Why Does Converting a Float to a Double Introduce Precision Issues?

Preserving Precision When Converting Float to Double

When attempting to convert a primitive float to a primitive double, users may encounter unexpected precision discrepancies. Simply casting the float to a double introduces spurious precision, as illustrated in the example below:

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

However, parsing a float as a string and then parsing the resulting string as a double yields the desired precision:

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

Understanding the Precision Discrepancy

It's crucial to note that the apparent gain in precision is not genuine. The float did not accurately represent the intended number initially. The converted double accurately reflects the float's original value, while toString() reveals the "extra" precision that was always present but hidden.

For instance, consider the following fictitious scenario:

float f = 0.1F;
double d = f;

In this case, the value of f could be precisely 0.100000234523. d will hold the same value but when converted to a string, it will assume higher precision and display the additional digits that were already present.

String Conversion as a Workaround

Converting to a string and back can result in a double value closer to the intended number than the original float, but this workaround is valid only if the string value accurately represents the desired number.

Alternative Data Type Considerations

Users should evaluate whether float/double are suitable for their use case. If precise decimal values are required (e.g., for financial calculations), BigDecimal may be a more appropriate data type.

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