driver-legacy/src/main/com/mongodb/QueryBuilder.java
public QueryBuilder put(final String key) {
        _currentKey = key;
        if (_query.get(key) == null) {
            _query.put(_currentKey, new NullObject());
        }
        return this;
    }
public QueryBuilder put(final String key) {
        _currentKey = key;
        if (_query.get(key) == null) {
            _query.put(_currentKey, new NullObject());
        }
        return this;
    }
Adds a new key to the query if not present yet. Sets this key as the current key.

@param key MongoDB document key
@return {@code this}