問題描述

我有一個原始浮點數，我需要一個原始雙精度數.簡單地將浮點數轉換為 double 會給我帶來奇怪的額外精度.例如:

I have a primitive float and I need as a primitive double. Simply casting the float to double gives me weird extra precision. For example:

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

但是，如果我不進行強制轉換，而是將浮點數輸出為字符串，并將字符串解析為雙精度，我會得到我想要的:

However, if instead of casting, I output the float as a string, and parse the string as a double, I get what I want:

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

有沒有更好的方法而不是去字符串然后返回?

Is there a better way than to go to String and back?

推薦答案

并不是你實際上獲得了額外的精度 - 而是浮點數沒有準確地代表你的目標數字起初.double is 準確地代表了原來的浮點數；toString 正在顯示已經存在的額外"數據.

It's not that you're actually getting extra precision - it's that the float didn't accurately represent the number you were aiming for originally. The double is representing the original float accurately; toString is showing the "extra" data which was already present.

例如(這些數字不正確，我只是在編造)假設你有:

For example (and these numbers aren't right, I'm just making things up) suppose you had:

float f = 0.1F;
double d = f;

那么 f 的值可能正好是 0.100000234523.d 將具有完全相同的值，但是當您將其轉換為字符串時，它會相信"它的精確度更高，因此不會提前四舍五入，您會看到已經存在但對您隱藏的額外數字".

Then the value of f might be exactly 0.100000234523. d will have exactly the same value, but when you convert it to a string it will "trust" that it's accurate to a higher precision, so won't round off as early, and you'll see the "extra digits" which were already there, but hidden from you.

當您轉換為字符串并返回時，您最終會得到一個比原始浮點數更接近字符串值的雙精度值 - 但這只是 如果 你真的相信字符串值是您真正想要的.

When you convert to a string and back, you're ending up with a double value which is closer to the string value than the original float was - but that's only good if you really believe that the string value is what you really wanted.

您確定 float/double 是此處使用的合適類型，而不是 BigDecimal?如果您嘗試使用具有精確十進制值的數字(例如貨幣)，那么 BigDecimal 是更合適的 IMO 類型.

Are you sure that float/double are the appropriate types to use here instead of BigDecimal? If you're trying to use numbers which have precise decimal values (e.g. money), then BigDecimal is a more appropriate type IMO.

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