When defining static class

My current understanding is that some of the tools
recently did months of Excel operations, multiple functions can realistically

class static Excel{
  public static toCsv(arg..){}
  public static read(arg..){}
  public static write(arg..){}
  ...
  }

then looked at the POI source code examples XLS2CSVmra, suddenly puzzled
why this class is not static
then thought

SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
  sdf.format(new Date());

if I am to write, would write the class

SimpleDateFormat.format(new Date(),"yyyy-MM-dd")

But the large cattle did not say so.
suddenly feel connected "when defining static class " This can not understand simple questions
good lost , help
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in another class internally defined class that does not want to rely on the instantiation of the outer class instantiation , this time can be defined as a static class ( ie , internal static class ) .
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http://blog.sina.com.cn/s/ blog_605f5b4f0100zbps.html to give a reference here to the more detailed
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directly define static constants .
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this trouble speaking , visual LZ is to ask static method , but wander static inner classes to go. .
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SimpleDateFormat should not be tools, only internal static class category , the landlord said should be a static method. .
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  //可以利用SimpleDateFormat来写自己的工具类，比如
  public class DateUtil {
  	
  	public static String getStringDate(){
  		SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
  		return sdf.format(new Date());
  	}
  }
  

use of static methods as well as " single mode" code written much, much more to see , a lot of things naturally understand
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SimpleDateFormat first new a new object in the call methods .
accordance with the landlord was thinking : SimpleDateFormat.format (new Date (), "yyyy-MM-dd") so format () method is a static method
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class static Excel . . .
LZ good creativity
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wrong , do not care about these details, to understand better
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novice passing . . . . . .

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static class is doing, there is demand for it ?