Why Does Converting a Float to a Double Sometimes Appear to Lose Precision?

Preserving Precision While Converting Float to Double

Problem

Casting a float primitive to a double primitive can introduce apparent precision loss. This phenomenon is often attributed to the double's "extra" precision, but it's actually due to the float's limited ability to represent certain numbers accurately.

In the following example, casting a float to a double results in the introduction of digits:

float temp = 14009.35F;
System.out.println(Float.toString(temp)); // 14009.35
System.out.println(Double.toString((double)temp)); // 14009.349609375

Parsing Solution

One workaround is to convert the float to a string and then parse the string as a double, which retains the original value:

System.out.println(Double.toString(Double.parseDouble(Float.toString(temp)))); // 14009.35

Precision Considerations

It's important to note that the "extra" digits are not actually added by the double primitive. The float simply did not represent the number accurately. The double faithfully represents the original float's value.

The conversion to string and back results in a double value that's closer to the string representation, but it's important to verify that this is actually desirable.

BigDecimal Alternative

In situations where precise decimal values are required (e.g., monetary calculations), consider using the BigDecimal type instead of float or double, as it ensures more accurate representation of decimal numbers.

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