java - NullPointerException 用字符串长度初始化变量

Question

我们被告知要实现一种方法来检测字符串是否是具有迭代和递归的回文。我成功地通过迭代实现了该方法，但是当我尝试递归执行该方法时，我收到了此错误。

Exception in thread "main" java.lang.NullPointerException
at recursion.RecursivePallindrome.isPallindrome(RecursivePallindrome.java:14)
at recursion.RecursivePallindrome.main(RecursivePallindrome.java:44)

我目前不知道出了什么问题。

package recursion;

public class RecursivePalindrome {

static String word;
public RecursivePalindrome(String a)
{
    a = word;
}

public static boolean isPalindrome()
{
    int start = 0;
    int end = word.length()-1; //Line 14
        char a = word.charAt(start);
        char z = word.charAt(end);
            Character.toLowerCase(a); Character.toLowerCase(z);

    if(start >= end)
    {
        if(Character.isLetter(a) && Character.isLetter(z))
        {
            if(a == z)
            {
                a++;
                z--;
            }
            else
                return false;
        }
        else if(!Character.isLetter(a))
            start++;
        else if(!Character.isLetter(z))
            end--;
        else
            return false;
    }

    return true;
}

public static void main(String[] args)
{
    new RecursivePalindrome("testing");
    if(isPalindrome())
        System.out.println("Is!");
    else
        System.out.println("Is not!");
}
}

Answer 1

尝试:

public RecursivePalindrome(String a) {
    word = a;
}

这段代码可以工作，但是它不是递归!

package recursion;

public class RecursivePalindrome {

    static String word;

    public RecursivePalindrome(String a) {
        word = a;
    }

    public static boolean isPalindrome() {
        int start = 0;
        int end = word.length() - 1;
        char a = word.charAt(start);
        char z = word.charAt(end);
        Character.toLowerCase(a);
        Character.toLowerCase(z);

        while (start < end) {
            if (Character.isLetter(a) && Character.isLetter(z)) {
                if (a == z) {
                    start++;
                    end--;
                    a = word.charAt(start);
                    z = word.charAt(end);
                } else {
                    return false;
                }
            } else if (!Character.isLetter(a)) {
                start++;
            } else if (!Character.isLetter(z)) {
                end--;
            } else {
                return false;
            }
        }

        return true;
    }

    public static void main(String[] args) {
        new RecursivePalindrome("testing");
        if (isPalindrome())
            System.out.println("Is!");
        else
            System.out.println("Is not!");
    }
}