Simple question -- why can't I switch on a String in Java, and as far as anyone knows, is this functionality going to be put into a later Java version?
EDIT: If someone can point me to an article, or themselves explain why I can't do this, as in, the technical way Java's switch statement works, it would be most appreciated.
UPDATE: Along the lines of the post made by @erickson, this feature is confirmed to be implemented in Java 7.
Switches based on integers can be optimized to very efficent code. Switches based on other data type can only be compiled to a series of if() statements.
For that reason C & C++ only allow switches on integer types, since it was pointless with other types.
The designers of C# decided that the style was important, even if there was no advantage.
The designers of Java apparently thought like the designers of C.
Java can't switch on objects, only standard data types.
This is a discussion at java.net regarding requests to add the ability to switch on objects in future versions: http://forums.java.net/jive/thread.jspa?threadID=595
Right now the answer is unknown on whether or not it will be added in the future.
If you have a place in your code where you can switch on a String, then it may be better to refactor the String to be an enumeration of the possible values, which you can switch on. Of course, you limit the potential values of Strings you can have to those in the enumeration, which may or may not be desired.
Of course your enumeration could have an entry for 'other', and a fromString(String) method, then you could have
ValueEnum enumval = ValueEnum.fromString(myString);
switch (enumval) {
case MILK: lap(); break;
case WATER: sip(); break;
case BEER: quaff(); break;
case OTHER:
default: dance(); break;
}
Reminds me of this thread : http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/ddee6c336894e00d
James Curran succinctly says: "Switches based on integers can be optimized to very efficent code. Switches based on other data type can only be compiled to a series of if() statements. For that reason C & C++ only allow switches on integer types, since it was pointless with other types."
My opinion, and it's only that, is that as soon as you start switching on non-primitives you need to start thinking about "equals" versus "==". Firstly comparing two strings can be a fairly lengthy procedure, adding to the performance problems that are mentioned above. Secondly if there is switching on strings there will be demand for switching on strings ignoring case, switching on strings considering/ignoring locale,switching on strings based on regex.... I would approve of a decision that saved a lot of time for the language developers at the cost of a small amount of time for programmers.
Beside the above good arguments, I will add that lot of people today see switch as an obsolete remainder of procedural past of Java (back to C times).
I don't fully share this opinion, I think switch can have its usefulness in some cases, at least because of its speed, and anyway it is better than some series of cascading numerical else if I saw in some code...
But indeed, it is worth looking at the case where you need a switch, and see if it cannot be replaced by something more OO. For example enums in Java 1.5+, perhaps HashTable or some other collection (sometime I regret we don't have (anonymous) functions as first class citizen, as in Lua — which doesn't have switch — or JavaScript) or even polymorphism.