We all have used syntax’s like i += j
and i = i + j
thousands of times in our day to day programming. In first sight, they both look similar. In fact, they will result in same output in almost all of the cases in practical cases. But, to surprise you they are not similar. In run-time, they are treated differently when i and j are of different types. Let’s look at the example below:
int i = 5; double d1 = (double)i + 4.5; //necessary to satisfy compiler i += 4.5; System.out.println(i); System.out.println(d1); Output: 9 9.5
Weird. Isn’t it. Both are expected to be same as operation is same. Why they have different values? Let’s find out.
Reason
Java language specification says following:
A compound assignment expression of the form E1 op= E2
is equivalent to E1 = (T)((E1) op (E2))
, where T is the type of E1, except that E1 is evaluated only once.
So effectively our original example code can be re-written as below:
int i = 5; double d1 = (double) i + 4.5; i = (int)(i + 4.5); //Result converted to int System.out.println(i); System.out.println(d1); Output: 9 9.5
So the value 9 is nothing but a result of loss of precision while converting from double to int.
Lesson Learned
Always use the compound assignment operator [i += j] very carefully. You should use only when you are dealing with similar data types. In different data types, result can be incorrect.
Happy Learning !!
References
http://stackoverflow.com/questions/8710619/java-operator
http://docs.oracle.com/javase/specs/jls/se5.0/html/expressions.html#15.26.2
Niket kumar
I think the example which you have given for these concepts not match this specification. it should be something like
int i = 5;
double d1 = 4.5;
i=i+d1;//this will give compile time error
i += d1;
HIMANSU NAYAK
Hi Lokesh,
Haven’t seen this great catch.
“where T is the type of E1” is the T anything to do with the Generic Type T and also please tell us if this behaviour found in all the version of Java or only on 1.5 above.
HIMANSU NAYAK
not able to find the version in spec, let me install diffrent version of java and check.
Lokesh Gupta
T has not related to generic type T. It’s a symbol to denote the type information in formula.