
55 iconst_4
56 istore_1
57 goto 62 (+5)
60 iconst_5
61 istore_1
62 iload_1
63 ireturn
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can be seen: 4 invokevirtual #7 <xx/enumt/SeasonEnum.ordinal>

That is, switchwhen acting on the enumeration, enumeration calls the ordinalmethod that ultimately intvalue comparison

lookupswitch 和 tableswitch

tableswitch Using an array data structure can be positioned by the jump destination line of the subscripts
lookupswitch The number of lines to maintain the relationship between a key-value, a jump by one by comparing the index to find matches to be of
tableswitchThan lookupswitchbetter lookup performance
switchStatement casewhen the branch condition value sparse, tableswitchspatial low usage instructions, may be used in this case lookupswitchinstruction

in conclusion

switch The syntax is actually only valid for integer parameters
breakAt the bytecode level generates a gotostatement casewithout tape breakdirectly to the next time a caselogical
switchActing on the enumeration actually uses the enumeration ordinalmethod returns a value as a basis for comparison
switchActing on the string when the character string hashcodeas a basis for comparison, hashcodethe same is further used equalsto compare strings content
The java virtual machine specification, java virtual machine tableswitchand the lookupswitchinstructions only support an int conditions, it switchdoes not support longLong