Alex Rivera | Logout

Best way to handle multiple constructors in Java

Asked 2009-02-24T14:10:22.553
95

I've been wondering what the best (i.e. cleanest/safest/most efficient) way of handling multiple constructors in Java is? Especially when in one or more constructors not all fields are specified:

public class Book
{

    private String title;
    private String isbn;

    public Book()
    {
      //nothing specified!
    }

    public Book(String title)
    {
      //only title!
    }

    ...     

}

What should I do when fields are not specified? I've so far been using default values in the class so that a field is never null, but is that a "good" way of doing things?

java constructor field
Edit
Report

1 Answer

14

You should always construct a valid and legitimate object; and if you can't using constructor parms, you should use a builder object to create one, only releasing the object from the builder when the object is complete.

On the question of constructor use: I always try to have one base constructor that all others defer to, chaining through with "omitted" parameters to the next logical constructor and ending at the base constructor. So:

class SomeClass
{
SomeClass() {
    this("DefaultA");
    }

SomeClass(String a) {
    this(a,"DefaultB");
    }

SomeClass(String a, String b) {
    myA=a;
    myB=b;
    }
...
}

If this is not possible, then I try to have an private init() method that all constructors defer to.

And keep the number of constructors and parameters small - a max of 5 of each as a guideline.

answered 2009-02-24T17:57:18.950

Your Answer