如何在Java中格式化数字?

时间:2008-09-08 19:59:45

标签: java number-formatting

如何在Java中格式化数字?
什么是“最佳实践”?

在格式化之前,我是否需要对数字进行舍入?

  

32.302342342342343 => 32.30

     

.7323 => 0.73

9 个答案:

答案 0 :(得分:116)

this thread开始,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

DecimalFormat()似乎是最有活力的方式,在阅读其他代码时也很容易理解。

答案 1 :(得分:73)

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);

答案 2 :(得分:14)

请注意,从NumberFormat(以及大多数其他Format后代)继承的类不会同步。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。

答案 3 :(得分:11)

回合数字,是的。这是main example source

/*
 * 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]);
        }
     }
 }

答案 4 :(得分:8)

试试这个:

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

简单而有效。

答案 5 :(得分:5)

使用DecimalFormat

答案 6 :(得分:4)

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

答案 7 :(得分:2)

正如罗伯特在他的回答中指出的那样:DecimalFormat既不是同步的,也不是API保证线程安全(它可能取决于你正在使用的JVM版本/供应商)。

使用Spring的Numberformatter代替,这是线程安全的。

答案 8 :(得分:1)

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值是345.9372,则以下将是 结果是: 345.937 同样,如果将值.7697传递给方法,则结果如下: 0.770