Javaswitch⽀持类型详解
switch⽀持哪些类型？
（1）最早时，只⽀持int、char、byte、short这样的整型的基本类型或对应的包装类型Integer、Character、Byte、Short常量，包装类型最终也会经过拆箱为基本类型，本质上还是只⽀持基本类型
（2）JDK1.5开始⽀持enum，原理是给枚举值进⾏了内部的编号，进⾏编号和枚举值的映射
（3）JDK1.7开始⽀持String，但不允许为null，原理是借助 hashcode( ) 来实现。
官⽅⽂档中对于switch有如下描述。
Compilation of switch statements uses the tableswitch and lookupswitch instructions. The tableswitch instruction is used when the cases of the switch can be efficiently represented as indices into a table of target offsets. The default target of
the switch is used if the value of the expression of the switch falls outside the range of valid indices.
The Java Virtual Machine's tableswitch and lookupswitch instructions operate only on int data. Because operations
on byte, char, or short values are internally promoted to int, a switch whose expression evaluates to one of those types is compiled as though it evaluated to type int. If the chooseNear method had been written using type short, the same Java Virtual Machine instructions would have been generated as when using type int. Other numeric types must be narrowed to
type int for use in a switch.
Where the cases of the switch are sparse, the table representation of the tableswitch instruction becomes inefficient in terms of space. The lookupswitch instruction may be used instead. The lookupswitch instruction pairs int keys (the values of the case labels) with target offsets in a table. When a lookupswitch instruction is executed, the value of the expression of
the switch is compared against the keys in the table. If one of the keys matches the value of the expression, execution continues at the associated target offset. If no key matches, execution continues at the default target.
The Java Virtual Machine specifies that the table of the lookupswitch instruction must be sorted by key so that implementations may use searches more efficient than a linear scan. Even so, the lookupswitch instruction must search its keys for a match rather than simply perform a bounds check
and index into a table like tableswitch. Thus,
a tableswitch instruction is probably more efficient than a lookupswitch where space considerations permit a choice.
⼤概意思就是以下⼏点：
1、
2、Java虚拟机的 tableswitch 和 lookupswitch 指令只能作⽤在int类型的数据
3、byte、char 和 short类型可以安全转换为int，所以switch（key）的key类型可以为byte、char和short。long和double等类型是不能安全的转换为int，所以key的类型不能为long和double
4、如果cases对应的值⽐较稀疏时，使⽤tableswitch 会导致空间的利⽤率低下，此时可以使⽤
5、lookupswitch指令时，要求按照key进⾏排序，⽅便对key的搜索进⾏优化，⽐如可以采⽤⼆分查等进⾏优化。
从字节码层⾯查看下包装类型时处理⽅式。
public void integerSwitch(Integer integer) {
switch (integer) {
case 1:
System.out.println(1);
break;
default:
break;
}
}
⽣成的字节码如下（有省略）：
public void integerSwitch(java.lang.Integer);
descriptor: (Ljava/lang/Integer;)V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_1
1: invokevirtual #2 // Method java/lang/Integer.intValue:()I
4: lookupswitch { // 1
1: 24
default: 34
}
24: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
