@Override public Map<String, String> putAll(@NotNull Map<String, String> entries) { if (entries == null) { throw new IllegalArgumentException("Cannot set null entries"); } Map<String, String> oldValues = new HashMap<>(entries.size()); for (String key : entries.keySet()) { String oldValue = put(key, entries.get(key)); oldValues.put(key, oldValue); } return oldValues; } }