Accuracy of BigDecimal Division

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BigDecimal The accuracy of division

In the use of BigDecimal Division time , Encounter a ghost animal problem , The accuracy of this calculation , Result use return 0, Of course, I finally found that my posture was wrong , So record , Avoid repeating the mistakes later

I. Ask questions

In the use of BigDecimal When doing high-precision Division , I encountered a small problem when I didn't pay attention , as follows

@Test
public void testBigDecimal() {
    BigDecimal origin = new BigDecimal(541253);
    BigDecimal now = new BigDecimal(12389431);

    BigDecimal val = origin.divide(now, RoundingMode.HALF_UP);
    System.out.println(val);

    origin = new BigDecimal(541253);
    now = new BigDecimal(12389431.3);
    val = origin.divide(now, RoundingMode.HALF_UP);
    System.out.println(val);

    origin = new BigDecimal(541253.4);
    now = new BigDecimal(12389431);
    val = origin.divide(now, RoundingMode.HALF_UP);
    System.out.println(val);
}
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What is the output above ？

0
0
0.043686703610520937021487456961257
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Why are the first two 0 Well , If it's directly 541253 / 12389431 = 0 It's understandable , however BigDecimal Isn't it a high-precision calculation , To be reasonable, there should not be such a problem of division

We know that BigDecimal When triggering , You can specify parameters to keep decimals , If you add this , Will it be different ？

BigDecimal origin = new BigDecimal(541253);
BigDecimal now = new BigDecimal(12389431);

BigDecimal val = origin.divide(now, 5, RoundingMode.HALF_UP);
System.out.println(val);
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The output is :

0.04369
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So after the decimal point is specified , No problem , So make a bold guess , Is it some of the above case in , because scale When the value is not specified , The default values are different , Which leads to different accuracy of the final result ？

Simple in-depth source code analysis , The way of execution is origin.divide(now, RoundingMode.HALF_UP);, So this scale Parameters are aimed origin object , And this object , We can only analyze its structure , Because there is no other place to use

II. Source location

1. Shaping parameter transmission structure

Analyze the following line , Go straight to source

BigDecimal origin = new BigDecimal(541253);
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The obvious int Pass parameter structure , Go in and have a brief look

// java.math.BigDecimal#BigDecimal(int)
public BigDecimal(int val) {
    this.intCompact = val;
    this.scale = 0;
    this.intVal = null;
}

public BigDecimal(long val) {
    this.intCompact = val;
    this.intVal = (val == INFLATED) ? INFLATED_BIGINT : null;
    this.scale = 0;
}
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so, Clearly know the default scale by 0, That is to say, when origin Positive number , Division with it , Unrealistic designation scale When parameters are , The final returns are all without decimals , Take the same look , also long The way of transmitting reference , BigInteger It's the same thing

2. Floating point parameter transfer

The next step is floating point scale The default value confirms , This structure is a little more complex than the previous one , Source code is not posted , Too long , I don't quite understand what I did , Directly in a more obscene way , Get into debug Pattern , Step by step

@Test
public void testBigDecimal() {
    BigDecimal origin = new BigDecimal(541253.0);
    BigDecimal now = new BigDecimal(12389431.1);
    BigDecimal tmp = new BigDecimal(0.0);
}
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according to debug Result , first ,scale by 0; the second scale by 29, Third scale by 0

3. String The ginseng

It's still a bunch of logic , Also use one-step debug Try... In a different way

@Test
public void testBigDecimal() {
    BigDecimal origin = new BigDecimal("541253.0");
    BigDecimal now = new BigDecimal("12389431.1");
    BigDecimal t = new BigDecimal("0.0");
}
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The top three scale All are 1

4. Summary

• about BigDecimal In Division , It is best to specify its scale Parameters , Or there might be a hole
• about BigDecimla Of scale The principle of initialization , Need to take a closer look BigDecimal How did it happen

II. other

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