27: iconst_1
28: invokevirtual #4 // Method java/io/PrintStream.println:(I)V
31: goto 34
34: return
可以看到 1: invokevirtual #2 // Method java/lang/Integer.intValue:()I 调⽤ intValue 进⾏⾃动拆箱。从字节码层⾯看下switch是怎么⽀持枚举类型的。
我们定义如下枚举：
/**
* 定义性别枚举类
*/
public enum SexEnum {
MALE(1, "男"), FEMALE(6, "⼥");
private int key;
private String name;
SexEnum(int key, String name) {
this.key = key;
this.name = name;
}
}
从字节码中可以看出枚举SexEnum继承java.lang.Enum<T>
java.lang.Enum类中有如下实例变量：
/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
java switch case string* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;
表⽰是⼀个序数，或者叫序号，定位枚举常量的位置，起始值是0。从源码中可知，，有三个地⽅可以初始化，⽽且⼀经初始化后值不能再改变。
（1）定义时就初始化
（2）⾮静态代码块
（3）构造函数
我们看下结合SexEnum枚举的字节码看下ordinal是在哪⾥进⾏赋值的？
就需要先了解下枚举中的 MALE 和 FEMALE是怎么表⽰的？看下SexEnum⽣成的字节码，是由两个由static final 修饰的类变量
MALE 和 FEMALE来表⽰的，
由static final 修饰的类变量在定义的时候进⾏初始化 或者 在静态代码块中进⾏初始化。此处定义的时候没进⾏初始化，所以我们再从字节码的层⾯看下静态代码块的逻辑
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=6, locals=0, args_size=0
0: new #4 // class com/Ycb/jvm/switchtest/SexEnum
3: dup
4: ldc #9 // String MALE
6: iconst_0
7: iconst_1
8: ldc #10 // String 男
10: invokespecial #11 // Method "<init>":(Ljava/lang/String;IILjava/lang/String;)V 13: putstatic #12 // Field MALE:Lcom/Ycb/jvm/switchtest/SexEnum;
16: new #4 // class com/Ycb/jvm/switchtest/SexEnum
19: dup
20: ldc #13 // String FEMALE
22: iconst_1
23: bipush 6
25: ldc #14 // String ⼥
27: invokespecial #11 // Method "<init>":(Ljava/lang/String;IILjava/lang/String;)V 30: putstatic #15 // Field FEMALE:Lcom/Ycb/jvm/switchtest/SexEnum;
33: iconst_2
34: anewarray #4 // class com/Ycb/jvm/switchtest/SexEnum
37: dup
38: iconst_0
39: getstatic #12 // Field MALE:Lcom/Ycb/jvm/switchtest/SexEnum;
42: aastore
43: dup
44: iconst_1
45: getstatic #15 // Field FEMALE:Lcom/Ycb/jvm/switchtest/SexEnum;
48: aastore
49: putstatic #1 // Field $VALUES:[Lcom/Ycb/jvm/switchtest/SexEnum;
52: return
这⼀部分就是设置MALE静态变量的值，框住的两个地⽅
（1）6：iconst_0 感觉就是 MALE 的 ordinal 的赋值
（2）7：iconst_1 应该就是 定义MALE时，MALE(1,:"男") 中的1
这⼀部分就是设置FEMALE静态变量的值，框住的两个地⽅
（1）22：iconst_1 感觉就是 FEMALE 的 ordinal 的赋值
（2）23: bipush 6 应该就是 定义FEMALE时，FEMALE(6,"⼥") 中的6
知道了ordinal值的定义和赋值时机之后我们看下，switch是怎么只是枚举类型的
定义如下代码：
public void enumSwitch(SexEnum sexEnum) {
switch (sexEnum) {
case MALE:
System.out.println("男");
break;
case FEMALE:
System.out.println("⼥");
break;
default:
break;
}
}
反编译⽣成的class⽂件，得到如下内容
package com.Ycb.jvm.switchtest;
import com.Ycb.jvm.switchtest.SexEnum;
import com.Ycb.jvm.switchtest.SexEnumTest.1;
public class SexEnumTest {
public void enumSwitch(SexEnum sexEnum) {
switch(1.$SwitchMap$com$Ycb$jvm$switchtest$dinal()]) {
case 1:
System.out.println("MALE");
break;
case 2:
System.out.println("FEMALE");
}
}
}
核⼼就是这⼀句：switch(1.$SwitchMap$com$Ycb$jvm$switchtest$dinal()])
1.$SwitchMap$com$Ycb$jvm$switchtest$SexEnum[ ] 这个数组是什么？原来是会⾃动⽣成SexEnumTest$1.class⽂件，我们反编译后看下⾥⾯的内容如下：
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