How to implement RowMapper using java lambda expression

RowMapper is a interface with a single abstract method (not inheriting from a method of Object), so it can be considered a functional interface. Its functional method takes a ResultSet and a int, and returns an object.

The first problem with the code is that the type of the object returned is a generic type of the interface. As currently used with RowMapper, you're using a raw-type, which you shouldn't do. The second issue is that the lambda expression does not return any object, so it cannot conform to the functional method which except an object to be returned.

As such, a corrected code would be:

RowMapper<Person> rowMapper = (rs, rowNum) -> {
    Person p = new Person();
    p.setName(rs.getString("personName"));
    p.setAddress(rs.getString("address"));
    p.setAge(rs.getInt("age"));
    return p;
};

@Tunaki is correct. Here is the shorthand version:

RowMapper<Person> rowMapper = (rs, rowNum) -> new Person(rs.getString("personName"), rs.getString("address"),rs.getInt("age")) ;

The brackets and the return aren't needed as they are implied.

RowMapper using lambda expression example:

return jdbcTemplate.query(" select Scale_Point,Scale_Head from TEval_Scale ", new Object[] {},
                (resultSet, rowNum) ->{

                    TEvalScale tEvalScale = new TEvalScale();
                    tEvalScale.setScalePoint(resultSet.getInt("Scale_Point"));
                    tEvalScale.setScaleHead(resultSet.getString("Scale_Head"));
                    return tEvalScale;

                });

In addition to @Tunaki answer.

RowMapper<Person> rowMapper = (rs, rowNum) -> {
    Person p = new Person();
    p.setName(rs.getString("personName"));
    p.setAddress(rs.getString("address"));
    p.setAge(rs.getInt("age"));
    return p;
};

Map<String, Object> paramMap = new HashMap<>();
paramMap.put("person_id", personId);

If you want to get single Person :

Person person = getJdbcTemplate().queryForObject(queryString, paramMap, rowMapper);

If you want to get List of Person :

List<Person> personList = getJdbcTemplate().query(queryString, paramMap, rowMapper);