Java BigDecimal class application example code analysis

1. Introduction

The main design goal of float and double types is for scientific calculations and engineering calculations. They perform binary floating-point operations, which are carefully designed to provide fast approximate calculations with high accuracy over a wide range of values. However, they do not provide completely accurate results and should not be used where accurate results are required. However, business calculations often require accurate results, and BigDecimal comes in handy at this time.

2. Introduction to knowledge points

• 1. Overview

• 2. Construction method

• 3. Operations of addition, subtraction, multiplication and division

• 4. Source code description

• 5. Summary

• 6. Refining exercises

##3. Detailed explanation of knowledge points

1. Overview

Why use BigDecimal?

Code demonstration:

package com.Test;

import Test2.MyDate;
import java.awt.*;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Properties;

public class Main {
private final static String name = "磊哥的java历险记-@51博客";
public static void main(String[] args) {
System.out.println(0.2 + 0.1);
System.out.println(0.3 - 0.1);
System.out.println(0.2 * 0.1);
System.out.println(0.3 / 0.1);
System.out.println("============="+name+"=============");
}
}

You think you saw it wrong, but the result is like this. What's the problem? The reason is that our computers are binary. There is no way to accurately represent floating point numbers in binary. Our CPU represents floating point numbers as consisting of two parts: the exponent and the mantissa. This representation method generally loses a certain degree of accuracy, and some floating point number operations will also produce certain errors. For example: the binary representation of 2.4 is not exactly 2.4. Instead, the closest binary representation is 2.3999999999999999. The value of a floating point number is actually calculated by a specific mathematical formula.

In fact, Java's float can only be used for scientific calculations or engineering calculations. In most commercial calculations, the java.math.BigDecimal class is generally used for precise calculations.

2. BigDecimal construction method

• (1) public BigDecimal(double val): Convert double representation to BigDecimal (note: not recommended)

• (2) public BigDecimal(int val): Convert int representation to BigDecimal

• (3) public BigDecimal(String val): Convert String Convert the representation to BigDecimal

Why is it not recommended to use the first construction method?

Code demonstration:

package com.Test;

import Test2.MyDate;

import java.awt.*;
import java.math.BigDecimal;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Properties;

public class Main {

private final static String name = "磊哥的java历险记-@51博客";

public static void main(String[] args){
BigDecimal bigDecimal =new BigDecimal(2);
BigDecimal bDouble = new BigDecimal(2.3);
BigDecimal bString = new BigDecimal("2.3");
System.out.println("bigDecimal="+ bigDecimal);
System.out.println("bDouble="+ bDouble);
System.out.println("bString="+ bString);
System.out.println("============="+name+"=============");
}
}

The running results are as follows:

Why does it appear? What about this situation?

3. Source code description

The result of the constructor method with parameter type double is somewhat unpredictable. One might think that writing newBigDecimal(0.1) in Java creates a BigDecimal that is exactly equal to 0.1 (the unscaled value of 1, whose scale is 1), but it is actually equal to 0.100000000000000005551115123

1257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that case, as any finite-length binary decimal). This way, the value passed into the constructor will not be exactly equal to 0.1 (although it will appear to be equal to that value).

The String constructor, on the other hand, is completely predictable: writing newBigDecimal("0.1") will create a BigDecimal that is exactly equal to the expected 0.1. Therefore, in comparison, it is generally recommended to use the String constructor in preference.

When double must be used as the source of BigDecimal, please use Double.toString(double) to convert to String, and then use the String constructor, or use the static method valueOf

of BigDecimal Code demonstration:

package com.Test;
import Test2.MyDate;
import java.math.BigDecimal;
public class Main {
private final static String name = "磊哥的java历险记-@51博客";
public static void main(String[] args) {
BigDecimal bDouble1 =BigDecimal.valueOf(2.3);
BigDecimal bDouble2 = new BigDecimal(Double.toString(2.3));
System.out.println("bDouble1="+ bDouble1);
System.out.println("bDouble2="+ bDouble2);
System.out.println("============="+name+"=============");
}
}

The results are as follows:

4. BigDecimal addition, subtraction, multiplication and division operations

For commonly used addition, subtraction, multiplication, and division, the BigDecimal class provides corresponding member methods.

