Java += compiler/jre bug?

Question

I encountered someting unexpected with the java += operator.

Apparently, this compiles:

int a = 0;
a += 3/2.0;
System.out.println(a);  // outputs "1"

While, this doesn't

int a = 0;
a = a + (3/2.0);   // eclipse compiler error "Type mismatch: cannot convert from double to int"
System.out.println(a);

Is this expected behaviour? I find it strange that the += operator does not report a "Type mismatch", because it is an 'add and assign' operator, where you add a double, which gives a double result, and is then assigned to an int variable. Instead it silently casts (and truncates) the result.

Have a look @ this [answer](http://stackoverflow.com/a/8710685/2024761) & this [too](http://stackoverflow.com/a/608728/2024761). — Rahul, Jun 11 '13 at 09:45
I should Cut&Paste the answer from there, so my rep goes skywards ;-) — TobiMcNamobi, Jun 11 '13 at 09:50
if gosling read this question then he would do FACEPALMS :P , he call it a bug in java , lol — anshulkatta, Jun 11 '13 at 09:53

score 3 · Answer 1 · edited May 23 '17 at 12:07

3

This is the way += has worked from C. It's "feature" not a bug

See Java's +=, -=, *=, /= compound assignment operators

BTW You can try this

char ch = '0';
ch *= 1.1; // ch = '4'

Also see a post of mine http://vanillajava.blogspot.com/2012/11/java-and-implicit-casting.html

edited May 23 '17 at 12:07

Community

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answered Jun 11 '13 at 09:47

Peter Lawrey

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C does the narrowing conversion in the assignment without complaint, so that's not C legacy, that's a compromise made when making Java stricter than C and C++. – Sebastian Redl Jun 11 '13 at 09:48
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The original [answer](http://stackoverflow.com/a/8710747/2024761)! – Rahul Jun 11 '13 at 09:50
@R.J Well spotted, but memory isn't so good some times ;) – Peter Lawrey Jun 11 '13 at 09:52
When java designers see simple `int d=1.3` as a compiler error instead of just a warning, how justified is it to make `int a+=3/2.0` compilable when it's equivalent sin? – pinkpanther Jun 11 '13 at 09:58
@pinkpanther I think it's an inconsistency which came about becuase they made assignment more strict but perhaps assumed that if you use an assignment operator you knew what you were doing. Given C allows implicit casts, they may not have thought it such a problem. – Peter Lawrey Jun 11 '13 at 10:01

Mik378 · Answer 2 · 2013-06-11T09:55:22.663

int a = a + (3/2.0); doesn't compile since:

int + double leads to a double
a is referenced as an int
cast from double to int missing

int a += 3/2.0; compiles since:

int + double leads to a double
a is referenced as an int
double is castable to an int and luckily, compiler adds an implicit cast similar to:

int a = (int)(a+ 3/2.0);. This is due to the special op= notation that is interpreted more cleverly from compiler than the basic assignment operator: =.

score 0 · Answer 3 · answered Jun 11 '13 at 09:48

This is not a bug , its a implicit conversion happened.

for eg.

Byte b=3;

b+=5;  //compiles

b=b+5; //doesnt compiles

Here what happens is

a=a+3/2.0;   // 3/2.0 produces double , it add int to double but cant convert it implicitly to int.

a += 3/2.0;  // here implicit conversion after addition of a to 3/2.0 happends from double to int

Java += compiler/jre bug?

3 Answers3