essay8.txt

Java Essay Serials 1 - Java Basics - 8, Can switch be applied for byte?long?String?
Answer: Switch statement work with byte and String, but can not work with long.

The following paragraph is abstracted from Sun's Java tutorial:
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.

for example:
	int ivar = 3;
	switch(ivar){
    	case 3 :
       		break; 
    	case 2 :
       		break; 
    	default : 
	}

Another example:
	String var = "JAN";
	switch (var) {
	case "JAN":
		break;
	case "FEB":
		break;
	default:				
	}

but if you change the var's data type to long, the compiler will complain an error:
Cannot switch on a value of type long. Only convertible int values, strings or enum variables are permitted.

Java compiler implements switch statement using int datatype, even for enum and string. A long type value can not be converted to an int, that is the reason why Java do not support long datatype for switch statement.

Now we have a look at how string work with switch.
For the following code snippet:
	String var = "JAN";
	switch (var) {
	case "JAN":
		break;
	case "FEB":
		break;
	default:				
	}
We can compile the above program and use a decompiler tool to decompile the program, here is the result:
    String var = "JAN";
    String str1;
    switch ((str1 = var).hashCode())
    {
    case 69475:
      if (str1.equals("FEB")) return; break;
    case 73207:
      if (str1.equals("JAN"))
        return;
    }
From the above code, we can see that Java compiler first use hashCode() to get the string's hash code (an int), then lookup the hashcode,
and because two different strings might have the same hash code, so the compiler will give a further check to use equals().

That is to say, the source code of switch(string) statement is converted to switch(int) statement internally.

We can go further to check the situation for enum.
Code example:
	ColorEnum color = ColorEnum.RED;
	switch (color) {
	case RED:
		break;
	case BLACK:
		break;
	default:	
	}
After decompile,the code likes the following:
    label753: ColorEnum color = ColorEnum.RED;
    switch ($SWITCH_TABLE$ColorEnum()[color.ordinal()])
    {
    case 1:
    case 2:
    }
Here we can see that the compiler uses enum.ordinal() to get an int value and then check the switch lookup table.
Again, the source code of the switch(enum) statement is converted to switch(int) statement.