Is BigDecimal safe to calculate the amount? Look at these five pits~~

I saw an article last week because it was not used in the amount calculation BigDecimal And the article that caused the failure , But unless in some very simple scenes , Settlement of remittance businesses will not be directly used BigDecimal To calculate the amount , For two reasons ：

1. BigDecimal There are still many hidden pits

2. BigDecimal The unit that did not provide the amount

1. BigDecimal Five easy pits in

1.1 new BigDecimal() still BigDecimal#valueOf()？

First look at the following code

BigDecimal bd1 = new BigDecimal(0.01);
BigDecimal bd2 = BigDecimal.valueOf(0.01);
System.out.println("bd1 = " + bd1);
System.out.println("bd2 = " + bd2);

The output to the console is ：

bd1 = 0.01000000000000000020816681711721685132943093776702880859375
bd2 = 0.01

The reason for this difference is 0.1 This digital computer cannot accurately express , Send BigDecimal The accuracy has been lost by , and BigDecimal#valueOf The implementation of is completely different

public static BigDecimal valueOf(double val) {
    // Reminder: a zero double returns '0.0', so we cannot fastpath
    // to use the constant ZERO.  This might be important enough to
    // justify a factory approach, a cache, or a few private
    // constants, later.
    return new BigDecimal(Double.toString(val));
}

It uses the corresponding string of floating-point numbers to construct BigDecimal object , Therefore, the accuracy problem is avoided . So we should try to use strings instead of floating-point numbers to construct BigDecimal object , If it doesn't work , Just use BigDecimal#valueOf() Methods! .

1.2 Equivalence comparison

BigDecimal bd1 = new BigDecimal("1.0");
BigDecimal bd2 = new BigDecimal("1.00");
System.out.println(bd1.equals(bd2));
System.out.println(bd1.compareTo(bd2));

The output of the console will be ：

false
0

The reason is ,BigDecimal in equals The implementation of the method will compare the accuracy of two numbers , and compareTo Method will only compare the size of the value .

1.3 BigDecimal Does not mean infinite accuracy

First look at this code

BigDecimal a = new BigDecimal("1.0");
BigDecimal b = new BigDecimal("3.0");
a.divide(b) // results in the following exception.

The result is an exception ：

java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

About this exception ,Oracle The official documents of have specific instructions

If the quotient has a nonterminating decimal expansion and the operation is specified to return an exact result, an ArithmeticException is thrown. Otherwise, the exact result of the division is returned, as done for other operations.

The main idea is , If the result of the quotient of division is an infinite decimal, but we expect to return the exact result , Then the program will throw an exception . Back to our example , We need to tell JVM We don't need to return exact results

BigDecimal a = new BigDecimal("1.0");
BigDecimal b = new BigDecimal("3.0");
a.divide(b, 2, RoundingMode.HALF_UP)// 0.33

1.4 BigDecimal Turn it back String Be careful

BigDecimal d = BigDecimal.valueOf(12334535345456700.12345634534534578901);
String out = d.toString(); // Or perform any formatting that needs to be done
System.out.println(out); // 1.23345353454567E+16

It can be seen that the results have been converted into scientific counting , Maybe this is not the expected result BigDecimal There are three methods that can be converted to the corresponding string type , Remember not to use the wrong ：

String toString();     //  Use scientific counting when necessary 
String toPlainString();   //  No scientific counting 
String toEngineeringString();  //  The method of recording numbers often used in engineering calculation , Similar to scientific counting , But ask for 10 The power of must be 3 Multiple 

1.5 The execution sequence cannot be changed （ The commutative law of multiplication fails ）

It is common sense that multiplication satisfies the commutative law , But in the world of computers , There will be situations that do not satisfy the commutative law of multiplication

BigDecimal a = BigDecimal.valueOf(1.0);
BigDecimal b = BigDecimal.valueOf(3.0);
BigDecimal c = BigDecimal.valueOf(3.0);
System.out.println(a.divide(b, 2, RoundingMode.HALF_UP).multiply(c)); // 0.990
System.out.println(a.multiply(c).divide(b, 2, RoundingMode.HALF_UP)); // 1.00

Don't underestimate this 0.01 The difference between , In Huijin , There will be a very large amount difference .

2. Best practices

About the amount calculation , Many business teams will be based on BigDecimal One more Money class , In fact, we can directly use a semi official Money class ：JSR 354 , Although not in Java 9 To become Java standard , It is likely to be integrated into subsequent Java Become the official library in version .

2.1 maven coordinate

<dependency>
    <groupId>org.javamoney</groupId>
    <artifactId>moneta</artifactId>
    <version>1.1</version>
</dependency>

2.2 newly build Money class

CurrencyUnit cny = Monetary.getCurrency("CNY");
Money money = Money.of(1.0, cny); 
//  perhaps  Money money = Money.of(1.0, "CNY");
//System.out.println(money);

2.3 Amount calculation

CurrencyUnit cny = Monetary.getCurrency("CNY");
Money oneYuan = Money.of(1.0, cny);
Money threeYuan = oneYuan.add(Money.of(2.0, "CNY")); //CNY 3
Money tenYuan = oneYuan.multiply(10); // CNY 10
Money fiveFen = oneYuan.divide(2); //CNY 0.5

2.4 More equal

Money fiveFen = Money.of(0.5, "CNY"); //CNY 0.5
Money anotherFiveFen = Money.of(0.50, "CNY"); // CNY 0.50
System.out.println(fiveFen.equals(anotherFiveFen)); // true

You can see , This class abstracts the amount explicitly , Increase the unit of the amount , It also avoids direct use BigDecimal Some pits .

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