Why Does Casting a Float to a Double Seem to Reduce Precision?

Preserving Precision When Converting Float to Double

In certain scenarios, simply casting a float to a double may result in seemingly incorrect precision. This unexpected behavior is not due to the double having increased precision but rather because the float's precision did not accurately reflect the intended value.

To understand this concept, consider the example:

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

While the float's string representation appears to have the desired precision, the double's string representation accurately displays the value stored in the float. The difference arises because the float's internal value was never truly 14009.35 but closer to 14009.349609375. Casting the float to a double merely exposes the underlying precision.

Converting to a string and back may seem like a viable solution, as it appears to maintain the desired precision:

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

However, this approach introduces a potential issue. If the string representation is not truly accurate, the resulting double will still be inaccurate, albeit with a different precision.

Therefore, it's crucial to evaluate the suitability of using float and double for specific numeric requirements. In cases where precise decimal values are necessary, BigDecimal may be a more appropriate choice. BigDecimal allows for arbitrary precision, eliminating the potential pitfalls of float and double representation limitations.

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