java += question

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.

Answer 1

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

Answer 2

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

Answer 3

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