格式化

朴灿烈づ我的快乐病毒、 2021-09-23 00:44 343阅读 0赞

## java.util.Formatter ##

--------------------

###### 源码 ######

public final class Formatter implements Closeable, Flushable {}

###### 例1 ######

import java.util.Formatter;
    public class COM {
    	public static void main(String[] args) {
    		 float x1= -12.3f;
    		 Formatter f = new Formatter(System.out);   //注意这里的区别
    		 f.format("%.3f//%5s", x1 ,"hello");
    		 System.out.println();
    		 f.format("%.3f// %5s", x1 ,"hello");
    		 System.out.println();
    		 f.format("%.3f//h%5s", x1 ,"hello");
    		 System.out.println();
    	}
    }
    Output:
    -12.300//hello
    -12.300// hello
    -12.300//hhello

import java.util.Formatter;
    public class COM {
    	public static void main(String[] args) {
    		 float x1= -12.3f;
    		 Formatter f = new Formatter(System.out);
    		 f.format("%.3f//%5s", x1 ,"hello");
    		 f.format("%.3f// %5s", x1 ,"hello");
    		 f.format("%.3f//h%5s", x1 ,"hello");
    	}
    }
    Output:
    -12.300//hello-12.300// hello-12.300//hhello

###### 例2 ######

import java.util.Formatter;
    public class COM {
    	public static void main(String[] args) {
    		 float x1= -12.3f;
    		 Formatter f = new Formatter();  // //注意这里的区别
    		 System.out.println(f.format("%.3f//%5s", x1 ,"hello"));
    		 System.out.println(f.format("%.3f//%5s", x1 ,"hello"));
    		 System.out.println(f.format("%.3f//%5s", x1 ,"hello"));
    	}
    }
    Output:
    -12.300//hello
    -12.300//hello-12.300// hello
    -12.300//hello-12.300// hello-12.300//hello

f.format输出了上次调用f.format后输出的。

###### 例3 ######

import java.util.Formatter;
    public class COM {
    	public static void main(String[] args) {
    		 int x = 9;
    		 float x1= -12.3f;
    		 Formatter f = new Formatter(System.out);
    		 f.format("%.3f// %5s", x1 ,"hello");
    		 System.out.println(); 
    		 f.format("%.3f//%6s", x1 ,"hello");
    		 System.out.println(); 
    		 // %6.2s：取出"he"，共2位，6有效，"he"前补4位空格
    		 f.format("%.3f//%6.2s", x1 ,"hello");
    		 System.out.println(); 
    		 // %6.7s表示："hello"中从前往后取7个字符，但一共占6位，
    		 // 如果6小于"hello"取7位后的字符个数，则6无效
    		 f.format("%.3f//%6.7s//%d", x1 ,"hello",x);
    		 System.out.println(); 
    		 // %4.7s：取出"hello"，共5位，4无效
    		 f.format("%.3f//%4.7s//%d", x1 ,"hello",x);
    		 System.out.println(); 
    		 f.format("%5s--%1$5s--%2$5s--%3$5s","hello","world","HELLO");
    	}
    }
    Output:
    -12.300// hello
    -12.300// hello
    -12.300//    he
    -12.300// hello//9
    -12.300//hello//9
    hello--hello--world--HELLO

f.format("%5s--%0$5s--%0$5s--%0$5s","hello","world","HELLO");  //出现异常
    // 因为%5s输出后，0$指向的是"world",不是,"hello"。
    Output:
    hello--world--HELLO--Exception in thread "main"
    
    f.format("%5s--%0$5s--%0$5s--%3$5s","hello","world","HELLO");
    f.format("%0$5s--%0$5s--%3$5s","hello","world","HELLO");
    Output:
    hello--world--HELLO--HELLO
    hello--world--HELLO

开始时，索引0指向第一个参数，所以（）内开始时的%5s，%0$5s是在输出第一个格式符下的数值；输出后，索引0的指向向后移一位。而指定参数时，从索引1开始。

## java.lang.String ##

--------------------

###### 源码 ######

public final class String implements java.io.Serializable,
    	 Comparable< String >, CharSequence {
        @Stable
        private final byte[] value;
        public static String format(String format, Object... args) {
            return new Formatter().format(format, args).toString();
        }
    }

###### 例1 ######

import java.util.Formatter;
    public class COM {
    	public static void main(String[] args) {
    		 String s="hello";
    		 Formatter f = new Formatter(System.out);
    		 String j=String.format("%s", s);  //new的对象，j是对象地址
    		 System.out.println(j); 
    		 System.out.println((j==s));   //==比较引用，equals比较内容
    		 System.out.println((j.equals(s))); 
    	}
    }
    Output:
    hello
    false
    true