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Does any one know as to why Java does not allow you to switch on numbers larger than integers? Is there any technical reason behind this?

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5 Answers 5

As said here, switch works with the byte, short, char, and int primitive data types. It also works with enumerated types and a few special classes that "wrap" certain primitive types: Character, Byte, Short, and Integer.

The Java switch statement is compiled into the JVM bytecode tableswitch or lookupswitch. Both of these bytecodes require that the case values be unique, 32-bit, integer, compile-time constants.

The tableswitch and lookupswitch instructions both include one default branch offset and a variable-length set of case value/branch offset pairs.
Both instructions pop the key (the value of the expression in the parentheses immediately following the switch keyword) from the stack.
The key is compared with all the case values:

  • If a match is found, the branch offset associated with the case value is taken.
  • If no match is found, the default branch offset is taken.

Even though the above represents implementation details, I believe the types used for switch are the one compatible with an efficient bytecode for control flow, and it may have been an important part for the reason of this choice.

As said in this Java Bug:

I suppose you could allow Java switch statement cases to include other types of values, and compile a switch statement that uses one of those kinds of values into other bytecode, such as a sequence of ifs and gotos. But then we'd have to examine all switch statements carefully in order to determine if it will be compiled into a fast switch bytecode or a slow sequence of if-elses.

As said in this other Java bug

'switch' statements can use function-table dispatch and/or binary-tree search to match the case statements faster than just testing them one by one in order.

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This is implementation details. Not the actual reason of this choice. –  Gilles Nov 21 '08 at 9:20

Scala, a language built on top of JVM, allows you to define your own case classes, that you can use in a switch statement. So, using longs or doubles or Strings in a switch statement is certainly possible.

I don't know however how complicated and how effective is it. With simple types, the compiler just calculates an offset to jump on a table. This is definitely not the case with more complex types.

I suppose the answer has do to with the time java was designed and the goals the designers were trying to achieve. Java initial goals were to design a "better C++", portable in many environments. I can understand why a switch on complex types didn't fit it in.

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My guess is, that long was dropped as a valid switch expression type because of/related to the fact that operations on long variables may not be atomic.

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I don't think that is the reason. If that is the case then if else also should have been restricted right? I think it has to do something with the addressing modes of a processor as VonC pointed out (Because switch is not compiled into the same byte code as if else structure). –  Abhi Nov 21 '08 at 12:46

If you find the native java switch construct is too much limiting give a glance to the lambdaj Switcher that allows to declaratively switch on any object by matching them with some hamcrest matchers.

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Maybe because it's really bad programming to have more than integer max cases in a switch?

If you want to switch over long variables, just projet them in the interger space.

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Please don't vote this answer down! It explains why the Java language designers thought it was reasonable to limit the switch statement to 32-bit types. –  palm3D Nov 21 '08 at 10:07
This is not a reason, it is speculation. Projecting a long into the integer space also defeats the purpose of using a long. –  Ken Gentle Nov 21 '08 at 12:25
That would be a pretty dumb reason, imagine having some long bit sequences you need to switch over. –  MightyPork Apr 21 at 19:32

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