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Assignment of values

Vrushali Gore
Ranch Hand

Joined: Oct 17, 2005
Posts: 40
line1 byte b;

line2 b += 10;
line3 b = b + 10;
line4 b = 'b' + 1;



In line2 how does compiler compiles the program without giving error while at line3 we would get in error. In case of expressions byte, short & char is promoted to int so by that rule line4 should give error but still it compiles well

How this happens?
Ernest Friedman-Hill
author and iconoclast
Marshal

Joined: Jul 08, 2003
Posts: 24183
    
  34

Hi,

Welcome to JavaRanch!

The compound assignment operators effectively include a cast to the appropriate type -- i.e.,

b += 10;

is actually not equivalent to

b = b + 10;

but rather

b = (byte) (b + 10);

So this explains line 2. Line 4 compiles because " 'b' + 1" is a constant expression which can be evaluated at compile time; the compiler can see that it is less than Byte.MAX_VALUE so there is no overflow, and the result is a byte constant.

Line 3, on the other hand, is evaluated at runtime. The compiler doesn't keep track of the values of variables on previous lines of code, so as far as it knows, the expression might overflow; therefore, this is an error.


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Nipun Devlekar
Greenhorn

Joined: Dec 07, 2005
Posts: 9
Really nice explanation
 
Consider Paul's rocket mass heater.
 
subject: Assignment of values