public V remove(Object key) {
        final int index = indexOfKey(key);
        if (index >= 0) {
            return removeAt(index);
        }

        return null;
    }
Remove an existing key from the array map.
@param key The key of the mapping to remove.
@return Returns the value that was stored under the key, or null if there
was no such key.
core/src/main/java/com/bluelinelabs/logansquare/util/SimpleArrayMap.java