Java 字符串格式化示例

本文向你展示如何通过 String.format() 来格式化Java中的字符串。

这是总结。

进一步阅读-Formatter JavaDoc

1. Simple

JavaStringFormat1.java

package com.mkyong;
public class JavaStringFormat1 {
    public static void main(String[] args) {
        String result = String.format("%s is %d", "mkyong", 38);     // mkyong is 38
        System.out.println(result);
        String result2 = String.format("%d + %d = %d", 1, 1, 1 + 1); // 1 + 1 = 2
        System.out.println(result2);
        String result3 = String.format("%s = %f", "PI", Math.PI);    // PI = 3.141593
        String result4 = String.format("%s = %.3f", "PI", Math.PI);  // PI = 3.142
        System.out.println(result3);
        System.out.println(result4);
    }
}

Output

mkyong is 38
1 + 1 = 2
PI = 3.141593
PI = 3.142

2. Argument

我们可以使用来重新排列参数%{index}${conversion}。

• %1$ –第一个论点
• %2$ –第二个论点
• %3$ –第三个论点
• %{index}$ – {index}参数

JavaStringFormat2.java

package com.mkyong;
public class JavaStringFormat2 {
    public static void main(String[] args) {
        String a1 = "hello1";
        String a2 = "hello2";
        Integer a3 = 333;
        String result = String.format("Test: %3$d, %1$s, %1$s, %2$s", a1, a2, a3);
        System.out.println(result);
    }
}

Output

Test: 333, hello1, hello1, hello2

3. String

JavaStringFormat3.java

package com.mkyong;
public class JavaStringFormat3 {
    public static void main(String[] args) {
        String input = "Hello World";
        // default
        String result1 = String.format("|%s|", input);              // |Hello World|
        // this %s has length of 20, format as %20s, pad 20 characters
        // default right-justified
        String result2 = String.format("|%20s|", "Hello World");    // |         Hello World|
        // left-justified
        String result3 = String.format("|%-20s|", "Hello World");   // |Hello World         |
        // max length = 5
        String result4 = String.format("|%.5s|", "Hello World");    // |Hello|
        // left pad with $
        String result5 = String.format("|%20s|", "Hello World")     // |$$$$$$$$$Hello$World|
                .replace(' ', '$');
        // left pad with $, ignore spaces in string
        String result6 = padLeft("Hello World", 20, "$");          // $$$$$$$$$Hello World
        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
        System.out.println(result4);
        System.out.println(result5);
        System.out.println(result6);
    }
    public static String padLeft(String str, int width, String padWith) {
        String result = "";
        String temp = String.format("%" + width + "s", str);
        if (temp.length() > str.length()) {
            result = temp.substring(0, temp.length() - str.length()).replace(" ", padWith);
        }
        result += str;
        return result;
    }
}

Output

|Hello World|
|         Hello World|
|Hello World         |
|Hello|
|$$$$$$$$$Hello$World|
$$$$$$$$$Hello World

4. Integer

JavaStringFormat4.java

package com.mkyong;
public class JavaStringFormat4 {
    public static void main(String[] args) {
        // default
        String result1 = String.format("|%d|", 100);                // |100|
        // this %d has length of 20, format as %20d, pad 20 characters
        // default right-justified
        String result2 = String.format("|%20d|", 100);              // |                 100|
        // left-justified
        String result3 = String.format("|%-20d|", 100);             // |100                 |
        // left pad with 0
        String result4 = String.format("|%020d|", 100);             // |00000000000000000100|
        // prefix with +
        String result5 = String.format("|%+20d|", 100);             // |                +100|
        // negative
        String result6 = String.format("|%20d|", -100);             // |                -100|
        // octal
        String result7 = String.format("|%20o|", 100);              // |                 144|
        // hex
        String result8 = String.format("|%20x|", 30);               // |                  1e|
        // prefix 0x with #
        String result9 = String.format("|%#20x|", 30);              // |                0x1e|
        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
        System.out.println(result4);
        System.out.println(result5);
        System.out.println(result6);
        System.out.println(result7);
        System.out.println(result8);
        System.out.println(result9);
    }
}

输出量

|100|
|                 100|
|100                 |
|00000000000000000100|
|                +100|
|                -100|
|                 144|
|                  1e|
|                0x1e|

5. Floating Points（浮点数）

JavaStringFormat5.java

package com.mkyong;
public class JavaStringFormat5 {
    public static void main(String[] args) {
        double pi = Math.PI;
        // default
        String result1 = String.format("%f", pi);       // 3.141593
        // 2 decimal points
        String result2 = String.format("%.2f", pi);     // 3.14
        String result3 = String.format("%e", pi);       // 3.141593e+00
        String result4 = String.format("%a", pi);       // 0x1.921fb54442d18p1
        // right
        String result5 = String.format("|%20f|", pi);   // |            3.141593|
        // left
        String result6 = String.format("|%-20f|", pi);  // |3.141593            |
        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
        System.out.println(result4);
        System.out.println(result5);
        System.out.println(result6);
    }
}

Output

3.141593
3.14
3.141593e+00
0x1.921fb54442d18p1
|            3.141593|
|3.141593            |

6.常见问题

6.1 将十进制转换为二进制（4个字节），并用0填充。

  // 1100100
  String result1 = String.format("%s", Integer.toBinaryString(100));
  // 00000000000000000000000001100100
  String result2 = String.format("%32s", Integer.toBinaryString(100)).replace(" ", "0");
  // 00000000000000011110001001000000
  String result3 = String.format("%32s", Integer.toBinaryString(123456)).replace(" ", "0");

参考文献

• Formatter JavaDoc

扩展阅读

JAVA字符串格式化-String.format()的使用