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;
    }
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}
Adds a new key to the query if not present yet . Sets this key as the current key .