Typecasting Float in Java

Question

A.) Does precision loss happens when one converts a float to a double in Java ?

B.) If I typecast a float to a float does that result in any precision loss or does Java simply intelligently ignores/skips such kind of typecasting ?

• Dawood ibn Kareem: Every number that can be exactly represented by a float can also be exactly represented by a double. Is that what you're asking?
• user2031271: Yes that was one part of my question. Second part is if I typecast a float to a float does that result in any precision loss and or does Java ignores any such kind of typecasting to same type ?
• Dawood ibn Kareem: Hah, OK, I assumed that was a typo. Yeah, casting float to float does nothing.
• Dawood ibn Kareem: Beware decimal representations that are more precisely approximated by a double than by a float though. So 1.1 != (double) 1.1F for example.
• Dawood ibn Kareem: The Java Language Specification is the normative reference for such questions.

Answer 1

Question A is covered by the Java Language Specification, Widening Primitive Conversion.

"Conversions widening from float to double in strictfp expressions also preserve the numeric value exactly; however, such conversions that are not strictfp may lose information about the overall magnitude of the converted value."

The non-strictfp issue relates to numbers with extreme exponent values. The issue is discussed in FP-strict Expressions.

"Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow."

If you want to be sure that conversions from float to double will be exact, use strictfp.

Question B is a question about Identity Conversions. I'm not sure whether an identity conversion can trigger a change in value set, involving the same strictfp vs. non-strict issue as for Question A.

Answer 2

Assume a double is a 64-bit bucket and a float a 32-bit bucket. If you empty the contents of a float into the double-bucket surely it will fit with more space to spare. However converting from double to float won't be the same because the float-bucket is just too small to hold the bigger content of a double. In case of float to float, they are all of the same size so you surely won't lose nothing and will fit perfectly into each other's buckets.