Question

Why are constructor calls in Java allowed only once per instance? If would be useful to set multiple instance variables in one call rather than calling several setters.

Answer 1

If you want to set several variables at once just define a function:

public class Person {
  public Person(String firstName, String lastName, Date dateOfBirth) {
    set(firstName, lastName, dateOfBirth);
  }

  public void set(String firstName, String lastName, String dateOfBirth) {
    ...
  }
}

By definition an object is only constructed once, hence the constructor is only called once.

One thing worth noting: it’s more common to favour immutability, so:

public class Person {
  private final String firstName;
  private final String lastName;
  private final Date dateOfBirth;

  public Person(String firstName, String lastName, Date dateOfBirth) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.dateOfBirth = dateOfBirth == null ? null new Date(dateOfBirth.getTime());
  }

  public String getFirstName() { return firstName; }
  public String getLastName() { return lastName; }
  public Date getDateOfBirth() { return dateOfBirth == null ? null : new Date(dateOfBirth.getTime()); }

  public Person withFirstName(String firstName) {
    return new Person(firstName, lastName, dateOfBirth);
  }

  public Person withLastName(String lastName) {
    return new Person(firstName, lastName, dateOfBirth);
  }

  public Person withDateOfBirth(Date dateOfBirth) {
    return new Person(firstName, lastName, dateOfBirth);
  }
}

because a lot of concurrency issues simply go away when you do this. Not just concurrency issues too. String, BigDecimal, BigInteger and some other standard classes are immutable (immutable means once instantiated its state can never change). Date isn’t. You see in the above code I have to constantly defensively copy the date of birth? That’s because Date is not immutable. If you didn’t a caller could do this:

public class Person {
  private final Date dateOfBirth;

  ...

  public Date getDateOfBirth() { return dateOfBirth; }
}

Person person = new Person(...);
Date date1 = person.getDateOfBirth();
date1.setTime(1000000000L);
System.out.println(person.getDateOfBirth());

The date of birth will have changed. That problem is caused solely by Date being mutable. That’s another reason to favour immutability.