The Curious Case of Enums and Autoboxing

Prior to the release of JDK 5, the expression in a switch statement must be either a char, byte, short, or int; otherwise, compilation error occurs.

 switch (expression) { 
... 
} 

However as of JDK 5, Sun introduced enumeration type and auto-boxing and because of this, the switch statement also supports these two concepts.

Below is sample code for JDK 5 and later:

1:  public class CaseEnum {  
2:      enum ResultEnum { OK, CANCEL };  
3:      public void test1() {  
4:          ResultEnum result = ResultEnum.OK;  
5:          switch (result) {  
6:          case OK:  
7:          case CANCEL:  
8:          }  
9:      }  
10:      public void test2() {  
11:          switch (new Integer(2)) {  
12:          case 1:  
13:          case 2:  
14:          }  
15:      }  
16:      public void test3() {  
17:          switch (new Byte((byte) 3)) {  
18:          case 1:  
19:          case 2:  
20:          case 3:  
21:          }  
22:      }  
23:  }  

Compiling with JDK6 and disassembling this class reveals:

1:  Compiled from "CaseEnum.java"  
2:  public class CaseEnum extends java.lang.Object{  
3:  public CaseEnum();  
4:   Code:  
5:    0:     aload_0  
6:    1:     invokespecial     #1; //Method java/lang/Object."<init>":()V  
7:    4:     return  
8:  public void test1();  
9:   Code:  
10:    0:     getstatic     #2; //Field CaseEnum$ResultEnum.OK:LCaseEnum$ResultEnum;  
11:    3:     astore_1  
12:    4:     getstatic     #3; //Field CaseEnum$1.$SwitchMap$CaseEnum$ResultEnum:[I  
13:    7:     aload_1  
14:    8:     invokevirtual     #4; //Method CaseEnum$ResultEnum.ordinal:()I  
15:    11:     iaload  
16:    12:     lookupswitch{ //2  
17:            1: 40;  
18:            2: 40;  
19:            default: 40 }  
20:    40:     return  
21:  public void test2();  
22:   Code:  
23:    0:     new     #5; //class java/lang/Integer  
24:    3:     dup  
25:    4:     iconst_2  
26:    5:     invokespecial     #6; //Method java/lang/Integer."<init>":(I)V  
27:    8:     invokevirtual     #7; //Method java/lang/Integer.intValue:()I  
28:    11:     lookupswitch{ //2  
29:            1: 36;  
30:            2: 36;  
31:            default: 36 }  
32:    36:     return  
33:  public void test3();  
34:   Code:  
35:    0:     new     #8; //class java/lang/Byte  
36:    3:     dup  
37:    4:     iconst_3  
38:    5:     invokespecial     #9; //Method java/lang/Byte."<init>":(B)V  
39:    8:     invokevirtual     #10; //Method java/lang/Byte.byteValue:()B  
40:    11:     tableswitch{ //1 to 3  
41:            1: 36;  
42:            2: 36;  
43:            3: 36;  
44:            default: 36 }  
45:    36:     return  
46:  }  

From line 14, javac adds an extra command to turn the enum into an integer value by invoking the enum "ordinal" method: CaseEnum$ResultEnum.ordinal:()I

From line 27, javac adds an extra command to unbox Integer to int by invoking the "intValue" method:
java/lang/Integer.intValue:()I

From line 39, again javac adds another extra command to unbox Byte to byte by invoking the "byteValue" method:
java/lang/Byte.byteValue:()B