float/int implicit conversion

Question

I'm doing multiplication and division of floats and ints and I forget the implicit conversion rules (and the words in the question seem too vague to google more quickly than asking here).

If I have two ints, but I want to do floating-point division, do I need only to cast one or both of the operands? How about for multiplication — if I multiply a float and an int, is the answer a float?

Answer 1

You can’t assign to an int result from division of a float by an int or vice-versa.

So the answers are:

If I have two ints, but I want to do floating point division…?

One cast is enough.

If I multiply a float and an int, is the answer a float?

Yes it is.

---

float f = 1000f;
int i = 3; 

f = i; // Ok
i = f; // Error

f = i/f; //Ok 0.003
f = f/i; //Ok 333.3333(3)

i = i/f; //Error
i = f/i; //Error

Answer 2

To demonstrate:

 int i1 = 5;
 float f = 0.5f;
 int i2 = 2;
 System.out.println(i1 * f);
 System.out.println(i1 / i2);
 System.out.println(((float) i1) / i2);

Result:

2.5
2 
2.5

Answer 3

In order to perform any sort of floating-point arithmetic with integers, you need to convert (read: cast) at least one of the operands to a float type.

Answer 4

If at least one of the operands to a binary operator is of floating-point type, then the operation is a floating-point operation, even if the other is integral.

(Source: Java language specifications - 4.2.4)

if I multiply a float and an int, is the answer a float?

System.out.println(((Object)(1f*1)).getClass());//class java.lang.Float

(If you use DrJava, you can simply type ((Object)(1f*1)).getClass() into the interactions panel. There's a plugin for DrJava for Eclipse too.)

Answer 5

The simple answer is that Java will perform widening conversions automatically but not narrowing conversions. So for example int->float is automatic but float->int requires a cast.