Inncer Class!

[Brian Smith]

hi folks,
can you please explain me with some examples why do we need inner class in java other than in eventlistener?
thanks.

[Ilja Preuss]

*Need* would be too strong a word. Everywhere you are using an inner class, you could replace it by a top level class. In fact, to the JVM all classes are top-level - inner classes are just syntactic sugar fully handled by the compiler.
It's just that sometimes a class is so small and so tightly coupled to another class, that writing it as an inner class is simply much more convenient.

[Chris Treglio]

I happen to have two of them in the class I'm working with right now ...
Here's a FileNameFilter

File logDir = new File(dir);
   
   FilenameFilter logFilesOnly = new FilenameFilter(){
   
   public boolean accept(File dir, String name){
   
   if (name != null && name.startsWith(pre))
   return true;
   else 
   return false;
   }
   };
   
   File[] logFiles = logDir.listFiles(logFilesOnly);

and here's a Comparator

public static Comparator byUser(final boolean or){

return new Comparator() {
public int compare(Object o1, Object o2){
Hit h1, h2;

// is either object null?
Integer objNull = nullComp(o1, o2);

if (objNull != null){
return objNull.intValue();
}

// is either object not castable to Hit?
if (! (o1 instanceof Hit || o2 instanceof Hit) ) {
return 0;
}

if (!(o1 instanceof Hit)){
return 1;
}

if (!(o2 instanceof Hit)){
return -1;
}

h1 = (Hit) o1;
h2 = (Hit) o2;


// is either object's user String null?
String user1 = h1.getUser();
String user2 = h2.getUser();

Integer uNull = nullComp(user1, user2);

if (uNull != null){
return uNull.intValue();
}

// now, which one comes first, depending on the user attribute?
if (or)
return user1.compareToIgnoreCase(user2);
else 
return -user1.compareToIgnoreCase(user2);


}
};


}

[Cindy Glass]

Perhaps you should read Getting in Touch with your Inner Class .
It explains why lazy folks prefer Inner classes so that even I can understand it.

[Chris Treglio]

Oh Cindy that's harsh.
Do you really think that using anonymous inner classes like the ones I posted above are lazy programming? I'm open to suggestions, but I always thought that they were good examples of encapsulation .... the chance of being able to reuse one outside the class is pretty slim, especially the Comparator.