variable1.indexOf（variable2.charAt（i））超出范围异常

Question

This is for my school assignment, and I'm supposed to create an encoder. The user enters a encryption key which is contained in the variable 'key', an example of an encryption key is, "abc". Using the alphabet, I'm supposed to add the letters in the key the user entered according to the order of the alphabet. For example, the key "abc" is supposed to add to 6. So according to the order of the alphabet 'a' is 1, b = 2, c = 3, so 1 + 2 + 3 = 6.

Now for this error, whenever I try to add the alphabet.indexOf(key.charAt(i)) it gives an out of bounds exception. How can I fix this?

 35   public static int option1(String input, String alphabet, String key)
 36     {
 37       
 38       int sum = 0;
 39         
 40       for (int i = 0; i < alphabet.length(); i++)
 41       { 
 42         
 43           **sum = sum + alphabet.indexOf(key.charAt(i));**
 44           
 45         }
 46                  
 47       System.out.println(sum);
 48        return sum;
 49     }

This gives an error of

java.lang.StringIndexOutOfBoundsException: String index out of range: 3
    at java.lang.String.charAt(Unknown Source)
    at Encryption.option1(Encryption.java:43)
    at Encryption.main(Encryption.java:21)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

The error is the sum statement where I add the index of a certain letter of the alphabet using a letter of another variable. Why is this giving the out of bounds exception? How can it be fixed?

Answer 1

If you have the key abc it can be broken down to a[0] b[1] c[2] so in the for loop when you get to i == 3 it is out of the range of your key. So you need to change your for loop to use key.length() rather than alphabet.length() like so:

for (int i = 0; i < key.length(); i++)
{     
    sum = sum + alphabet.indexOf(key.charAt(i));        
}

Answer 2

I'm guessing you meant to use key.length() in your for loop.

I guess i'll put my comment here too:

your "key" string is only length 3 if you input "abc"... However you are using "alphabet" length in your loop which is of length 26.. This is causing an error on trying to grab any character after the 3rd position of your string "key"

 35   public static int option1(String input, String alphabet, String key)
 36     {
 37       
 38       int sum = 0;
 39         
 40       for (int i = 0; i < key.length(); i++)
 41       { 
 42         
 43           **sum = sum + alphabet.indexOf(key.charAt(i));**
 44           
 45         }
 46                  
 47       System.out.println(sum);
 48        return sum;
 49     }

Answer 3

The above answers hint at this as well, but it's important to note that when a String is created, it actually creates a reference to a 1-D array. When creating or working with strings, it's good practice to think about what the size of the array is that's created rather than the size of the String (although in many cases this is interchangeable). That way you can avoid any "ArrayOutOfBounds" exceptions that come up in the future.