NullPointerException（当 String 初始化时）

发布于 2024-12-12 06:43:03 字数 1073 浏览 0 评论 0原文

以下是抛出 java.lang.NullPointerException 的代码片段。

else if(jRadioButton2.isSelected()) {
             // chrome selected
             String chrome_count_S="0";
             int chrome_count_I=0;
             FileWriter writer = new FileWriter("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             FileReader reader = new FileReader("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             BufferedReader br = new BufferedReader(reader);
             while((chrome_count_S = br.readLine()) != null) {
                  chrome_count_I = Integer.parseInt(chrome_count_S);
                  chrome_count_I++;
                  chrome_count_S = Integer.toString(chrome_count_I);
             }
             writer.write(chrome_count_S);
             writer.close();

当遇到此代码片段时，会抛出 NullPointerException 。如果我将 writer.write(chrome_count_S); 的参数替换为 writer.write("chrome_count_S"); IE 一个 String，我不没有任何例外。否则为什么当我初始化字符串 chrome_count_S 时会出现异常？

原文

Following is a snippet that throws java.lang.NullPointerException.

else if(jRadioButton2.isSelected()) {
             // chrome selected
             String chrome_count_S="0";
             int chrome_count_I=0;
             FileWriter writer = new FileWriter("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             FileReader reader = new FileReader("D:\\UnderTest\\MyFirstPoll\\build\\classes\\poll_count\\Chrome.txt");
             BufferedReader br = new BufferedReader(reader);
             while((chrome_count_S = br.readLine()) != null) {
                  chrome_count_I = Integer.parseInt(chrome_count_S);
                  chrome_count_I++;
                  chrome_count_S = Integer.toString(chrome_count_I);
             }
             writer.write(chrome_count_S);
             writer.close();

When this snippet is encountered NullPointerException is thrown. If I replace the argument of writer.write(chrome_count_S); to writer.write("chrome_count_S"); I.E. a String, I don't get any exception. Otherwise why do I get the exception when I have initialized the string chrome_count_S?

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禾厶谷欠 2024-12-19 06:43:03

当 readline() 为 null 时，while 循环停止，并将当前值写入变量 chrome_count_S。

while((chrome_count_S = br.readLine()) != null)

因此，在循环之后和执行 write 命令时，chrome_count_S 为 null。

===更新===

删除循环中的chrome_count_S行，并在写入过程中从chrome_count_I获取值：

while((chrome_count_S = br.readLine()) != null) {
    chrome_count_I = Integer.parseInt(chrome_count_S);
    chrome_count_I++;
}
writer.write(Integer.toString(chrome_count_I));

The while loop stops when the readline() is null and writes the current value to the variable chrome_count_S.

while((chrome_count_S = br.readLine()) != null)

So chrome_count_S is be null after the loop and at the the write command.

=== UPDATE ===

Remove the chrome_count_S row in the loop and take the value from the chrome_count_I during writing:

while((chrome_count_S = br.readLine()) != null) {
    chrome_count_I = Integer.parseInt(chrome_count_S);
    chrome_count_I++;
}
writer.write(Integer.toString(chrome_count_I));

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