Strange typecast while doing variableOfTypeA+=variableOfTypeB

Question

I am using Java and new to it.

When I try

int integerValue = 100;
long longValue = 100;
integerValue = integerValue + longValue;

I get "Type mismatch: cannot convert from long to int".

But

integerValue+=longValue;

works fine, which means it is doing the cast for me :)

Is it something that "+=" provides inherently? Any specific reason for that?

Edit: Oops!! Too common question! :) I should have thoroughly searched first, my bad!!

Duplicate of http://stackoverflow.com/questions/8272635/why-does-java-perform-implicit-type-conversion-from-double-to-integer-when-using — sw1nn, Mar 13 '12 at 14:37
possible duplicate of [Varying behavior for possible loss of precision](http://stackoverflow.com/questions/2696812/varying-behavior-for-possible-loss-of-precision) — Louis Wasserman, Mar 13 '12 at 14:50

score 3 · Answer 1 · answered Mar 13 '12 at 14:30

3

Yes, this is behaving exactly as section 15.26.2 of the JLS explains.

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.

answered Mar 13 '12 at 14:30

Jon Skeet

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score 2 · Answer 2 · answered Mar 13 '12 at 14:32

2

You have uncovered the Tweedledum puzzle from the Java Puzzlers book. Basically the compound assignment performs type cast and regular one doesn't. See JLS for details.

answered Mar 13 '12 at 14:32

MK.

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score 1 · Answer 3 · answered Mar 13 '12 at 14:31

1

Using the operation assignment operators, there is an implicit cast.

int i = 10;
i *= 5.5;
// same as
i *= (int) (i * 5.5);

or even

char ch = '5';
ch /= 1.1; // ch = '0'

answered Mar 13 '12 at 14:31

Peter Lawrey

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score 1 · Answer 4 · answered Mar 13 '12 at 14:31

That's correct. In your first example, you have to cast to int, because the result result of the left expression is a long value (only because longValue is long).

 integerValue = (long) (integerValue + longValue);  // this works

The += operator does the (same) casting implicity.

Both behaviours are specified in the Java Language Specification.

score 0 · Accepted Answer · edited May 23 '17 at 12:04

0

Thanks everyone!

All answers were good enough!

Better explanations and examples available at stackoverflow.com/questions/8710619/java-operator :)

edited May 23 '17 at 12:04

Community

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answered Mar 14 '12 at 10:43

instanceOfObject

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Strange typecast while doing variableOfTypeA+=variableOfTypeB

5 Answers5