System.out.format的使用

JDK5.0允许象C语言那样直接用printf()方法来格式化输出，并且提供了许多参数来格式化输入，调用也很简单：

System.out.format("Pi is approximately  %f", Math.Pi);

   System.out.printf("Pi is approximately  %f", Math.Pi);

 printf()和 format() 方法具有相同的功能. System.out 是 java.io.PrintStream的实例. PrintStream, java.io.PrintWriter, 和 java.lang.String 每个类都有四个新的格式化方法:

format( String format, Object... args);

printf( String format, Object... args);

format( Locale locale, String format, Object... args);

printf( Locale locale, String format, Object... args);

 

同时，以前的formatter类也提供了更完善的方法来格式化，例如：

formatter.format("Pi is approximately %1$f," +

        "and e is about %2$f", Math.PI, Math.E);

 

格式化元素的构成如下：

%[argument_index$][flags][width][.precision]conversion

其中：

argument_index是一个正整数，说明了参数的位置，1为取第一个参数

width表示输出的最小字母个数

precision代表数字的小数位数

conversion代表被格式化的参数的类型：

f  float,

t  time

d  decimal

o octal

x  hexadecimal

s  general

c  a Unicode character

以下是个例子：

package format;

 

   import java.util.Formatter;

 

   public class UsingFormatter {

 

     public static void main(String[] args) {

       if (args.length != 1) {

         System.err.println("usage: " +

           "java format/UsingFormatter ");

         System.exit(0);

       }

       String format = args[0];

 

       StringBuilder stringBuilder = new StringBuilder();

       Formatter formatter = new Formatter(stringBuilder);

       formatter.format("Pi is approximately " + format +

         ", and e is about " + format, Math.PI, Math.E);

       System.out.println(stringBuilder);

     }

   }

 

//控制台调用

java format/UsingFormatter %f

//输出

Pi is approximately 3.141593, and e is about 2.718282

//控制台调用

java format/UsingFormatter %.2f

//输出

Pi is approximately 3.14, and e is about 2.72

//控制台调用

java format/UsingFormatter %6.2f

//输出(有空格来填补长度)

Pi is approximately   3.14, and e is about   2.72

//控制台调用

java format/UsingFormatter  %1$.2f

//输出

Pi is approximately 3.14, and e is about 3.14

 

//改变区域设置

package format;

 

   import java.util.Formatter;

   import java.util.Locale;

 

   public class UsingFormatter {

 

     public static void main(String[] args) {

       if (args.length != 1) {

         System.err.println("usage: " +

           "java format/UsingFormatter <format string>");

         System.exit(0);

       }

       String format = args[0];

 

       StringBuilder stringBuilder = new StringBuilder();

       Formatter formatter = new Formatter(stringBuilder,

                                   Locale.FRANCE);

       formatter.format("Pi is approximately " + format +

         ", and e is about " + format, Math.PI, Math.E);

       System.out.println(stringBuilder);

     }

   }

//控制台调用

java format/UsingFormatter %.2f

//输出

Pi is approximately 3,14, and e is about 2,72

 

//采用format,printf的可替代写法

package format;

 

   public class UsingSystemOut {

 

     public static void main(String[] args) {

       if (args.length != 1) {

         System.err.println("usage: " +

           "java format/UsingSystemOut <format string>");

         System.exit(0);

       }

       String format = args[0];

 

       System.out.format("Pi is approximately " + format +

         ", and e is approximately " + format, Math.PI, Math.E);

     }

   }

//控制台调用

java format/UsingSystemOut %.2f%n

//输出

Pi is approximately 3.14

   , and e is about 2.72

 

对时间的格式化用字母t来代表，通常在t后面加上特殊的字符来显示时间的某个部分：

tr  hour and minute,

tA  the day of the week

tB  the name of the month

te  the number of the day of the month

tY  the year

//eg.

package format;

 

   import java.util.Calendar;

 

   public class FormattingDates  {

 

     public static void main(String[] args) {

       System.out.printf("Right now it is %tr on " +

                         "%<tA, %<tB %<te, %<tY.%n",

                         Calendar.getInstance());

     }

   }

//说明：“<”指示采用的参数为前一个被格式化的参数

//输出

Right now it is 01:55:19 PM on Wednesday, September 22, 2004.