blocks|key|3372476|text|在this+thread中，有不同的方法可以做到这一点：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3372477|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|code-block|syntax|javascript|3372478|3372479|f+=+(float)+(Math.round(n*100.0f)/100.0f);|3372480|3372481|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|3372482|DecimalFormat()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。|3372483|entityMap|0|LINK|mutability|MUTABLE|url|http://bytes.com/forum/thread16212.html|1|http://docs.oracle.com/javase/7/docs/api/java/text/DecimalFormat.html^0|1|B|0|0|0|0|0|0|0|0|F|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@$A|11|B|12|1|13]]|C|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|15|8|@]|9|@]|C|$]]|$1|J|3|K|5|F|7|16|8|@]|9|@]|C|$G|H]]|$1|L|3|-4|5|6|7|17|8|@]|9|@]|C|$]]|$1|M|3|N|5|F|7|18|8|@]|9|@]|C|$G|H]]|$1|O|3|P|5|6|7|19|8|@]|9|@$A|1A|B|1B|1|1C]]|C|$]]|$1|Q|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|R|$S|$5|T|U|V|C|$W|X]]|Y|$5|T|U|V|C|$W|Z]]]]

From <a href="http://bytes.com/forum/thread16212.html" rel="noreferrer">this thread</a>, there are different ways to do this:

<pre><code>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
</code></pre>

<hr>

<pre><code>f = (float) (Math.round(n*100.0f)/100.0f);
</code></pre>

<hr>

<pre><code>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
</code></pre>

The <a href="http://docs.oracle.com/javase/7/docs/api/java/text/DecimalFormat.html" rel="noreferrer">DecimalFormat()</a> seems to be the most dynamic way to do it, and it is also very easy to understand when reading others code.

blocks|key|3372488|text|你和String.format()将成为新的最好的朋友！|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3372489|https://docs.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html#syntax|3372490|+String.format("%25.2f",+(double)value);|code-block|syntax|javascript|3372491|entityMap|0|LINK|mutability|MUTABLE|url^0|2|F|0|0|25|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|G]]]]

You and <code>String.format()</code> will be new best friends!

<a href="https://docs.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html#syntax" rel="nofollow noreferrer">https://docs.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html#syntax</a>

<pre><code> String.format("%.2f", (double)value);
</code></pre>

blocks|key|364055|text|请注意，从NumberFormat+(和大多数其他格式的后代)派生的类是不同步的。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的(但危险的)实践。在实践中，它将几乎总是工作，直到它开始经历显著的负载。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|364056|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Be aware that classes that descend from NumberFormat (and most other Format descendants) are not synchronized. It is a common (but dangerous) practice to create format objects and store them in static variables in a util class. In practice, it will pretty much always work until it starts experiencing significant load.

blocks|key|3372505|text|四舍五入，是的。这是main+example+source。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3372506|/*
+*+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+%2B+"++"+%2B+pattern+%2B+"++"+%2B+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+%2B+"++"+%2B+output+%2B+"++"+%2B+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('%7C');
++++++++unusualSymbols.setGroupingSeparator('%5E');
++++++++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%2B%2B)+{
++++++++++++localizedFormat("###,###.###",+123456.789,+locales[i]);
++++++++}
+++++}
+}|code-block|syntax|javascript|3372507|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/docs/books/tutorial/i18n/format/decimalFormat.html#numberpattern^0|A|J|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Round numbers, yes. This is the <a href="http://java.sun.com/docs/books/tutorial/i18n/format/decimalFormat.html#numberpattern" rel="nofollow noreferrer">main example source</a>. 

<pre><code>/*
 * 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 &lt; locales.length; i++) {
 localizedFormat("###,###.###", 123456.789, locales[i]);
 }
 }
 }
</code></pre>

blocks|key|364152|text|试试这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|364153|String.format("%25.2f",+32.302342342342343);|code-block|syntax|javascript|364154|简单而高效。|364155|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Try this:

<pre><code>String.format("%.2f", 32.302342342342343);
</code></pre>

Simple and efficient.

blocks|key|368856|text|使用DecimalFormat。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|368857|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/javase/6/docs/api/java/text/DecimalFormat.html^0|2|D|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Use <a href="http://java.sun.com/javase/6/docs/api/java/text/DecimalFormat.html" rel="noreferrer">DecimalFormat</a>.

blocks|key|3372529|text|在标准库中有两种方法。一种是使用java.text.DecimalFormat。基于java.util.Formatter的其他更隐蔽的方法(String.format、PrintStream.printf等)应该会让C程序员感到满意(Ish)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3372530|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

There are two approaches in the standard library. One is to use java.text.DecimalFormat. The other more cryptic methods (String.format, PrintStream.printf, etc) based around java.util.Formatter should keep C programmers happy(ish).

blocks|key|364160|text|public+static+void+formatDouble(double+myDouble){
+NumberFormat+numberFormatter+=+new+DecimalFormat("##.000");
+String+result+=+numberFormatter.format(myDouble);
+System.out.println(result);
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|364161|例如，如果传递给formatDouble()方法的双精度值是345.9372，则结果如下:+345.937类似地，如果将值.7697传递给该方法，则结果如下：.770|unstyled|364162|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

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

For instance, if the double value passed into the formatDouble() method is 345.9372, the following will 
be the result:
345.937
Similarly, if the value .7697 is passed to the method, the following will be the result:
.770

blocks|key|369010|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|369011|改用Spring的Numberformatter，它是线程安全的。|offset|length|369012|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.spring.io/spring/docs/3.0.x/api/org/springframework/format/number/NumberFormatter.html^0|0|9|F|0|0^^$0|@$1|2|3|-4|4|5|6|M|7|@]|8|@]|9|$]]|$1|A|3|B|4|5|6|N|7|@]|8|@$C|O|D|P|1|Q]]|9|$]]|$1|E|3|-4|4|5|6|R|7|@]|8|@]|9|$]]]|F|$G|$4|H|I|J|9|$K|L]]]]

As Robert has pointed out in his answer: DecimalFormat is neither synchronized nor does the API guarantee thread safety (it might depend on the JVM version/vendor you are using). 

Use Spring's <a href="http://docs.spring.io/spring/docs/3.0.x/api/org/springframework/format/number/NumberFormatter.html" rel="nofollow">Numberformatter</a> instead, which is thread safe.

How do I format a number in Java? 
What are the &quot;Best Practices&quot;?
Will I need to round a number before I format it?
<blockquote>
<code>32.302342342342343</code> =&gt; <code>32.30</code>
<code>.7323</code> =&gt; <code>0.73</code>
</blockquote>
etc.

How do I format a number in Java?

Java

如何在Java中格式化数字？什么是“最佳实践”？在格式化一个数字之前，我需要对它进行舍入吗？ 32.302342342342343 => 32.30.7323 => 0.73等。

问如何在Java中格式化数字？
EN

回答 9

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