How do I print an error message stating the user input index is too large?

Question

I am attempting to write a program that when a user inputs a word, then an index, which causes the program to display the character at the given index, or gives error telling the user that the index given is too large . Whenever I run the code and put an index that is too large, I get an error message from the java instead. Any help is appreciated!

import java.util.Scanner;
public class scratch {
    public static void main(String[] args) {

        Scanner reader = new Scanner(System.in);
        System.out.printf("Enter a word:");
        String word = reader.next();

        Scanner letter = new Scanner (System.in);
        System.out.printf("Enter an index:");
        int index = letter.nextInt();

        char inputIndex = word.charAt(index);
        int length = word.length();
        if (index < length - 1 ) {
            System.out.printf("In word \"%s\", the letter at index"
                + " \"%2d\" is \'%c\'.\n"
                ,word, index, inputIndex );

        } else {
            System.out.printf("too big");   
        }
        reader.close();
        letter.close();
    }
}

Error message: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3 at java.base/java.lang.StringLatin1.charAt(Unknown Source) at java.base/java.lang.String.charAt(Unknown Source) at scratch.main(scratch.java:15)

Consider using the same `Scanner` instance for all your reads, you don't need multiple ones. — Arnaud, Sep 17 '18 at 12:06
"I get an error message" - Error messages contain a lot of useful information to help you find out the cause of the problem. When you get an error message, then include the complete, exact error message in your question - don't just say "I get an error". That will make it easier to help you. — Jesper, Sep 17 '18 at 12:07
Here is the error message that I receive: Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3 at java.base/java.lang.StringLatin1.charAt(Unknown Source) at java.base/java.lang.String.charAt(Unknown Source) at scratch.main(scratch.java:15) — gloob, Sep 17 '18 at 12:09
Never put more information into comments, always update your question instead. — GhostCat, Sep 17 '18 at 12:10
Ah, okay. Sorry about that, obviously still super new to this. Thanks everyone for all the info! — gloob, Sep 17 '18 at 12:13
Someone might close this as DUP to https://stackoverflow.com/questions/5554734/what-causes-a-java-lang-arrayindexoutofboundsexception-and-how-do-i-prevent-it — GhostCat, Sep 17 '18 at 12:13
Possible duplicate of [What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?](https://stackoverflow.com/questions/5554734/what-causes-a-java-lang-arrayindexoutofboundsexception-and-how-do-i-prevent-it) — OrdoFlammae, Sep 17 '18 at 12:14

Sweeper · Accepted Answer · 2018-09-17T12:12:53.317

6

You should call charAt after the check:

if (index < length ) {
    char inputIndex = word.charAt(index); // move this line here
    System.out.printf("In word \"%s\", the letter at index"
            + " \"%2d\" is \'%c\'.\n"
            ,word, index, inputIndex );

} else {
    System.out.printf("too big");   
}

The exception is caused by trying to get a character at an index that is too large. So you should try to get the character after you have ensured that the index is not too large, right?

edited Sep 17 '18 at 12:12

answered Sep 17 '18 at 12:08

Sweeper

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Additionnally, the condition should either be `index < length` or `index <= length - 1`, and there's no reason to create two `Scanner`s when a single one would work better (wouldn't fail if all the input is provided at once). – Aaron Sep 17 '18 at 12:11
That worked! Ah, that makes sense. Thank you for your help – gloob Sep 17 '18 at 12:12
Yes, that is true about scanners, but that has nothing to do with the issue. – OrdoFlammae Sep 17 '18 at 12:12
Yes, sorry about the extra scanner. My classmate added it, and I wasn't sure whether or not it shout be there. Thank you. – gloob Sep 17 '18 at 12:18

score 1 · Answer 2 · answered Sep 17 '18 at 12:10

1

This is a perfect use for a try-catch block. Try accessing the index, catch the resulting exception if there is an error and print from there:

char inputIndex;
try {
    inputIndex = word.charAt(index);
} catch(IndexOutOfBoundsException e) {
    System.out.println("Out of bounds!");
}

answered Sep 17 '18 at 12:10

Paul Benn

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No, it is not. You use try/catch in Java for situations that are *exceptional*. Failing on "string doesnt contain what I think it should" is not in that category. You simply add up front checks, to avoid the code throwing up in the first place. – GhostCat Sep 17 '18 at 12:12
This *is* an exceptional situation - you are trying to access an index which doesn't exist. `java.lang` even has [a defined Exception](https://docs.oracle.com/javase/8/docs/api/java/lang/IndexOutOfBoundsException.html) for this, as I'm sure you're aware. – Paul Benn Sep 17 '18 at 12:21
In Java, you *check* an index before you use it. Other languages follow a different mind set, but again, in Java you dont use try/catch for something like that. When an index out of bonds shows up, you *fix* the code, and prevent the exception from happening. This is mainly about the best practice to use here. There is also a NullPointerException. Does that mean you should throw these, or catch them?! – GhostCat Sep 17 '18 at 12:23

score 1 · Answer 3 · answered Sep 17 '18 at 12:18

First of all - use one Scanner object. There is a correction in if condition as well. index should be less than or equal to length-1.

put char inputIndex = word.charAt(index); inside if statement.

Try this :

import java.util.*;



 public class Scratch { 

 public static void main(String[] args) {

    Scanner reader = new Scanner(System.in);
    System.out.printf("Enter a word:");
    String word = reader.next();

    //Scanner letter = new Scanner(System.in);
    System.out.printf("Enter an index:");
    int index = reader.nextInt();
    int length = word.length();
    if (index < length - 1) {
        char inputIndex = word.charAt(index);
        System.out.printf("In word \"%s\", the letter at index" + " \"%2d\" is \'%c\'.\n", word,
            index, inputIndex);

    } else {
        System.out.printf("too big");
    }
    reader.close();
    }
}

PS : Class name should be Scratch not scratch (java convention).

How do I print an error message stating the user input index is too large?

3 Answers3