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.