java中金額(浮點表示)的計算
java中進行金額的計算經常浮點數丟失精度,造成這種問題的原因應該與cpu對浮點數的計算方式有關,有下面的介紹:
從原理上來講,任何一門語言對於浮點數的計算都是不精確的。
因為現在的Computer都是基於二進位制數來儲存計算的。
例如計算8+3時,Computer會轉換為二進位制的加法1000+11=1011,然後再轉換為十進位制數為11。
這種演算法對於整數來說是不會產生誤差的(如果不超過計算範圍);
而對於浮點數計算有時就會產生誤差。因為有的浮點數轉換成為二進位制時是一個無窮迴圈小數。
例如十進位制的0.4,轉換成為二進位制為0.0110011001100110....,這樣,在0.4+0.3時就不能準確的算出是0.7,
而是經過一些舍入處理才能得出正確結果,但經過多次運算誤差產生的較大時,
即使經過一些舍入處理也不能得到精確的結果了。
既然這樣,那麼在java中為了保持精度需要怎樣做呢?
一句話:用BigDecimal進行運算;BigDecimal的建構函式有下面幾種:
BigDecimal(BigInteger val)
Translates a BigInteger into a BigDecimal.
BigDecimal(BigInteger unscaledVal, int scale)
Translates a BigInteger unscaled value and an int scale into a BigDecimal.
BigDecimal(double val)
Translates a double into a BigDecimal.
BigDecimal(String val)
Translates the String representation of a BigDecimal into a BigDecimal.
按照jdk幫助文件說明,如果採用double型別建構函式的話有可能丟失精度,推薦採用String型別的建構函式.
Note: the results of this constructor can be somewhat unpredictable.
One might assume that new BigDecimal(.1) is exactly equal to .1, but it is actually equal to
.1000000000000000055511151231257827021181583404541015625. This is so because .1 cannot be represented
exactly as a double (or, for that matter, as a binary fraction of any finite length).
Thus, the long value that is being passed in to the constructor is not exactly equal to .1,
appearances notwithstanding.
在用的過程中寫了一個幫助類Amount,以解決在日常開發中遇到的金額計算問題:
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