如何用Java格式化数字？
什么是"最佳做法"？

格式化之前是否需要四舍五入一个数字？

32.302342342342343 => 32.30
  
  .7323 => 0.73

等等。

解决方案

回答

通过此线程，可以有不同的方法来执行此操作：

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

DecimalFormat()似乎是最动态的方法，在读取其他代码时也很容易理解。

回答

我们和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);

回答

使用DecimalFormat。

回答

舍入数字，是的。这是主要的示例来源。

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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]);
        }
     }
 }

回答

标准库中有两种方法。一种是使用java.text.DecimalFormat。基于java.util.Formatter的其他更多隐秘方法(String.format，PrintStream.printf等)应使C程序员满意。

回答

请注意，源自NumberFormat的类(以及大多数其他Format的后代)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(但很危险)的做法。在实践中，它几乎总是可以工作，直到开始承受巨大的负载。