code: public void put(String key, String value) {
          Preconditions.checkNotNull(key, "key is null");
          Preconditions.checkNotNull(value, "value is null");
          Preconditions.checkArgument(key.indexOf('/') == -1, "key must not contain a slash");
          Preconditions.checkArgument(!children.containsKey(key), "key identifies a path, put operation not allowed");
          values.put(key, value);
      }
docs: Puts the {@code key-value} pair at the current {@code Context} level. The {@code key} must not contain any
  slashes ({@code '/'}).
  
  @throws NullPointerException if {@code key} or {@code value} is null
  @throws IllegalArgumentException if a child {@code Context} with the given {@code key} exists.