Making a collection of unique values

[Dave Bosky]

I have two collections, collection 1 contains all items and collection 2 contains items selected. I need to create a new collection of only items that haven't been selected. What is the easiest method to create a new collection that only contains unselected items? A little snippet would be cool too.

Thanks,
Dave

[Bear Bibeault]

Moving to Java in General(intermediate).

[Ernest Friedman-Hill]

The standard Collection classes have a "removeAll()" method which accepts another Collection as an argument; all the items in the first collection which appear in the second one are removed. So what you could do (if you need to preserve the full collection) is to construct a copy of it, and then use removeAll().

[Dave Bosky]

I tried to use the "Collection.removeAll()" method but both of my collections contain a collection of beans. How will collection2 know to remove the items that appear in collection1? I'm obviously missing something here. I'm fairly new to Java so excuse my lack of knowledge.

collection2.removeAll(collection1);

Thanks,
Dave

[Ernest Friedman-Hill]

Well, let's see. How will removeAll() know to do this? Because that's what it does. I'm not sure how else to answer this tautological question. For the sake of discussion, here's a little example program. Is your situation substantially different from this?

import java.util.*;
public class Sample {
  public static void main(String[] argv) {
 
    List collection1 = Arrays.asList(new String[] { "a", "b", "c", "d" });
    List collection2 = Arrays.asList(new String[] { "b", "c" });
   
    ArrayList complement = new ArrayList(collection1);
    complement.removeAll(collection2);
    System.out.println(complement);
  }
}

This program prints "[a, d]".

[Dave Bosky]

I have two collections that each contain a collection ItemBeans.
Each ItemBean has 2 getters/setters - ID(int) and Name(String).

Collection c1 - contains All Items(3 items).
ex: bean1 = ID:1 Name:Rock
bean2 = ID:2 Name aper
bean3 = ID:3 Name:Scissors

Collection c2 - contains only selected items(1 item).
ex: bean1 = ID:1 Name:Rock

I need to remove Collection c2 from Collection c1.
c1.removeAll(c2);

In this case I need to remove (ID:1 Name:Rock) from my collection.

I'm not sure how else to explain it. Like I mentioned in an earlier post, I'm still a little green in the Java world.

Thanks Again,
Dave

[Ernest Friedman-Hill]

As long as there's just one "copy" of a given ItemBean, this will just work, as given. If there are multiple copies -- i.e., two physically different ItemBeans that both represent "bean1", then for this to work, the ItemBean class has to properly override the equals() method, so that this method returns true when two ItemBeans are the same. Whenever you override equals(), you're supposed to do hashCode too. So...

class ItemBean {
 int id;
 String name;
 
 ...
 public boolean equals(Object o) {
   if (o.getClass() != getClass())
     return false;
   ItemBean other = (ItemBean) o;
   return name.equals(other.name) && id == other.id;
 }
 public int hashCode() {
   return name.hashCode();
 }
}

Now what I showed you should just work.