奇怪的java字符串数组空指针异常
这个问题是在实践测试中出现的：创建一个新的字符串数组，将其初始化为空，然后初始化第一个元素并打印它。为什么会出现空指针异常呢？为什么它不打印“一”？这与字符串不变性有关吗？
public static void main(String args[]) {
try {
String arr[] = new String[10];
arr = null;
arr[0] = "one";
System.out.print(arr[0]);
} catch(NullPointerException nex) {
System.out.print("null pointer exception");
} catch(Exception ex) {
System.out.print("exception");
}
}
谢谢！
This problem came up in a practice test: create a new string array, initialize it to null, then initializing the first element and printing it. Why does this result in a null pointer exception? Why doesn't it print "one"? Is it something to do with string immutability?
public static void main(String args[]) {
try {
String arr[] = new String[10];
arr = null;
arr[0] = "one";
System.out.print(arr[0]);
} catch(NullPointerException nex) {
System.out.print("null pointer exception");
} catch(Exception ex) {
System.out.print("exception");
}
}
Thanks!
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评论（3）
因为您使
arr引用了null，所以它抛出了NullPointerException。编辑：
让我用数字来解释一下：
在这一行之后：
堆中将为数组
arr保留 10 个位置：并在此行之后：
您将删除对数组的引用并使其引用
null：因此，当您调用此行时：
将抛出
NullPointerException。Because you made
arrreferring tonull, so it threw aNullPointerException.EDIT:
Let me explain it via figures:
After this line:
10 places will be reserved in the heap for the array
arr:and after this line:
You are removing the reference to the array and make it refer to
null:So when you call this line:
A
NullPointerExceptionwill be thrown.使用
arr = null;，您将删除对该对象的引用。因此您无法再使用
arr[anynumber]访问它。With
arr = null;you are deleting the reference to the object.So you can't access it anymore with
arr[anynumber].arr is null，然后...如果它确实打印了one，您不会感到惊讶吗？将字符串放入空值中？arr is null, and then... if it did printone, wouldn't you be surprised? put a string into a null?