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How do I remove duplicates in a List?

I have a List that may contain duplicates and I need to filter out the duplicates (so I don't get a foreign key violation when inserting records). Are there any Jakarta' Commons utilities to do this? I can see a few solutions to my problem:

Present it on the UI. This might be tough since I'm using a select box JavaScript library. Hmmm, I am using a custom setter for the String[] that gets sent in, maybe I can do it there - checking to see if the ArrayList I'm setting already contains the given String.
Filter it out in the business layer.
Use a type of List that doesn't allow duplicates, and just ignores a duplicate value when you try to add it.

I just thought of #1 while writing this post. It's cool how talking about your problems can help solve them - no wonder shrinks get paid so much. I'm still interested in any proposed utility classes.

Posted in Java at Mar 08 2003, 07:15:42 AM MST 5 Comments

Comments:

Why not use a Set instead? Sets only allow one instance of an object.

Posted by Lance on March 08, 2003 at 10:23 AM MST #

Matt,

You might try something like this in your formbean setter?

ArrayList list = new ArrayList();
 
list.add("CCCC");	
list.add("AAAA");	
list.add("BBBB");	
list.add("AAAA");	
list.add("CCCC");	
list.add("AAAA");	  
	    
Collections.sort(list);	    
boolean[] dupSet = new boolean[list.size()];
for (int i = 0; i < list.size() - 1;) { 
  dupSet[i] = ((String) list.get(i++)).equals(list.get(i));
}	
for (int i = dupSet.length -1; i >= 0; --i) {
   if (dupSet[i]) list.remove(i);
}		
dupSet = null;
		
		for (int i = 0; i < list.size(); i++) {
	       System.out.println(list.get(i).toString());
		}

Posted by Anonymous on March 08, 2003 at 10:55 AM MST #

There is always contains.

prop.add(someString);

..... in the bean....

public void add(Object obj)
{
   if(!this.prop.contains(obj))
      this.prop.add(obj);
}

No need to re-implement the wheel :)

Posted by Carl Fyffe on March 08, 2003 at 09:16 PM MST #

What about HashSet?

Posted by Anonymous on March 08, 2003 at 11:03 PM MST #

Using a Set seems to be the right approach. If the output has to be a List, add the items to a Set first and then use:

List result = (List) new ArrayList(mySet);

If you need the items to be ordered use TreeSet, else choose HashSet.

HTH
Fokko

Posted by Anonymous on March 09, 2003 at 10:28 AM MST #

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How do I remove duplicates in a List?