Palindrome using java stringbuilder

Question

public class testing {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        StringBuilder sb = new StringBuilder();
        StringBuilder sb1 = new StringBuilder();
        sb.append(str);
        sb1.append(str);
        sb1.reverse();
        if(sb == sb1) {
            System.out.println("yes");
        }else if(sb != sb1) {
            System.out.println("no");
        }
        System.out.println(sb.length());
        System.out.println(sb1.length());
        sc.close();
    }
}

Everything working fine including string reverse, but always getting no as result.

As an aside, don't use `if (condition) { ... } else if (!condition) { ... }`, unless you actually expect that condition to have changed. Just use `else`. — Andy Turner, Jun 03 '19 at 08:21
`sb == sb1` - you're comparing if it is the same object - it will never be. What's worse, you're comparing StringBuilders, not String. You should get String from them first and then compare using `equals()` method instead. — Amongalen, Jun 03 '19 at 08:22
@AndyTurner *"unless you actually expect that condition to have changed"* — What do you mean? Do you mean that the result of `sb == sb1` could have changed within the `if` branch? — MC Emperor, Jul 12 '21 at 10:09
@MCEmporer in this case, it can't have changed because they are local non-published variables; but in the general case of there is no guarantee that two syntactically-identical expressions evaluate to the same value. So, if you mean "if condition do this, otherwise that", use if/else, not if condition/else if not condition. — Andy Turner, Jul 12 '21 at 15:42

Chintan Mistry · Accepted Answer · 2021-07-12T09:17:14.583

0

You will always get no because StringBuilder creates new object & as you are using == for comparison, it compares reference only not content of StringBuilder object, so if you want to compare content you have to use .equals() method after applying .toString() on StringBuilder object.

edited Jul 12 '21 at 09:17

answered Jun 03 '19 at 08:25

Chintan Mistry

18
5

Had to check it to confirm - you can't compare `StringBuilder`s with `equals()` - will return false even if the content is the same. You have to get Strings from them before comparing. – Amongalen Jun 03 '19 at 08:34

Palindrome using java stringbuilder

1 Answers1