如何用Java格式化数字？

How do I format a number in Java?

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

通过此线程，可以有不同的方法来执行此操作：

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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

1	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

DecimalFormat()似乎是最动态的方法，在阅读其他代码时也很容易理解。

您和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

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

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，直到开始承受很大的负载。

舍入数字，是的。这是主要的示例来源。

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/*
* Copyright (c) 1995 - 2008 Sun Microsystems, Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* - Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* - Neither the name of Sun Microsystems nor the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS"AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

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]);
}
}
}

尝试这个：

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

简单高效。

使用DecimalFormat。

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。

正如罗伯特(Robert)在回答中指出的那样：DecimalFormat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)。

请改用Spring的Numberformatter，它是线程安全的。

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

例如，如果传递给formatDouble()方法的double值为345.9372，则以下内容将
结果：
345.937
同样，如果将值.7697传递给该方法，则结果为：
.770