如何在 Java 中格式化数字？

Question

如何在 Java 中格式化数字？
什么是“最佳实践”？

在格式化之前我需要对数字进行四舍五入吗？

32.302342342342343 => 32.30

.7323 => 0.73

ETC。

Answer 1

从 这个线程, ，有不同的方法可以做到这一点：

double r = 5.1234;
System.out.println(r); // r is 5.1234

int decimalPlaces = 2;
BigDecimal bd = new BigDecimal(r);

// setScale is immutable
bd = bd.setScale(decimalPlaces, BigDecimal.ROUND_HALF_UP);
r = bd.doubleValue();

System.out.println(r); // r is 5.12

---

f = (float) (Math.round(n*100.0f)/100.0f);

---

DecimalFormat df2 = new DecimalFormat( "#,###,###,##0.00" );
double dd = 100.2397;
double dd2dec = new Double(df2.format(dd)).doubleValue();

// The value of dd2dec will be 100.24

这 小数格式() 似乎是最动态的方式，而且在阅读其他代码时也很容易理解。

Answer 2

你和 String.format() 将成为新的最好的朋友！

http://java.sun.com/j2se/1.5.0/docs/api/java/util/Formatter.html#syntax

 String.format("%.2f", (double)value);

Answer 3

请注意，从 NumberFormat（以及大多数其他 Format 后代）派生的类不会同步。创建格式对象并将其存储在 util 类的静态变量中是一种常见（但危险）的做法。在实践中，它几乎总是可以工作，直到它开始承受很大的负载。

Answer 4

整数，是的。这是 主要示例来源.

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import java.util.*;
import java.text.*;

public class DecimalFormatDemo {

    static public void customFormat(String pattern, double value ) {
        DecimalFormat myFormatter = new DecimalFormat(pattern);
        String output = myFormatter.format(value);
        System.out.println(value + "  " + pattern + "  " + output);
    }

    static public void localizedFormat(String pattern, double value,                                       Locale loc ) {
        NumberFormat nf = NumberFormat.getNumberInstance(loc);
        DecimalFormat df = (DecimalFormat)nf;
        df.applyPattern(pattern);
        String output = df.format(value);
        System.out.println(pattern + "  " + output + "  " + loc.toString());
    }

    static public void main(String[] args) {

        customFormat("###,###.###", 123456.789);
        customFormat("###.##", 123456.789);
        customFormat("000000.000", 123.78);
        customFormat("$###,###.###", 12345.67);
        customFormat("\u00a5###,###.###", 12345.67);

        Locale currentLocale = new Locale("en", "US");

        DecimalFormatSymbols unusualSymbols = new DecimalFormatSymbols(currentLocale);
        unusualSymbols.setDecimalSeparator('|');
        unusualSymbols.setGroupingSeparator('^');
        String strange = "#,##0.###";
        DecimalFormat weirdFormatter = new DecimalFormat(strange, unusualSymbols);
        weirdFormatter.setGroupingSize(4);
        String bizarre = weirdFormatter.format(12345.678);
        System.out.println(bizarre);

        Locale[] locales = {
            new Locale("en", "US"),
            new Locale("de", "DE"),
            new Locale("fr", "FR")
        };

        for (int i = 0; i < locales.length; i++) {
            localizedFormat("###,###.###", 123456.789, locales[i]);
        }
     }
 }

Answer 5

尝试这个：

String.format("%.2f", 32.302342342342343);

简单高效。

Answer 6

使用 十进制格式.

Answer 7

标准库中有两种方法。一种是使用 java.text.DecimalFormat。基于 java.util.Formatter 的其他更神秘的方法（String.format、PrintStream.printf 等）应该会让 C 程序员高兴（ish）。

Answer 8

正如罗伯特在他的回答中指出的：DecimalFormat 既不同步，API 也不保证线程安全（它可能取决于您使用的 JVM 版本/供应商）。

使用Spring的 数字格式化程序 相反，这是线程安全的。

Answer 9

public static void formatDouble(double myDouble){
 NumberFormat numberFormatter = new DecimalFormat("##.000");
 String result = numberFormatter.format(myDouble);
 System.out.println(result);
}

例如，如果将双重值传递到formatDouble（）方法为345.9372，则以下是结果：345.937同样，如果值.7697传递给该方法，则以下是结果：.770