• (1) public BigDecimal add(BigDecimal value); addition

• (2) public BigDecimal subtract(BigDecimal value); //Subtraction

• (3) public BigDecimal multiply(BigDecimal value); //Multiplication (4) public BigDecimal divide(BigDecimal value); Division

Code demonstration:

package com.Test;
import Test2.MyDate;
import java.math.BigDecimal;
public class Main {
private final static String name = "磊哥的java历险记-@51博客";
public static void main(String[] args){
BigDecimal a = new BigDecimal("4.5");
BigDecimal b = new BigDecimal("1.5");
System.out.println("a+ b =" + a.add(b));
System.out.println("a- b =" + a.subtract(b));
System.out.println("a* b =" + a.multiply(b));
System.out.println("a/ b =" + a.divide(b));
System.out.println("============="+name+"=============");
}
}

One thing to note here is the division operation divide.

BigDecimal division may not be divisible. , such as 4.5/1.3, then the error java.lang.ArithmeticException:Non-terminating decimal expansion; no exact representable decimal result.

In fact, the divide method can pass three parameters:

 public BigDecimal divide(BigDecimal divisor, int scale, introundingMode)     第一参数表示除数， 第二个参数表示小数点后保留位数，   第三个参数表示舍入模式，只有在作除法运算或四舍五入时才用到舍入模式，有下面这几种

• （1）ROUND_CEILING    //向正无穷方向舍入 

• （2）ROUND_DOWN    //向零方向舍入 

• （3）ROUND_FLOOR    //向负无穷方向舍入 

• （4）ROUND_HALF_DOWN    //向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向下舍入, 例如1.55 保留一位小数结果为1.5 

• （5）ROUND_HALF_EVEN    //向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，如果保留位数是奇数，使用ROUND_HALF_UP，如果是偶数，使用ROUND_HALF_DOWN 

• （6）ROUND_HALF_UP    //向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向上舍入, 1.55保留一位小数结果为1.6 

• （7）ROUND_UNNECESSARY    //计算结果是精确的，不需要舍入模式 

• （8）ROUND_UP    //向远离0的方向舍入

按照各自的需要，可传入合适的第三个参数。四舍五入采用 ROUND_HALF_UP

需要对BigDecimal进行截断和四舍五入可用setScale方法，例：

代码演示：

public static void main(String[] args) {
BigDecimal a = newBigDecimal("4.5635");
//保留3位小数，且四舍五入
a = a.setScale(3,RoundingMode.HALF_UP);
System.out.println(a);
}

注：减乘除其实最终都返回的是一个新的BigDecimal对象，因为BigInteger与BigDecimal都是不可变的（immutable）的，在进行每一步运算时，都会产生一个新的对象

代码演示：

package com.Test;
import Test2.MyDate;
import java.math.BigDecimal;
public class Main {
private final static String name = "磊哥的java历险记-@51博客";
public static void main(String[] args){
BigDecimal a = new BigDecimal("4.5");
BigDecimal b = new BigDecimal("1.5");
a. add(b);
System.out.println(a); //输出4.5. 加减乘除方法会返回一个新的
System.out.println("============="+name+"=============");
}
}

5、总结

• (1)商业计算使用BigDecimal。

•  (2)尽量使用参数类型为String的构造函数。

• (3)BigDecimal都是不可变的（immutable）的，在进行每一步运算时，都会产wf 所以在做加减乘除运算时千万要保存操作后的值。

•  (4)我们往往容易忽略JDK底层的一些实现细节，导致出现错误，需要多加注意。

6、精炼练习

在银行结算或支付中，我们经常会用到BigDecimal的相关方法。

6.1  题目

• （1）使用BigDecimal创建出浮点类型的数字

• （2）使用BigDecimal进行加减乘除运算   

6.2 实验步骤

• （1）声明一个类Test

• （2）在Test类中，完成BigDecimal的构造和方法使用

代码演示：

package com.Test;
import Test2.MyDate;
import java.math.BigDecimal;
public class Main {
private final static String name = "磊哥的java历险记-@51博客";
public static void main(String[] args){
BigDecimal number = new BigDecimal("3.14");
//加法
System.out.println(number.add(new BigDecimal("1")));
//减法
System.out.println(number.subtract(new BigDecimal("1")));
//乘法
System.out.println(number.multiply(new BigDecimal("2")));
//除法
System.out.println(number.multiply(new BigDecimal("3.14")));
System.out.println("============="+name+"=============");
}
}

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