java += question

Why does:

public class Addition { 
public static void main() { 
int a = 0; 
double b = 1.0; 
a = a + b;
System.out.println(a); 
} }

not compile but:

public class Addition { 
public static void main() { 
int a = 0; 
double b = 1.0; 
a  += b; 
System.out.println(a); 
} }

compiles.

From stackoverflow

• int = int + double is essentially

  int = double + double

  and you cannot do that without casting...

  The int += double forces the result to an int while the other one requires casting.

  So a = (int)(a + b);

  should compile.

  Edit: as requested in the comments... here is a link to more reading (not the easiest read, but the most correct info): http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.26.2

  hhafez : could you provide some link on that for further reading? Thanks

  Johannes Schaub - litb : i think the "deep" reason is because it's disallowed to assign while narrowing: byte = int is disallowed and int = double too. would one do a simple byte a; a += 2; and fail to compile, people would throw shoes at java. but i would still have liked extra rules that make it work without that cast :(

  DefLog : I'm not certain there is a deep reason, but the Java language specification explicitly defines the implicit cast: http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html#5304

• In Java += operator has an implicit cast to the left hand type. This goes for all composed operators.

  matt b : I think this is a more concise answer

• double + int returns double, so double = double + int is legitimate, see JLS 5.1.2 Widening Primitive Conversion on the other hand int = double + int is "Narrowing Primitive Conversion" and requires explicit cast