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Groups > comp.lang.java.programmer > #30579

a question about factory method

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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 this 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 singleton, 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 following a trend, e.g. String.valueOf(xxx).

I guess most people will do what I do here.

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a question about factory method John <xsli2@yahoo.com> - 2014-12-18 05:57 -0800
  Re: a question about factory method Robert Klemme <shortcutter@googlemail.com> - 2014-12-18 15:28 +0100
  Re: a question about factory method Silvio <silvio@internet.com> - 2014-12-18 15:43 +0100
  Re: a question about factory method Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-12-18 10:37 -0500
    Re: a question about factory method John <xsli2@yahoo.com> - 2014-12-18 08:04 -0800
      Re: a question about factory method Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-12-18 12:04 -0500
        Re: a question about factory method John <xsli2@yahoo.com> - 2014-12-18 10:43 -0800
          Re: a question about factory method Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-12-18 14:24 -0500
            Re: a question about factory method Robert Klemme <shortcutter@googlemail.com> - 2014-12-19 01:39 +0100
  Re: a question about factory method Roedy Green <see_website@mindprod.com.invalid> - 2014-12-18 13:43 -0800

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