/*  File Err.java

    Part of Natural Number Example
    H. Conrad Cunningham, Professor

1234567890123456789012345678901234567890123456789012345678901234567890

2004-01-17: Original
2016-10-26: Formatting changes, compilation check

Class Err represents an error condition.  It is a leaf subclass in the
Composite pattern and is designed and implemented with the Singleton
pattern to ensure that exactly one instance is created.

Perhaps in a more sophisticated version, Err would carry an indicator
of what type of error is needed.  In that case, a Singleton could not
be used.

 */

public class Err extends Nat
{   
    private Err () { }  // private constructor for Singleton

    public Nat add(Nat n)
    {   return this;  }  // Err + n = Err

    public Nat sub(Nat n)
    {   return this;   }  // Err - n = Err

    public boolean isLess(Nat n)
    {   return false;   }  // Not comparable.  Probably bad design.

    // equals is tricky because of Err nesting down inside Succ
    // structures.  (As noted elsewhere, it is normally best not to
    // use instanceof calls in OO code, but it is often necessary in
    // comparisons.)
    
    public boolean equals(Object n)
    {   if (n == this)           {   return true;    } // Err = Err
        if (n == Zero.getZero()) {   return false;   } // Err != 0
        if (!(n instanceof Nat)) {   return false;   }
        return (equals(((Succ)n).pred()));  // Err = Succ(Err)
    }

    public String toString()  // method from class Object
    {   return "ERR";   }

    // Set up a Singleton for this type
    public static Err getErr() 
    {   return theErr;   }

    private static Err theErr = new Err();
}