jvm比较String与StringBuffer.reverse（）总是失败

Question

I had a problem where I was to find all he substrings of a string that are palindromes. Input would always be 1 word. The test input was aabaa. I decided to try be clever and make a string buffer of my substring then use the reverse method to compare with the original using String.equals. It didn't work.

import java.util.*

fun main(args: Array<String>) {
    val scan = Scanner(System.`in`)
    val input = scan.next()

    val found = ArrayList<String>()

    for (i in 0..input.length - 1) {
        for (j in 0..input.length - i) {
            val sub = input.substring(i, i + j)

            if (!found.contains(sub)) {
                // println(sub)
                found.add(sub)
                val rev = StringBuffer(sub).reverse()

                if (sub.equals(rev)) {
                    println(rev)
                }
            }
        }
    }
}

When I uncomment the first print statement the output look like this using test input aabaa

a
aa
aab
aaba
aabaa
ab
aba
abaa
b
ba
baa

So I am getting the correct substrings, but the last if statement never resolves true and I don't know why this is.

Answer 1

sub is a String. rev is a StringBuffer. They can't be equal, since they don't even have the same type.

Additional notes:

• don't use StringBuffer. Use StringBuilder. StringBuffer is needlessly synchronized, and should not be used anymore (just like Vector shouldn't be used and ArrayList should be instead).
• for (i in 0..input.length - 1) can be written for (i in 0 until input.length) which is more elegant
• input.substring(i, i + j) can't be right: at the end of the two loops, i would be length - 1, and j would be length - 1, and you would thus take a substring between length - 1 and 2 * length - 2.
• to store unique results, use a HashSet, not an ArrayList. Calling contains() on a HashSet is O(1), whereas it's O(n) on an ArrayList.
• in Kotlin, unlike in Java, you can use a == b to test if a is equal to b, even if a and b are references.