BigDecimal Method returns 模糊的结果，为什么会这样？

Question

代码：

package tk.vivekpatani.www;

import java.math.BigDecimal;
import java.util.Scanner;

public class SimpleCalc {

  public static void main(String[] args) {


    // TODO Auto-generated method stub
    Scanner sc = new Scanner(System.in);
    try{
    System.out.println("Welcome to the Calculator!");


    System.out.println("Enter Number 1:");
    double num1 = sc.nextDouble();

    System.out.println("Enter Number 2:");
    double num2 = sc.nextDouble();

    BigDecimal bd1 = new BigDecimal(num1);
    BigDecimal bd2 = new BigDecimal(num2);

    BigDecimal result = addition(bd1,bd2);

    System.out.println("Result:"+result.toString());
    }
    finally{
      sc.close();
    }

  }
  static BigDecimal addition(BigDecimal input1, BigDecimal input2){

    BigDecimal result = input1.add(input2);
    return result;

  }


}

结果：

Welcome to the Calculator!
Enter Number 1:
90
Enter Number 2:
10.2
Result:100.199999999999999289457264239899814128875732421875

我正在使用 Eclipse IDE Mars 和 Java 1.8.

Answer 1

double 由于其使用的精度无法正确处理 0.2。

尝试这样做来创建数字，您需要使用字符串构造 BigDecimal，这样您就不会遗留 double 精度问题。

    System.out.println("Enter Number 1:");
    String num1 = sc.nextLine();
    BigDecimal bd1 = new BigDecimal(num1);

    System.out.println("Enter Number 2:");
    String num2 = sc.nextLine();
    BigDecimal bd2 = new BigDecimal(num2);

Answer 2

引用 BigDecimal(double) 构造函数的 javadoc：

The results of this constructor can be somewhat unpredictable. [...] When a double must be used as a source for a BigDecimal, [...] use the static valueOf(double) method.

所以，总是使用 BigDecimal.valueOf(double)，从不使用 new BigDecimal(double)。

Scanner sc = new Scanner(System.in);

System.out.print("Enter Number 1: ");
double num1 = sc.nextDouble();

System.out.print("Enter Number 2: ");
double num2 = sc.nextDouble();

BigDecimal bd1 = BigDecimal.valueOf(num1);
BigDecimal bd2 = BigDecimal.valueOf(num2);

System.out.println("Result: " + bd1.add(bd2));

输出

Enter Number 1: 90
Enter Number 2: 10.2
Result: 100.2

BigDecimal Method returns 模糊的结果，为什么会这样？

BigDecimal Method returns vague results, why does this happen?

java

eclipse

double

bigdecimal

java.util.scanner