java - 执行方法时发生 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我正在尝试了解 Java 的一些基础知识。我在下面输入的代码编译得很好，但无法运行。每次运行代码时，它都会显示 NullPointerException发生错误。

class testOnStrings {
    public static void main (String args []){
        String input, output;

        // isWordPalindrome
        input = "forever eating cheese";
        boolean boolOutput = isWordPalindrome(input);
        System.out.println("\n" + input + (boolOutput ? " is " : " is not ") + "a word palindrome");

        input = "fall leaves when leaves fall";
        boolOutput = isWordPalindrome(input);
        System.out.println("\n" + input + (boolOutput ? " is " : " is not ") + "a word palindrome");

        input = null;
        boolOutput = isWordPalindrome(input);
        System.out.println("\n" + input + (boolOutput ? " is " : " is not ") + "a word palindrome");
    }

    static String reverseWords(String input) {
        // My code
        String words[] = input.split(" ");
        String reverseWord = "";
        for (int pointer = words.length - 1; pointer >= 0; pointer--)
            reverseWord += words [pointer] + " ";
        return reverseWord;
    }

    static boolean isWordPalindrome(String input) {
        // My code
        String reverseWordInput = reverseWords(input);
        boolean isPalindrome = input.equals(reverseWordInput);
        return isPalindrome;
    }
}

运行代码时出错。

Exception in thread "main" 
java.lang.NullPointerException
    at idle.reverseWords(idle.java:21)
    at idle.isWordPalindrome(idle.java:30)
    at idle.main(idle.java:15)

编译成功但代码未运行，请帮忙

Answer 1

您已将input指定为null，因此出现了问题

input = null;
boolOutput = isWordPalindrome(input);

为输入分配空字符串值，以防您不需要任何内容​​

input = "";
boolOutput = isWordPalindrome(input);

引用:

Difference between null and empty string