Sample code showing how even after a Guava transform, the underlying collection may still be altered.

WatchMe.java

import static com.google.common.collect.Collections2.transform;

import java.util.Collection;
import java.util.HashSet;
import java.util.Set;

import com.google.common.base.Function;
import com.google.common.collect.Lists;
import com.google.common.collect.Sets;


public class WatchMe {

	public static void main(String[] args) {
		Set<Noisy> s1 = Sets.newHashSet();
		Set<Noisy> s2 = Sets.newHashSet();
		
		
		s1.add(new Noisy("n1"));
		s1.add(new Noisy("n2"));
		s1.add(new Noisy("n3"));
		s1.add(new Noisy("n4"));
		s1.add(new Noisy("n5"));
		
		s2.add(new Noisy("n2"));
		s2.add(new Noisy("n3"));
		s2.add(new Noisy("n4"));
		
		System.out.println("Set 1:" + s1);
		System.out.println("Set 2:" + s2);

		Collection<String> xformed1 = transform(s1, new Function<Noisy,String>() {
			public String apply(Noisy input) {
				return input.humanValue();
			}});
		
		Collection<String> xformed2 = transform(s2, new Function<Noisy,String>() {
			public String apply(Noisy input) {
				return input.humanValue();
			}});
		
		System.out.println("X-Formed 1:" + xformed1);
		System.out.println("X-Formed 2:" + xformed2);
		
		System.out.println(" ** Call xformed1.retainAll(xformed2)");
		xformed1.retainAll(xformed2);
		System.out.println("X-Formed 1 after retainAll(xformed2):" + xformed1);
		System.out.println("Set 1 after xformed1.retainAll(xformed2):" + s1);
		
		System.out.println("\n\nMy initial though was that xformed1 would be altered to filter out");
		System.out.println("the contents of xformed 2 but what's occurring is that the UNDERLYING");
		System.out.println("Set is actually being altered. ");
		
	}
}

WatchMeOutput.txt

Set 1:[n2, n1, n4, n3, n5]
Set 2:[n2, n4, n3]
X-Formed 1:[The value = n2, The value = n1, The value = n4, The value = n3, The value = n5]
X-Formed 2:[The value = n2, The value = n4, The value = n3]
 ** Call xformed1.retainAll(xformed2)
X-Formed 1 after retainAll(xformed2):[The value = n2, The value = n4, The value = n3]
Set 1 after xformed1.retainAll(xformed2):[n2, n4, n3]


My initial though was that xformed1 would be altered to filter out
the contents of xformed 2 but what's occurring is that the UNDERLYING
Set is actually being altered.