Why is this happenning with switch (Java)?

Question

Its pretty much clear that switch statements can take string value in Java and something like this :

String s="diljit"
switch(s){
 ..
 ....}

will always compile.. s is a object of type String.... but on the other hand switch statement cannot take objects of other classes? Why is this happening and How will i validate this question with a suitable answer..

Can switch statement take objects or not?

score 8 · Answer 1 · answered Nov 25 '13 at 06:02

8

Can switch statement take objects or not?

No you can't use any arbitrary object in switch statements. This is specified in the language itself. Even String is only allowed from Java 7 onwards. From JLS §14.11:

The type of the Expression must be char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type (§8.9), or a compile-time error occurs.

answered Nov 25 '13 at 06:02

Rohit Jain

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To add to what Rohit said, if you want to switch based on values of objects, you can use enums do achieve that. Put all the variables of your object on which you want to switch into an enum and switch based on the enum. – TheLostMind Nov 25 '13 at 06:11

Mikhail · Answer 2 · 2013-11-25T07:49:21.597

Switch statements with strings are compiled with hashCode comparison, so the code:

switch(s){
    case "1":
    case "2":
    case "3":
}

after compilation looks like:

switch(s.hashCode()){
    case "1".hashCode():
    case "2".hashCode():
    case "3".hashCode():
}

Actually 7th JVM did not add anything specific about working with Strings in switches. Just a little compilation trick. Its possible to compare Strings by hashCode(), because this function is overridden and is based on object's content. This information is present at compile time. While its legal for Strings such approach is absolutely unacceptable for arbitrary object, because hashCode() returns a random number.

That's how it looks in bytecode:

11: tableswitch   { // 49 to 51
            49: 36        // "1".hashCode() 
            50: 50        // "2".hashCode() 
            51: 64        // "3".hashCode() 
       default: 75
  }
36: aload_2       
37: ldc           #4                  // String 1
39: invokevirtual #5                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
42: ifeq          75
45: iconst_0      
46: istore_3      
47: goto          75
50: aload_2       
51: ldc           #6                  // String 2
53: invokevirtual #5                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
56: ifeq          75
59: iconst_1      
60: istore_3      
61: goto          75
64: aload_2       
65: ldc           #7                  // String 3
67: invokevirtual #5                  // Method java/lang/String.equals:(Ljava/lang/Object;)Z
70: ifeq          75
73: iconst_2      
74: istore_3      
75: iload_3       
76: tableswitch   { // 0 to 2
         0: 104
         1: 104
         2: 104
   default: 104
}
104: return

Switch with Strings is compiled to switch with ints. If accidentally two hash codes are equal, strings are compared with equals() method. Compiling switches from JVM specification.

score 0 · Answer 3 · answered Nov 25 '13 at 06:10

String is very special in java. String is designed to be in between a primitive and a Class.

Making available to each primitive, java allowing (from 7) String also in switch (using **equals** method internally).

So String is allowing in switch. But not the every Object.

Why is this happenning with switch (Java)?

3 Answers3