Empty Private Constructor Class: Why?

[Tom Marler]

I am new to this forum, thanks for providing the platform! I have been starting to study Java and ran into a line of code from an example out of a book I could not make sense of, maybe somebody has an idea.

public class OuterClassName 
{
  private OuterClassName() {}
  // ...
  public static final class InnerClassName implements XYZ 
  {
     private InnerClassName() {}
     // ...
  }
  // ...
}

Why did they specify a private Constructor which is empty? They did it both for the outer class and also for the inner.

Thanks!
Tom

[Dan Din]

Welcome, Tom!

I think the example want to demonstrate the relation between the Outer and Inner classes - in my code, i can create an instance to the outer class from the inner class (line 8), and vice-verse (line 18), even the constructor are private and cannot be accessed from outside.

I look forward for other explanation.

D.

public class PrivateOuter {
	private PrivateOuter() {}  

    public static final class InnerClassName {   
    	private InnerClassName() {}  
         
        void getOuter() {
        	PrivateOuter po = new PrivateOuter();
        	po.doSmth();
        } 
         
        void doSmth() { 
        	System.out.println("Inner doSmth");
        }
    }
      
    void getInner() {
    	InnerClassName pi = new PrivateOuter.InnerClassName();
    	pi.doSmth();
    }

    public void doSmth() {
    	System.out.println("Outer doSmth");
    }  
 
}

[Seetharaman Venkatasamy]

Tom Marler wrote:

public class OuterClassName 
{
  private OuterClassName() {}
  // ...
  public static final class InnerClassName implements XYZ 
  {
     private InnerClassName() {}
     // ...
  }
  // ...
}

Why did they specify a private Constructor which is empty? They did it both for the outer class and also for the inner.

the whole purpose of this idiom is you cant create an object as well as cant subclass for both Outer and Inner classes from other classes.

[Seetharaman Venkatasamy]

And Welcome to JavaRanch

[Unnar Björnsson]

This class is probably designed to be a singleton or static method provider, in either case it should not be instantiated by outside call, therefore the default constructor is overridden as private.

[Seetharaman Venkatasamy]

Unnar Björnsson wrote:therefore the default constructor is overridden as private.

I think, you meant *marked* as private. constructor can not be overridden