Java使用BigDecimal确保数值计算精度的最佳实践指南

// 错误示范
BigDecimal bd = new BigDecimal(0.1); 
System.out.println(bd); // 输出: 0.1000000000000000055511151231257827021181583404541015625

BigDecimal bd1 = new BigDecimal("0.1");          // 安全
BigDecimal bd2 = BigDecimal.valueOf(0.1);        // 内部转字符串，也安全

BigDecimal a = new BigDecimal("1");
BigDecimal b = new BigDecimal("3");
a.divide(b); // Non-terminating decimal expansion; no exact representable decimal result.

BigDecimal result = a.divide(b, 4, RoundingMode.HALF_UP);
// 或
BigDecimal result2 = a.divide(b, new MathContext(10, RoundingMode.HALF_UP));

new BigDecimal("1.0").equals(new BigDecimal("1")); // false

new BigDecimal("1.0").compareTo(new BigDecimal("1")) == 0; // true

Set<BigDecimal> set = new HashSet<>();
set.add(new BigDecimal("1.0"));
set.add(new BigDecimal("1")); // 被视为两个不同元素！
System.out.println(set.size()); // 输出: 2

BigDecimal normalized = bd.stripTrailingZeros();
set.add(normalized);

BigDecimal bd = new BigDecimal("100");
System.out.println(bd.stripTrailingZeros().toString()); // 输出: 1E+2

System.out.println(bd.stripTrailingZeros().toPlainString()); // 输出: 100

BigDecimal bd = new BigDecimal("1.234");
bd.setScale(2, RoundingMode.HALF_UP); // 无效！
System.out.println(bd); // 仍是 1.234

bd = bd.setScale(2, RoundingMode.HALF_UP);

BigDecimal small = new BigDecimal("0.000000123");
System.out.println(small.toString());        // 1.23E-7
System.out.println(small.toPlainString());   // 0.000000123

BigDecimal trailing = new BigDecimal("100.00");
System.out.println(trailing.toString());        // 100.00
System.out.println(trailing.toPlainString());   // 100.00

BigDecimal large = new BigDecimal("1E+10");
System.out.println(large.toString());        // 1E+10
System.out.println(large.toPlainString());   // 10000000000

import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;
import java.util.HashSet;
import java.util.Set;

public class BigDecimalPitfallsTest {

    public static void main(String[] args) {
        System.out.println("=== 1. 构造函数陷阱 ===");
        testConstructor();

        System.out.println("\n=== 2. 除法异常 ===");
        testDivision();

        System.out.println("\n=== 3. equals vs compareTo ===");
        testEqualsVsCompareTo();

        System.out.println("\n=== 4. Set 中重复问题 ===");
        testSetBehavior();

        System.out.println("\n=== 5. stripTrailingZeros 与 toPlainString ===");
        testStripAndToString();

        System.out.println("\n=== 6. setScale 必须重新赋值 ===");
        testSetScale();

        System.out.println("\n=== 7. toString vs toPlainString ===");
        testToStringFormats();
    }

    private static void testConstructor() {
        BigDecimal bad = new BigDecimal(0.1);
        BigDecimal good1 = new BigDecimal("0.1");
        BigDecimal good2 = BigDecimal.valueOf(0.1);
        System.out.println("new BigDecimal(0.1): " + bad);
        System.out.println("new BigDecimal(\"0.1\"): " + good1);
        System.out.println("BigDecimal.valueOf(0.1): " + good2);
    }

    private static void testDivision() {
        BigDecimal a = new BigDecimal("1");
        BigDecimal b = new BigDecimal("3");
        try {
            a.divide(b);
        } catch (ArithmeticException e) {
            System.out.println("除法异常: " + e.getMessage());
        }
        BigDecimal safe = a.divide(b, 6, RoundingMode.HALF_UP);
        System.out.println("安全除法结果: " + safe);
    }

    private static void testEqualsVsCompareTo() {
        BigDecimal x = new BigDecimal("1.0");
        BigDecimal y = new BigDecimal("1");
        System.out.println("x.equals(y): " + x.equals(y)); // false
        System.out.println("x.compareTo(y) == 0: " + (x.compareTo(y) == 0)); // true
    }

    private static void testSetBehavior() {
        Set<BigDecimal> set = new HashSet<>();
        set.add(new BigDecimal("1.0"));
        set.add(new BigDecimal("1"));
        System.out.println("Set 大小（未标准化）: " + set.size()); // 2

        Set<BigDecimal> normalizedSet = new HashSet<>();
        normalizedSet.add(new BigDecimal("1.0").stripTrailingZeros());
        normalizedSet.add(new BigDecimal("1").stripTrailingZeros());
        System.out.println("Set 大小（标准化后）: " + normalizedSet.size()); // 1
    }

    private static void testStripAndToString() {
        BigDecimal bd = new BigDecimal("100");
        System.out.println("stripTrailingZeros().toString(): " + bd.stripTrailingZeros().toString());
        System.out.println("stripTrailingZeros().toPlainString(): " + bd.stripTrailingZeros().toPlainString());
    }

    private static void testSetScale() {
        BigDecimal bd = new BigDecimal("1.234");
        bd.setScale(2, RoundingMode.HALF_UP); // 无效果
        System.out.println("未重新赋值: " + bd); // 1.234

        bd = bd.setScale(2, RoundingMode.HALF_UP);
        System.out.println("重新赋值后: " + bd); // 1.23
    }

    private static void testToStringFormats() {
        BigDecimal[] cases = {
            new BigDecimal("0.000000123"),
            new BigDecimal("100.00"),
            new BigDecimal("12345678901234567890")
        };
        for (BigDecimal bd : cases) {
            System.out.println("原始: " + bd);
            System.out.println("  toString():        " + bd.toString());
            System.out.println("  toPlainString():  " + bd.toPlainString());
            System.out.println();
        }
    }
}