如何在Java中格式化数字？

如何在Java中格式化数字？
什么是"最佳实践"？

在格式化之前,我需要对数字进行舍入吗？

32.302342342342343 => 32.30

.7323 => 0.73

等等

1> Espo..：

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从这个线程,有不同的方法来做到这一点:

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

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

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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

该DecimalFormat的()似乎是这样做的最有活力的方式,并且它也很容易阅读别人的代码时,理解.

2> Jeff Atwood..：

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你和String.format()将成为新的好朋友!

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

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

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如果来自其他任何人,这个答案就不会有这么多的赞成.`String.format`用于格式化字符串,而不是数字.

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实际上,String.format就像C的printf.它可以格式化几种数据类型.

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如果要将舍入值转换为字符串,这是其中一种方法.其他的是使用`DecimalFormat`.并且`DecimalFormat`比`String.format`略快.简单的`System.currentTimeMillis`差异揭示了这一点.

3> Robert J. Wa..：

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请注意,从NumberFormat(以及大多数其他Format后代)下降的类不会同步.创建格式对象并将它们存储在util类中的静态变量中是一种常见(但危险)的做法.在实践中,它几乎总是有效,直到它开始经历重大负载.

4> Vinko Vrsalo..：

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回合数,是的.这是主要的示例源.

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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]);
        }
     }
 }

5> 小智..：

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试试这个:

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

简单高效.

6> ckpwong..：

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使用DecimalFormat.