public class MyNum {
    private int _value;

    public MyNum( int value ) {
        _value = value;
    }

    public MyNum negate() {
         return new MyNum( -_value );
    }

    public String toString() {
        return "" + _value;
    }
}

public class MyNumCategory {
    public static MyNum getMyNum( Integer num ) {
        return new MyNum( num );
    }

    public static MyNum negate( MyNum num ) {
        return num.negate();
    }

    public static void main( String[] args ) {
        System.out.println( getMyNum(1234).negate() );
    }
}

Groovy code if unary negation could be overloaded:
    use( MyNumCategory ) {
         def value = 1234.myNum;
         def negatedValue = -value;

         println "value = ${value}"
         println "negatedValue = ${negatedValue}"
    }

Output:
   value = 1234
   negatedValue = -1234