方法一：格式化输出
double one=5;System.out.printf("%.02f", one); //5.00方法二：BigDecimal.setScale()
BigDecimal.setScale()这个方法是用于格式化小数点
setScale(0)表示保留整数
setScale(1)表示保留一位小数，默认用四舍五入
setScale(1,BigDecimal.ROUND_DOWN)直接删除多余的小数位，如2.35会变成2.3
setScale(1,BigDecimal.ROUND_UP)进位处理，2.35变成2.4
setScale(1,BigDecimal.ROUND_HALF_UP)四舍五入，2.35变成2.4
setScale(1,BigDecimal.ROUND_HALF_DOWN)四舍五入，2.35变成2.3，如果是5则向下舍
BigDecimal num1 = new BigDecimal(2.225667); //这种写法不允许，会造成精度损失
BigDecimal num = new BigDecimal(“2.225667”); //一般都会这样写最好
double one1 = 5.459;BigDecimal two1 = new BigDecimal(one1);double three1 = two1.setScale(2,BigDecimal.ROUND_HALF_UP).doubleValue();System.out.println(three1); //5.46方法三：DecimalFormat
double one2 = 5.459;DecimalFormat format2 = new DecimalFormat("#.00");String str2 = format2.format(one2);System.out.println(str2); //5.46double four2 = Double.parseDouble(str2);System.out.println(four2); //5.46方法四：String.format
double one3 = 5.459;String str3 = String.format("%.2f",one3);System.out.println(str3); //5.46double four3 = Double.parseDouble(str3);System.out.println(four3); //5.46方法五：NumberFormat
double one4 = 5.459;NumberFormat format4 = NumberFormat.getInstance();format4.setMaximumFractionDigits(2);String str4 = format4.format(one4);System.out.println(str4); //5.46double two4 = Double.parseDouble(str4);System.out.println(two4); //5.46补充：
- Double.valueOf(String s)返回保存用参数字符串s表示的double值的Double对象
- Double.parseDouble返回一个初始化为指定String表示的值的新double
- 如果要变成int,则用Integer.parseInt()==
方法二到四遇到的问题：
不能将double 5转换成double 5.00输出
import java.math.BigDecimal;import java.text.DecimalFormat;import java.text.NumberFormat;public class test1 { public static void main(String[] args) { double one1 = 5; BigDecimal two1 = new BigDecimal(one1); double three1 = two1.setScale(2,BigDecimal.ROUND_HALF_UP).doubleValue(); System.out.println(three1); //5.0 double one2 = 5; DecimalFormat format2 = new DecimalFormat("#.00"); String str2 = format2.format(one2); System.out.println(str2); //5.00 double four2 = Double.parseDouble(str2); System.out.println(four2); //5.0 double one3 = 5; String str3 = String.format("%.2f",one3); System.out.println(str3); //5.00 double four3 = Double.parseDouble(str3); System.out.println(four3); //5.0 double one4 = 5; NumberFormat format4 = NumberFormat.getInstance(); format4.setMaximumFractionDigits(2); String str4 = format4.format(one4); System.out.println(str4); //5 double two4 = Double.parseDouble(str4); System.out.println(two4); //5.0 }}package code;import java.math.BigDecimal;import java.text.DecimalFormat;import java.text.NumberFormat;public class test1 { public static void main(String[] args) { double one1 = 5.0; BigDecimal two1 = new BigDecimal(one1); double three1 = two1.setScale(2,BigDecimal.ROUND_HALF_UP).doubleValue(); System.out.println(three1); //5.0 double one2 = 5.0; DecimalFormat format2 = new DecimalFormat("#.00"); String str2 = format2.format(one2); System.out.println(str2); //5.00 double four2 = Double.parseDouble(str2); System.out.println(four2); //5.0 double one3 = 5.0; String str3 = String.format("%.2f",one3); System.out.println(str3); //5.00 double four3 = Double.parseDouble(str3); System.out.println(four3); //5.0 double one4 = 5.0; NumberFormat format4 = NumberFormat.getInstance(); format4.setMaximumFractionDigits(2); String str4 = format4.format(one4); System.out.println(str4); //5 double two4 = Double.parseDouble(str4); System.out.println(two4); //5.0 }}package code;import java.math.BigDecimal;import java.text.DecimalFormat;import java.text.NumberFormat;public class test1 { public static void main(String[] args) { double one1 = 5.00; BigDecimal two1 = new BigDecimal(one1); double three1 = two1.setScale(2,BigDecimal.ROUND_HALF_UP).doubleValue(); System.out.println(three1); //5.0 double one2 = 5.00; DecimalFormat format2 = new DecimalFormat("#.00"); String str2 = format2.format(one2); System.out.println(str2); //5.00 double four2 = Double.parseDouble(str2); System.out.println(four2); //5.0 double one3 = 5.00; String str3 = String.format("%.2f",one3); System.out.println(str3); //5.00 double four3 = Double.parseDouble(str3); System.out.println(four3); //5.0 double one4 = 5.00; NumberFormat format4 = NumberFormat.getInstance(); format4.setMaximumFractionDigits(2); String str4 = format4.format(one4); System.out.println(str4); //5 double two4 = Double.parseDouble(str4); System.out.println(two4); //5.0 }}不管是5或者是5.000…最后的结果都是5.0
来源地址：https://blog.csdn.net/m0_62122789/article/details/127490280
