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Class with private constructor

5 replies on 1 page. Most recent reply: Mar 15, 2004 5:09 AM by Vincent O'Sullivan

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Walter Rieppi

Posts: 5
Nickname: walter2004
Registered: Mar, 2004

Class with private constructor Posted: Mar 11, 2004 12:16 AM
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Hi people,
i need to resolve thi question. I have this class
Class car()
{
private car ()
{
....
.....
}
}

In main program i need to create an car object. I can't use that constructor because its private. There is anther way to create an object of that class or not ????
Thanks Walter


Adam Duffy

Posts: 168
Nickname: adamduffy
Registered: Feb, 2003

Re: Class with private constructor Posted: Mar 11, 2004 9:26 AM
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I'm guessing something like the Factory Design Pattern?

public class car() {
  private car () {
    // do something
  }
 
  public car getInstance() {
    return( new car() );
  }
}
 


Adam

David Ramsey

Posts: 34
Nickname: dlramsey
Registered: Apr, 2002

Re: Class with private constructor Posted: Mar 11, 2004 2:06 PM
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Minor correction, since the factory method has to be static to get an instance otherwise it needs an instance to be called first:

public class car() {
  private car () {
    // do something
  }
 
  public static car getInstance() {
    return( new car() );
  }
}

mausam

Posts: 243
Nickname: mausam
Registered: Sep, 2003

Re: Class with private constructor Posted: Mar 11, 2004 8:47 PM
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Since you are planning to invoke the class constructor from the main of the same class you can easily do that.

Walter Rieppi

Posts: 5
Nickname: walter2004
Registered: Mar, 2004

Re: Class with private constructor Posted: Mar 15, 2004 12:24 AM
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Thanks all. I tried both solution with my Eclipse. Why i have an compilation error if the class Sundae makeSundae is not a static class???

public class Sundae
{
private Sundae()
{}

public static Sundae makeSundae()
{
return (new Sundae()) ;
}
}

Since, i can create an object only if the makeSundae is a static. Anyone can me explain this ???
Best Regards Walter

Vincent O'Sullivan

Posts: 724
Nickname: vincent
Registered: Nov, 2002

Re: Class with private constructor Posted: Mar 15, 2004 5:09 AM
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> Thanks all. I tried both solution with my Eclipse. Why i
> have an compilation error if the class Sundae makeSundae
> is not a static class???

I think David has already answered this, above. If a method is not static then it can only be called via an instance of the class. For example:
Sundae s2 = s1.makeSundae();
This means that you need an existing instance to create a new instance. That's OK except that you won't be able to create a first instance to start things off (because the constructor is private). Therefore, the makeSundae method needs to be static so that you can write the following:
Sundae s = Sundae.makeSundae();


Vince.

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