比较Java中的两个字符串

Question

I am trying to compare to strings by using an algorithm as you see below My code does not work .. eclipse does not show any error before I run the code

    public class MysteryClass {
    public static void mystery(String n) {
        String k= "alla";
            if (k.charAt(k.length())==n.charAt(n.length())) {
                System.out.println("palindrom");
            } else {
                System.out.println("not palindrom");

            }
        }

    public static void main(String[] args) {
        MysteryClass.mystery("alla");
    }
}

but we I run the code I get

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4
    at java.lang.String.charAt(String.java:658)
    at shapes.MysteryClass.mystery(MysteryClass.java:6)
    at shapes.MysteryClass.main(MysteryClass.java:15)

How to fix that?? thanks

Answer 1

Two issues with your code:-

1)By your logic, tigert is also palindrome but it is actually not.Palindrome is when the word is completely reversed and it is still the same.Currently,you are checking only the first and last character

2)String is internally held as a char array and arrays have indexing from 0 to length -1

Answer 2

The error is caused by the statement if (k.charAt(k.length())==n.charAt(n.length())) Above k.length() returns 4. the statement then resolves to:

(k.charAt(4) 

This will through error because the function charAt counts from the index 0. So it counts the characters at index 0, 1, 2, 3 and you are asking it to fetch character at index 4 which does not exist. A suggested alternative is : (k.charAt(k.length() -1)==n.charAt(n.length()-1))

Answer 3

Not sure what you want, but your StringIndexOutOfBoundsException is cause by access an array's index which is does not exist.

Take your String k = "alla" as example, k.length() will return 4 because "alla" have four characters, and you are accessing k.charAt(4) , the "alla" break into as follows:

a l l a  // string
0 1 2 3  // index

As you can see the last index of "alla" is 3 , that's why you get StringIndexOutOfBoundsException . But you can solve it using .length() -1 as follow:

 if (k.charAt(k.length()-1)==n.charAt(n.length()-1)) {
            System.out.println("palindrom");
        } else {
            System.out.println("not palindrom");

Alternative:

If you want to compare two string , you can use .equals method too, as follow:

 if (k.equals(n)) {
            System.out.println("palindrom");
        } else {
            System.out.println("not palindrom");