How to throw Exceptions

[Olivier Legat]

import java.text.ParseException;
abstract class Expression {
 
    public static Expression simpleParseExpression (String expr)
    {
        //Code here is irrelevant.
    }
 
    //@param expr Contain single-digit numbers separated by operations (+,-,*,/)
    //            eg: 1+2-3*4/5
    public static Expression parseExpression (String expr)throws ParseException
    {   
        //expr.length must always be odd
        if(expr.length()%2==0) throw new ParseException("incorrect length. String.length() must be odd", 0);
        
        //check that the first character is a number
        char[] c = expr.toCharArray();
        if(!isANumber(c[0])) throw new ParseException("illegal start of expression. number expected", 0);
        
        //check the rest of the expression to see if it's in the correct format
        for(int i=1; i<=c.length-1; i=i+2){
            if( !isAnOperation(c[i]) ) throw new ParseException("invalid operation. only +,-,*,/ can apply",i);
            if( !isANumber(c[i+1]) ) throw new ParseException("not a number.",i+1);
        }
        //return a Expression formatted into a BinaryTree
        return simpleParseExpression(expr);
    }
 
    private static boolean isANumber(char c){
        return (c=='0' || c=='1' || c=='2' || c=='3' || c=='4' 
             || c=='5' || c=='6' || c=='7' || c=='8' || c=='9');
    }
    private static boolean isAnOperation(char c){
        return (c=='+' || c=='-' || c=='*' || c=='/');
    }
}

Hello everyone,

I have a project to do and my professor wants us to create our own methods that can throw an exception. Not having any idea how to do this I check Java's official site:
http://java.sun.com/docs/books/tutorial/essential/exceptions/throwing.html
I looked at the first example on that page and this something similar to parseExpression(String) without putting throws ParseException. But the complier gave me an error "unreported Exception".
I reread the criteria that we were given and realized that our professor gave us the method signature for parseExpression(String) and that it included "throws ParseException" at the end. So I added that as you can see in the code and it got rid of the syntax errors.

However when I call the method without putting it inside a try{} it compiles are runs, which it isn't meant to, the compiler should normally tell me "expection must be caught or throw". And thus this method doesn't throw a ParseException when there is an error in String expr's format.

Help please I've only got until the end of this week to complete this.

Thanks in advance.

[Sunil Kumar]

It should give an error unless you are again using throws in the method where you are calling parseExpression method.
FYI, All exceptions that extends RuntimeException doesn't require any try/catch block.

[Olivier Legat]

No it doesn't give an error, that's the problem. Here is the class I used to test:

public class TestClass
{
    public static void main(String[]args){
        //Test simpleParseExpression("<#><opp><#>");
        Expression e = Expression.simpleParseExpression("2/2+3/2-4");
        System.out.println(e.result());
        
        //Test parseExpression(String) with an error
        try{
            Expression x = Expression.simpleParseExpression("2fd/2+3/2-4f");
            System.out.println(x.result());
        }
        catch(Exception exc){
            System.out.println(exc.getMessage());
        }
    }
}

The first one gives me -2.0 as in should. And the second one gives me -2.5, when it should be giving me an error.

[Olivier Legat]

Ok I found my error, it was a problem with my testing code. In the second test I wrote simpleParseExpression instead of parseExpression.

All is good Thanks for your help Sunil