Guava precondition checks, empty List

Question

I would like to know what is the best pattern for precondition checks on a list from which I will need to pick the first item.

In words, I suppose the list should not be null and its size should be > 1.

I find Guava's checkPositionIndex not helpful in this respect. On the contrary, I find it counter-intuitive, see the example below which bombs on an empty list because I use checkPositionIndex rather than checkArgument, as outlined after the guard which does not trigger.

It seems like checking position 0 is not sufficient to validate the argument even if I .get(0) from it?

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkPositionIndex;
import java.util.List;
import com.google.common.collect.Lists;
public class HowShouldIUseCheckPositionIndex {
  private static class ThingAMajig {
    private String description;
    private ThingAMajig(String description) {
      this.description = description;
    }
    @Override
    public String toString() {
      return description;
    }
  }
  private static void goByFirstItemOfTheseAMajigs(List<ThingAMajig> things) {
    checkNotNull(things);
    // Check whether getting the first item is fine
    checkPositionIndex(0, things.size()); // Looks intuitive but...
    System.out.println(things.get(0)); // Finally, help the economy!
    checkArgument(things.size() > 0); // This would have worked :(
  }
  public static void main(String[] args) {
    List<ThingAMajig> fullList =
        Lists.newArrayList(new ThingAMajig(
            "that thingy for the furnace I have been holding off buying"));
    List<ThingAMajig> emptyList = Lists.newLinkedList();
    goByFirstItemOfTheseAMajigs(fullList);
    // goByFirstItemOfTheseAMajigs(emptyList); // This *bombs*
  }
}

Answer 1

You should use checkElementIndex() instead.

checkPositionIndex() ensures that the given position is a valid position to insert new element to (i.e. you can do add(0, obj) on empty list), not a valid index to get an element from.

Answer 2

You don't need to do the check at all, in point of fact.

The list.get(0) call itself will already throw basically exactly the same error message.

But if you did want to do the check explicitly, yes, use checkElementIndex or checkArgument(!list.isEmpty()).