/*  File Zero.java

    Part of Natural Number Example
    H. Conrad Cunningham, Professor

1234567890123456789012345678901234567890123456789012345678901234567890

2004-01-17: Original
2016-10-26: Formatting changes, compilation check
    
    Class Zero represents the natural number value zero.  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.

*/

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

    public Nat add(Nat n) 
    {   return n;   }  // 0 + n = n

    public Nat sub(Nat n)
    {   if (n == this) {   return this;  }  // 0 - 0 = 0
        return Err.getErr();                // undefined
    }

    public boolean isLess(Nat n) 
    {   return (n != this);   }  // 0 < n

    public boolean equals(Object n)
    {   return (n == this);   }  // exactly one value of Zero

    public String toString()     // method from class Object
    {   return "0";   }

    // Set up Singleton pattern for this type
    public static Zero getZero() 
    {   return theZero;   }

    private static Zero theZero = new Zero();
}