Pit using BigDecimal

BigDecimal The pit of

BigDecimal It is often used by us to calculate some scenes that need accurate calculation , For example, calculation of amount . however ,BigDecimal There are also many unknown pits . below , Let's briefly introduce some common pits .

1、 Use valueOf() replace new BigDecimal

public static void main(String[] args) {
    
    BigDecimal bigDecimal1 = new BigDecimal(0.01);
    BigDecimal bigDecimal2 = BigDecimal.valueOf(0.01);
    System.out.println(bigDecimal1);
    System.out.println(bigDecimal2);
}

The output is ：

You can see , direct new BigDecimal Pass in 0.01 When , because 0.01 This digital computer cannot accurately express , Cause it to be passed into BigDecimal Precision has been lost in . There is an error in the final output . and valueOf Is different , His bottom will Double Convert to String, Ensure that his accuracy will not be lost .

public static BigDecimal valueOf(double val) {
    
    return new BigDecimal(Double.toString(val));
}

summary ： Try to use string form to construct BigDecimal object , If it doesn't work , Then please also use BigDecimal.valueOf() Method to convert the value , To ensure our accuracy .

2、 Use compareTo instead of equals Compare

public static void main(String[] args) {
    
    BigDecimal bigDecimal1 = new BigDecimal("1.0");
    BigDecimal bigDecimal2 = new BigDecimal("1.00");
    System.out.println(bigDecimal2.equals(bigDecimal1));
    System.out.println(bigDecimal2.compareTo(bigDecimal1));
}

Running results ：

Running results , Use equals Compare two BigDecimal The value of will be equal to false, because BigDecimal Of equals Will compare the accuracy of two numbers , and compareTo Method will only compare the size of two numbers .

summary ： Use compareTo instead of equals Compare the two BigDecimal Value .

3、BigDecimal Not infinite precision

public static void main(String[] args) {
    
    BigDecimal bigDecimal1 = new BigDecimal("1.0");
    BigDecimal bigDecimal2 = new BigDecimal("3.0");
    bigDecimal1.divide(bigDecimal2);
}

The operation results are as follows ：

This is because ,1 / 3 be equal to Infinitely recurring decimals （0.33333… ）. This is the time , We must tell JVM, We don't need the most accurate results . Change the code to ：

public static void main(String[] args) {
    
    BigDecimal bigDecimal1 = new BigDecimal("1.0");
    BigDecimal bigDecimal2 = new BigDecimal("3.0");
    System.out.println(bigDecimal1.divide(bigDecimal2, RoundingMode.HALF_UP));
}

Running results ：

4、 Turn into String Use the right method

public static void main(String[] args) {
    
    BigDecimal bigDecimal = BigDecimal.valueOf(12345678902132123113213.12345678912345678);
    // When necessary, , Use scientific counting 
    System.out.println(bigDecimal.toString());
    // No scientific counting 
    System.out.println(bigDecimal.toPlainString());
    // The method of recording numbers often used in engineering calculation , It's like scientific counting , But the requirement is 10 The power of must be 3 Multiple 
    System.out.println(bigDecimal.toEngineeringString());
}

Running results ：

summary ：

1. toString()： If necessary , Use scientific counting .
2. toPlainString()： No scientific counting
3. toEngineeringString()： The method of recording numbers often used in engineering calculation , It's like scientific counting , But the requirement is 10 The power of must be 3 Multiple