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switch comparison vs == comparison

Larry Olson
Ranch Hand

Joined: Feb 03, 2009
Posts: 142
(1)
byte b = 10;
switch(b)
{
case 128:
System.out.println("here");
}
results in a compile time error.

But

(2)
byte b = 10;
integer i = 128;
System.out.println(b == i);
runs fine and produces a result of false.

Question:

Why byte can't be compared to int in (1), whereas byte can be compared to int in (2) ?
Alternatively, why code in (1) cannot compare byte and int values (but will result in loss of precision), whereas code in (2) could compare a byte and int fine ?
Ankit Garg
Sheriff

Joined: Aug 03, 2008
Posts: 9305
    
  17

From the JLS
Every case constant expression associated with a switch statement must be assignable to the type of the switch Expression.

There is no such restriction for if conditions (or I must say the == operator). When you compare byte == int, the byte is promoted to int, but this doesn't happen in switch statements...


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Larry Olson
Ranch Hand

Joined: Feb 03, 2009
Posts: 142
I see. So it is the Java language specification that tells this. But it seems unnatural to force the switch statement to this behavior whereas it isn't the case while using == operator. One of those weird, strange quirks one has to remember
Ankit Garg
Sheriff

Joined: Aug 03, 2008
Posts: 9305
    
  17

If you think about it, it is logical. The == operator allows comparison between byte and int because you might compare two variables i.e. something like this
So the compiler cannot tell whether i's value will be in the range of byte or not. But in the case of a switch statement, you cannot use variables as case values, so at the compile time you already know whether the case value is assignable to the switch expression or not. What is the whole point of allowing a case which can never be true?? In the example you showed, the case 128 can never be true, so what is the point of allowing it?? By flagging this as an error, the compiler helps you avoid such circumstances as its definitely a logical error...
Abimaran Kugathasan
Ranch Hand

Joined: Nov 04, 2009
Posts: 2066

A case constant must evaluate to the same type as the switch expression can use, in your case, can 128 to be evaluated to byte? Definitely No!


|BSc in Electronic Eng| |SCJP 6.0 91%| |SCWCD 5 92%|
Lucas Smith
Ranch Hand

Joined: Apr 20, 2009
Posts: 804
    
    1

When you use object for switch comparison what kind of evaluation will be performed? Stay tuned :P


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Abimaran Kugathasan
Ranch Hand

Joined: Nov 04, 2009
Posts: 2066

In objects, enum only can be checked with switch - case.
 
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