java基础---->Java的格式化输出

　　在JavaSe5中，推出了C语言中printf()风格的格式化输出。这不仅使得控制输出的代码更加简单，同时也给与Java开发者对于输出格式与排列更大的控制能力。今天，我们开始学习Java中的格式化输出。

System.out.format()

由于内容比较简单，我们通过实例来加以说明。项目结构如下：

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" 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Java Se5引入的format方法可用于PrintStream或PrintWriter对象，其中也包括System.out对象。

package com.tomhu.format;

public class FormatTest1 {

    public static void main(String[] args) {
        int x = 5;
        double y = 3.141592;

        // 一般方式
        System.out.println("x = " + x + ", y = " + y);
        // printf()方式
        System.out.printf("x = %d, y = %f\n", x, y);
        // format()方式
        System.out.format("x = %d, y = %f\n", x, y);
    }
}

输出的结果如下：

x = 5, y = 3.141592
x = 5, y = 3.141592
x = 5, y = 3.141592

可以看到，format与printf是等价的，它们只需要一个简单的格式化字符串，加上一串参数即可，每个参数对应一个格式修饰符。

public PrintStream printf(String format, Object ... args) {
    return format(format, args);
}

在format的具体代码中，其实就是调用Formatter的format方法：formatter.format(Locale.getDefault(), format, args);

public PrintStream format(String format, Object ... args) {
    try {
        synchronized (this) {
            ensureOpen();
            if ((formatter == null)
                || (formatter.locale() != Locale.getDefault()))
                formatter = new Formatter((Appendable) this);
            formatter.format(Locale.getDefault(), format, args);
        }
    } catch (InterruptedIOException x) {
        Thread.currentThread().interrupt();
    } catch (IOException x) {
        trouble = true;
    }
    return this;
}

Formatter类

在Java中，所有新的格式化功能都由Formatter类处理，上述的printf与format也是。可以将Formatter看作是一个翻译器，它将你的格式化字符串与数据翻译成需要的结果。当你创建一个Formatter对象的时候 ，需要向其构造器传递一些信息，告诉它最终的结果将向哪里输出

package com.tomhu.format;

import java.util.Formatter;

public class FormatTest2 {

    public static void main(String[] args) {
        String name = "huhx";
        int age = 22;

        Formatter formatter = new Formatter(System.out);
        formatter.format("My name is %s, and my age is %d ", name, age);
        formatter.close();
    }
}

它的输出结果如下：

My name is huhx, and my age is 22 

格式化说明符

在插入数据时，如果想要控制空格与对齐，就需要精细复杂的格式修饰符，以下是其抽象的语法：

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

The optional argument_index is a decimal integer indicating the position of the argument in the argument list. The first argument is referenced by "1$", the second by "2$", etc.

The optional flags is a set of characters that modify the output format. The set of valid flags depends on the conversion.

The optional width is a non-negative decimal integer indicating the minimum number of characters to be written to the output.

The optional precision is a non-negative decimal integer usually used to restrict the number of characters. The specific behavior depends on the conversion.

The required conversion is a character indicating how the argument should be formatted. The set of valid conversions for a given argument depends on the argument's data type.

最常见的应用是控制一个域的最小尺寸，这可以通过指定width来实现。Formatter对象通过在必要时添加空格，来确保一个域至少达到某个长度。在默认的情况下，数据是右对齐的，通过"-"标志可以改变对齐的方向。

与width相对的是precision(精确度)，它用来指明最大尺寸。width可以应用各种类型的数据转换，并且其行为方式都一样。precision则不一样，不是所有类型的数据都能使用precision，而且，应用于不同的类型的数据转换时，precision的意义也不同。

• precision应用于String时，它表示打印String时输出字符的最大数量
• precision应用于浮点数时，它表示小数点要显示出来的位数。默认是6位小数，如果小数位数过多则舍入，过少则在尾部补零。
• 由于整数没有小数部分，所以precision不能应用于整数。如果你对整数应用precision，则会触发异常

package com.tomhu.format;

import java.util.Formatter;

public class FormatTest3 {
    static Formatter formatter = new Formatter(System.out);

    public static void printTitle() {
        formatter.format("%-15s %-5s %-10s\n", "huhx", "linux", "liuli");
        formatter.format("%-15s %-5s %-10s\n", "zhangkun", "yanzi", "zhangcong");
        formatter.format("%-15s %-5s %-10s\n", "zhangkun", "yanzhou", "zhangcong");
    }

    public static void print() {
        formatter.format("%-15s %5d %10.2f\n", "My name is huhx", 5, 4.2);
        formatter.format("%-15.4s %5d %10.2f\n", "My name is huhx", 5, 4.1);
    }

    public static void main(String[] args) {
        printTitle();
        System.out.println("----------------------------");
        print();
        formatter.close();
    }
}

它的输出结果如下：

huhx            linux liuli
zhangkun        yanzi zhangcong
zhangkun        yanzhou zhangcong
----------------------------
My name is huhx     5       4.20
My n                5       4.10

Formatter转换

下面的表格包含了最常用的类型转换：

类型转换字符

d	整数型(10进制 )	e	浮点数(科学计数)
c	Unicode字符	x	整数(16进制)
b	Boolean值	h	散列码(16进制)
s	String	%	字符"%"
f	浮点数(10进制)

String.format()是一个static方法，它接受与Formatter.format()方法一样的参数，但返回一个String对象。当你只需要用format方法一次的时候，String.format()还是很方便的。

package com.tomhu.format;

public class FormatTest4 {

    public static void main(String[] args) {
        int age = 22;
        String name = "huhx";

        String info = String.format("My name is %s and my age is %d", name, age);
        System.out.println(info);
    }
}

它的输出结果如下：

My name is huhx and my age is 22

其实String.format方法的实质还是Formatter.format()，只不过是做了简单封装而已：

public static String format(String format, Object... args) {
    return new Formatter().format(format, args).toString();
}

简单的十六进制转换工具

package com.tomhu.format;

public class FormatTest5 {

    public static String format(byte[] data) {
        StringBuilder builder = new StringBuilder();
        int n = 0;
        for(byte b: data) {
            if (n %16 == 0) {
                builder.append(String.format("%05x: ", n));
            }
            builder.append(String.format("%02x ", b));
            n ++;
            if (n % 16 == 0) {
                builder.append("\n");
            }
        }
        builder.append("\n");
        return builder.toString();
    }

    public static void main(String[] args) {
        String string = "my name is huhx, welcome to my blog";
        System.out.println(format(string.getBytes()));
    }
}

输出结果如下：

00000: 6d 79 20 6e 61 6d 65 20 69 73 20 68 75 68 78 2c
00010: 20 77 65 6c 63 6f 6d 65 20 74 6f 20 6d 79 20 62
00020: 6c 6f 67 

友情链接

作者： huhx 
出处： www.cnblogs.com/huhx
格言：你尽力了，才有资格说自己的运气不好。
版权：本文版权归作者huhx和博客园共有，欢迎转载。但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文链接，否则保留追究法律责任的权利。