a question about factory method

Hi:

I am reading a very good web site
http://www.javapractices.com/topic/TopicAction.do?Id=21

I have a question about the following code:

public final class ComplexNumber {

  /**
  * Static factory method returns an object of this class.
  */
  public static ComplexNumber valueOf(float aReal, float aImaginary) {
    return new ComplexNumber(aReal, aImaginary);
  }

  /**
  * Caller cannot see this private constructor.
  *
  * The only way to build a ComplexNumber is by calling the static
  * factory method.
  */
  private ComplexNumber(float aReal, float aImaginary) {
    fReal = aReal;
    fImaginary = aImaginary;
  }

  private float fReal;
  private float fImaginary;

  //..elided
}

The code uses a private constructor and the factory method. I also notice t=
his class is an immutable class. If I write this class, I would simply have=
 a public constructor, like:

public final class ComplexNumber {

  public ComplexNumber(float aReal, float aImaginary) {
    fReal = aReal;
    fImaginary = aImaginary;
  }

  private float fReal;
  private float fImaginary;

  //..elided
}

Could you explain to me why their code is better than mine? If theirs is si=
ngleton, I would agree. But in this case, it does not make sense for using =
singleton. The only thing I can think of is using their code is more like f=
ollowing a trend, e.g. String.valueOf(xxx).

I guess most people will do what I do here.