Formatting格式化

Stream objects that implement formatting are instances of either PrintWriter, a character stream class, or PrintStream, a byte stream class.实现格式化的流对象是PrintWriter（字符流类）或PrintStream（字节流类）的实例。

Like all byte and character stream objects, instances of PrintStream and PrintWriter implement a standard set of write methods for simple byte and character output. 与所有字节和字符流对象一样，PrintStream和PrintWriter的实例为简单的字节和字符输出实现了一组标准的write方法。In addition, both PrintStream and PrintWriter implement the same set of methods for converting internal data into formatted output. 此外，PrintStream和PrintWriter都实现了将内部数据转换为格式化输出的相同方法集。Two levels of formatting are provided:提供了两个级别的格式设置：

• print and println format individual values in a standard way.以标准方式print和println单个值。
• format formats almost any number of values based on a format string, with many options for precise formatting.format基于格式字符串格式化几乎任意数量的值，并提供许多用于精确格式化的选项。

The print and println Methodsprint方法和println方法

Invoking print or println outputs a single value after converting the value using the appropriate toString method. 调用print或println在使用适当的toString方法转换值后输出单个值。We can see this in the Root example:我们可以在Root示例中看到这一点：

public class Root {
    public static void main(String[] args) {
        int i = 2;
        double r = Math.sqrt(i);
        
        System.out.print("The square root of ");
        System.out.print(i);
        System.out.print(" is ");
        System.out.print(r);
        System.out.println(".");

        i = 5;
        r = Math.sqrt(i);
        System.out.println("The square root of " + i + " is " + r + ".");
    }
}

Here is the output of Root:下面是Root的输出：

The square root of 2 is 1.4142135623730951.
The square root of 5 is 2.23606797749979.

The i and r variables are formatted twice: the first time using code in an overload of print, the second time by conversion code automatically generated by the Java compiler, which also utilizes toString. i和r变量被格式化两次：第一次使用print的重载中的代码，第二次使用Java编译器自动生成的转换代码，Java编译器也使用toString。You can format any value this way, but you don't have much control over the results.您可以用这种方式格式化任何值，但对结果没有太多控制权。

The format Methodformat方法

The format method formats multiple arguments based on a format string. format方法基于格式字符串格式化多个参数。The format string consists of static text embedded with format specifiers; except for the format specifiers, the format string is output unchanged.格式字符串由嵌入格式说明符的静态文本组成；除格式说明符外，格式字符串输出不变。

Format strings support many features. 格式字符串支持许多功能。In this tutorial, we'll just cover some basics. 在本教程中，我们将只介绍一些基础知识。For a complete description, see format string syntax in the API specification.有关完整说明，请参阅API规范中的格式字符串语法。

The Root2 example formats two values with a single format invocation:Root2示例使用单个format调用格式化两个值：

public class Root2 {
    public static void main(String[] args) {
        int i = 2;
        double r = Math.sqrt(i);
        
        System.out.format("The square root of %d is %f.%n", i, r);
    }
}

Here is the output:以下是输出：

The square root of 2 is 1.414214.

Like the three used in this example, all format specifiers begin with a % and end with a 1- or 2-character conversion that specifies the kind of formatted output being generated. 与本例中使用的三个一样，所有格式说明符都以%开头，以1或2个字符的转换结束，该转换指定生成的格式化输出的类型。The three conversions used here are:这里使用的三种转换是：

• d formats an integer value as a decimal value.将整数值格式化为十进制值。
• f formats a floating point value as a decimal value.将浮点值格式化为十进制值。
• n outputs a platform-specific line terminator.输出特定于平台的线路终止符。

Here are some other conversions:以下是一些其他转换：

• x formats an integer as a hexadecimal value.将整数格式化为十六进制值。
• s formats any value as a string.将任何值格式化为字符串。
• tB formats an integer as a locale-specific month name.将整数格式化为特定于区域设置的月份名称。

There are many other conversions.还有许多其他的转换。

In addition to the conversion, a format specifier can contain several additional elements that further customize the formatted output. 除了转换之外，格式说明符还可以包含几个额外的元素，这些元素可以进一步自定义格式化输出。Here's an example, Format, that uses every possible kind of element.这里有一个例子，Format，它使用了所有可能的元素。

public class Format {
    public static void main(String[] args) {
        System.out.format("%f, %1$+020.10f %n", Math.PI);
    }
}

Here's the output:以下是输出：

3.141593, +00000003.1415926536

The additional elements are all optional. 附加元素都是可选的。The following figure shows how the longer specifier breaks down into elements.下图显示了较长说明符如何分解为元素。

Elements of a Format Specifier.格式说明符的元素。

The elements must appear in the order shown. Working from the right, the optional elements are:元素必须按显示的顺序显示。从右侧操作时，可选元素包括：

• Precision. 精度。For floating point values, this is the mathematical precision of the formatted value. 对于浮点值，这是格式化值的数学精度。For s and other general conversions, this is the maximum width of the formatted value; the value is right-truncated if necessary.对于s和其他常规转换，这是格式化值的最大宽度；如有必要，将右截断该值。
• Width. 宽度。The minimum width of the formatted value; the value is padded if necessary. 格式化值的最小宽度；如有必要，将填充该值。By default the value is left-padded with blanks.默认情况下，该值用空格填充。
• Flags specify additional formatting options. 标志指定其他格式选项。In the Format example, the + flag specifies that the number should always be formatted with a sign, and the 0 flag specifies that 0 is the padding character. 在Format示例中，+标志指定数字应始终使用符号进行格式设置，0标志指定0是填充字符。Other flags include - (pad on the right) and , (format number with locale-specific thousands separators). 其他标志包括-（右边的pad）和,（使用特定于区域设置的千位分隔符格式化数字）。Note that some flags cannot be used with certain other flags or with certain conversions.请注意，某些标志不能与某些其他标志或某些转换一起使用。
• The Argument Index allows you to explicitly match a designated argument. 参数索引允许显式匹配指定的参数。You can also specify < to match the same argument as the previous specifier. 您还可以指定<与前面的说明符匹配相同的参数。Thus the example could have said: 因此，这个例子可以说：System.out.format("%f, %<+020.10f %n", Math.PI);