Question

I’m trying to understand how to use immutable classes in scala as a replacement for mutable java classes.

My example use case is searching for contacts in a database.
When programming imperatively you would pull each field from the database table and set it on the java object.

How would you do this in Scala using immutable types and functional style?

I know I could use a constructor and create a new instance of the object passing all required data to it, but for large complex data structures this doesn’t seem elegant.

What is the “Scala” / “Functional” way to accomplish this?
Or what are some best practices related to constructing complex immutable types?

public List<Contact> search(String firstName, String lastName) throws SQLException {
    List<Contact> contacts = new ArrayList<Contact>();

    Connection con = null;
    PreparedStatement ps = null;
    ResultSet rs = null;
    try {
        con = dataSource.getConnection();

        ps = con.prepareStatement("select * from contacts where first_name like ? and last_name like ?");
        ps.setString(1, "%" + firstName + "%");
        ps.setString(1, "%" + lastName + "%");
        rs = ps.executeQuery();

        while (rs.next()) {
            Contact contact = new Contact();
            contact.setFirstName(rs.getString("first_name"));
            contact.setMiddleName(rs.getString("middle_name"));
            contact.setLastName(rs.getString("last_name"));
            Date birthday = rs.getDate("birthday");
            if (birthday != null) {
                contact.setBirthday(new Date(birthday.getTime()));
            }
            contacts.add(contact);
        }

    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        rs.close();
        ps.close();
        con.close();
    }
    return contacts;
}

Contact POJO

import java.util.Date;

public class Contact {
    private String firstName;
    private String middleName;
    private String lastName;
    private Date birthday;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getMiddleName() {
        return middleName;
    }

    public void setMiddleName(String middleName) {
        this.middleName = middleName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
}

Answer 1

You can also have a mutable builder which makes the immutable object you are looking for. There’s nothing wrong with having temporary mutable objects if they stay in their creation scope and they are not propagated elsewhere. The Scala standard library implementation use plenty of those.

Here is a simple example (although this solution is more useful when constructors are big):

// Here is the immutable class
case class Person( name: String, age: Int, married: Boolean )

// Here comes the mutable builder
class PersonBuilder {
  private var name: Option[String] = None
  private var age: Option[Int] = None
  private var married: Option[Boolean] = None

  def setName( n: String ) = { name = Some(n); this }
  def setAge( a: Int ) = { age = Some(a); this }
  def setMarried( m: Boolean ) = { married = Some(m); this }

  def build() = {
    val person = for( n <- name; a <- age; m <- married ) 
                   yield { Person( n, a, m ) }
    person getOrElse { throw new IllegalStateException( /*msg*/ ) }
  }
}

It can be used as:

val builder = new PersonBuilder
builder setName "Alex"
builder setAge 42
builder setMarried false
val person = builder build  // The immutable instance