Question

given the following two classes

Within a file named Rational.java, define a public class named Rational such that …

• the Rational class has two private instance variables, numerator and denominator, both of type int
• the Rational class defines two public accessor methods, numerator() and denominator()
• the Rational class defines a constructor that has one String parameter
  • a throws clause indicates that this constructor could potentially throw a MalformedRationalException
  • this constructor throws a MalformedRationalException in the following circumstances …
    1. When the argument passed to the constructor is null.
    2. When any of the following applies to the String argument that is passed to the constructor …
       • this String does not contain exactly one '/' character
       • not all characters of this String ( other than the one '/' ) are decimal digits
       • within this String, there is not at least one decimal digit preceding the '/' character
       • within this String, there is not at least one decimal digit following the '/' character

I am having issues with getting the MalformedRationalException to throw different messages any of the four examples errors are submitted in the command line

------------------------------------------------------------------------------------------------------------------------------------------------------

public class Main
{
public static void main ( String[] args )
{
System.out.println ( "args[0] is ........ \"" + args[0] + "\"" ) ;
try
{
Rational r = new Rational ( args[0] ) ;
System.out.println ( "A Rational object was successfully constructed from \"" + args[0] +"\"." ) ;
System.out.println ( "Numerator is ...... " + r.numerator() ) ;
System.out.println ( "Denominator is .... " + r.denominator() ) ;
} // try
catch ( MalformedRationalException mre )
{
System.out.println ( "A Rational object could not be constructed from \"" + args[0] +"\"." ) ;
System.out.println ( mre.getMessage() ) ;
} // catch
} // main
} // Main

-------------------------------------------------------------------------------------------------------------------------------------------------

public class MalformedRationalException extends Exception
{
public MalformedRationalException ( String message )
{ super ( message ) ; }
} // MalformedRationalException

---------------------------------------------------------------------------------------------------------------------------------------------------

Answer 1

Please find the answer below, I have created the constructor with the messages provided in the question. You can change them according to our needs. I have mentioned all the details in the comments.

Rational.java

public class Rational {
    //private members of the class
    private int numerator;
    private int denominator;

    //public methods to get the values
    public int numerator(){
        return numerator;
    }
    public int denominator(){
        return denominator;
    }

    //constructor
    Rational(String value) throws MalformedRationalException {
        //first check if the value is null or string is empty
        if(value == null || value.isEmpty()){
            throw new MalformedRationalException("String passed is NULL");
        }
        //check for the index of "/", it should present
        //check for single "/" is using the comparison of first and last occurrence of the "/"
        else if(value.indexOf("/") == -1 || value.indexOf("/")!=value.lastIndexOf("/")){
            throw new MalformedRationalException("String Does not contain exactly one '/' character");
        }
        else{
            //get the index of "/"
            int slashIndex = value.indexOf("/");
            //check for the only digits in the string
            for (int i = 0; i < value.length(); i++) {
                if(i!= slashIndex) {
                    if (!Character.isDigit(value.charAt(i))) {
                        throw new MalformedRationalException("Not all characters of this String ( other than the one '/' ) are decimal digits");
                    }
                }
            }
            //check for the digits before and after "/"
            boolean before = false;
            boolean after = false;
            //first check for the digit before the "/"
            for (int i = 0; i < slashIndex ; i++) {
                if(Character.isDigit(value.charAt(i))){
                    before = true;
                }
            }
            //first check for the digits after the "/"
            for (int i = slashIndex+1; i < value.length() ; i++) {
                if(Character.isDigit(value.charAt(i))){
                    after = true;
                }
            }

            //throw errors accordingly
            if(!before){
                throw new MalformedRationalException("There is not at least one decimal digit preceding the '/' character");
            }

            if(!after){
                throw new MalformedRationalException("There is not at least one decimal digit following the '/' character");
            }
        }

        //if everything is good then assign the values accordingly
        int slashIndex = value.indexOf("/");
        numerator = Integer.parseInt(value.substring(0,slashIndex));
        denominator = Integer.parseInt(value.substring(slashIndex+1));
    }
}

MalformedRationalException.java

public class MalformedRationalException extends Exception {
    public MalformedRationalException(String message){
        super(message);
    }
}

Main.java

public class Main {
    public static void main ( String[] args )
    {
        System.out.println ( "args[0] is ........ \"" + args[0] + "\"" ) ;
        try
        {
            Rational r = new Rational (args[0]) ;
            System.out.println ( "A Rational object was successfully constructed from \"" + args[0] +"\"." ) ;
            System.out.println ( "Numerator is ...... " + r.numerator() ) ;
            System.out.println ( "Denominator is .... " + r.denominator() ) ;
        } // try
        catch ( MalformedRationalException mre )
        {
            System.out.println ( "A Rational object could not be constructed from \"" + args[0] +"\"." ) ;
            System.out.println ( mre.getMessage() ) ;
        } // catch
    } // main
}

Output:

Legal input:

Input: 1212

Input: 12//12

Input: /12

Input: 12/