Why does ImmutableSet allow duplicates, but ImmutableMap does not

Question

I noticed (and appreciated!) that Guava's ImmutableMap.Builder fails to build if duplicate keys have been added to the builder. Yet the same behavior (adding duplicate elements) succeeds with ImmutableSet.

Is there a reason for this difference, and any good way to construct an ImmutableSet with the same failure behavior?

Test case:

import static org.testng.Assert.*;
import org.testng.annotations.Test;

import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;

public class ImmutableDuplicatesTest
{
    @Test(expectedExceptions=IllegalArgumentException.class) // Note failure
    public void mapDuplicates() {
        ImmutableMap.Builder<String, String> map = ImmutableMap.builder();
        map.put("a", "a");
        map.put("b", "b");
        map.put("a", "c");
        assertEquals(map.build().size(), 2);
    }

    @Test // Passes normally
    public void setDuplicates() {
        ImmutableSet.Builder<String> set = ImmutableSet.builder();
        set.add("a");
        set.add("b");
        set.add("a");
        assertEquals(set.build().size(), 2);
    }
}

Answer 1

Yes, this behavior is deliberate. Here's one way of thinking about it: Sets are frequently created from other Collections, especially List, which may have duplicates. It'd be very awkward -- and inefficient -- to require users to write ImmutableSet.copyOf(Sets.newHashSet(element)) if there might be duplicates. On the other hand, Maps are typically built from other Maps, which can't have duplicate keys.

If you want to forbid duplicate elements, your best bet is something like

Set<E> set = new LinkedHashSet<E>();
for (E e : input) {
  if (!set.add(e)) {
    throw new IllegalArgumentException();
  }
}
return ImmutableSet.copyOf(set);