Java 什么是NullPointerException，如何修复它？
什么是空指针异常java.lang.NullPointerException？是什么导致它们Java 什么是NullPointerException，如何修复它？,java,nullpointerexception,Java,Nullpointerexception,什么是空指针异常java.lang.NullPointerException？是什么导致它们 String a = null; System.out.println(a.toString()); // NullPointerException will be thrown 可以使用哪些方法/工具来确定原因，从而阻止异常导致程序过早终止？NullPointerException是指当您尝试使用指向内存中无位置null的引用时发生的异常，就像它正在引用对象一样。对空引用调用方法或尝试访问空引用的字
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class Example {
public static void main(String[] args) {
Object obj = null;
obj.hashCode();
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
这是一个技术问题，但我认为值得一提：指向null的引用与指向无效内存位置的C指针不同。空指针实际上不指向任何地方，这与指向恰好无效的位置有细微的区别。这就像您试图访问空对象一样。考虑下面的例子：
TypeA objA;
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class School {
Student student;
public School() {
try {
student.getId();
}
catch(Exception e) {
System.out.println("Null pointer exception");
}
}
}
一般来说，这是因为某些东西没有正确初始化。当您声明一个引用变量（即对象）时，实际上是在创建一个指向对象的指针。考虑下面的代码，其中声明原始类型INT:< /P>的变量
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
Integer num;
num = new Integer(10);
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public void doSomething(SomeObject obj) {
// Do something to obj, assumes obj is not null
obj.myMethod();
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
doSomething(null);
private static String[] foo = new String[2];
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
// Throws an NPE with a custom error message if obj is null
Objects.requireNonNull(obj, "obj must not be null");
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
/**
* @param obj An optional foo for ____. May be null, in which case
* the result will be ____.
*/
public void doSomething(SomeObject obj) {
if(obj == null) {
// Do something
} else {
// Do something else
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class Some {
private int id;
public int getId(){
return this.id;
}
public setId( int newId ) {
this.id = newId;
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
Some reference = new Some(); // Point to a new object of type Some()
Some otherReference = null; // Initiallly this points to NULL
reference.setId( 1 ); // Execute setId method, now private var id is 1
System.out.println( reference.getId() ); // Prints 1 to the console
otherReference = reference // Now they both point to the only object.
reference = null; // "reference" now point to null.
// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );
// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...
这是需要知道的一件重要事情-当上面的示例中没有对对象的更多引用时，当reference和otherReference都指向null时，则该对象是不可访问的。我们无法使用它，因此该对象已准备好进行垃圾收集，在某个时刻，VM将释放该对象使用的内存并分配另一个。在Java中，除基本类型之外的所有内容都是以类的形式出现的
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
如果不提供初始化部分，则会出现NullPointerException。当应用程序在需要对象的情况下尝试使用null时，会引发null指针异常。这些措施包括：
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
引用：空指针异常是指您正在使用对象而未对其进行初始化
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class Student {
private int id;
public int getId() {
return this.id;
}
public setId(int newId) {
this.id = newId;
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class School {
Student student;
public School() {
try {
student.getId();
}
catch(Exception e) {
System.out.println("Null pointer exception");
}
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class School {
Student student;
public School() {
try {
student = new Student();
student.setId(12);
student.getId();
}
catch(Exception e) {
System.out.println("Null pointer exception");
}
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String[] phrases = new String[10];
String keyPhrase = "Bird";
for(String phrase : phrases) {
System.out.println(phrase.equals(keyPhrase));
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String[] phrases = new String[] {"The bird", "A bird", "My bird", "Bird"};
String keyPhrase = "Bird";
for(String phrase : phrases) {
System.out.println(phrase.equals(keyPhrase));
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
Exception in thread "main" java.lang.NullPointerException
at Printer.printString(Printer.java:13)
at Printer.print(Printer.java:9)
at Printer.main(Printer.java:19)
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
private String name = "";
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
printString((name == null) ? "" : name);
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
public class Test {
public static void main(String[] args) {
String foo = null;
int length = foo.length(); // HERE
}
}
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
$ javac Test.java
$ java Test
Exception in thread "main" java.lang.NullPointerException
at Test.main(Test.java:4)
$
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
Exception in thread "main" java.lang.NullPointerException
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
at Test.main(Test.java:4)
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
int length = foo.length(); // HERE
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
return args[pos].length();
int length = test(foo, 1);
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
return args[pos].length();
int length = test(foo, 1);
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
doSomething(null);
private static String[] foo = new String[2];
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
但另一方面，Android有一些常见的平台特定的NPE原因。一种非常常见的情况是getViewById意外返回null。我的建议是搜索有关意外空返回值的原因的Q&A。已经有很多解释来解释它是如何发生的以及如何修复它的，但是您还应该遵循最佳实践来避免出现错误
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
String a = null;
System.out.println(a.toString()); // NullPointerException will be thrown
我理解你在那里写的所有东西，但这只是因为我已经编写了一段时间的代码，并且知道什么是“指针”和“引用”，什么是null。当我试图深入到这样的解释中时，我的学生们会斜视着我，因为没有足够的背景。@mmr：谢谢你的反馈，你的观点是正确的。在互联网上很难真正判断某人在哪里，在什么程度上开始解释是安全的。我将再次尝试修改它。在实践中获取NullPointerException的一种更常见的方法是在使用它之前忘记显式地将成员变量初始化为null以外的值。对于局部变量，编译器将捕获此错误，但在本例中不会。也许这会对你的答案有所帮助？@EJP我认为你的观点是正确的，所以我更新了答案，使之更清晰，并避免在回答的地方说“零分”。@StevePowell我很久以前就表示我不希望我的答案改变。请尊重原作者的意图。虽然这是一个很好的例子，但我可以问一下，它给所有其他答案都没有涵盖的问题增加了什么？在这里使用“未初始化”这个词是不合适的。您显示的示例实际上已初始化，并且已用null初始化。对于未初始化的变量，编译器会向您投诉。+1对您有好处
有一个例子，包括通过stacktrace；说明为什么阅读它对调试NPE很重要是很重要的。为什么当有人就您提到的调试错误发帖时，我们几乎总是寻找stacktrace…这是如何工作的？我研究这个话题已经有一段时间了，但什么也找不到。我相信像你这样了不起的老师能在一秒钟内教给我！非常感谢-@RuchirBaronia调试器允许您逐行遍历程序，查看调用了哪些方法以及变量是如何更改的。IDE应该有一些工具来实现这一点。例如，请参见。@RuchirBaronia您可以在stacktrace中看到的任何NullPointerException周围的方法上设置断点，并根据您期望的值检查变量的值。如果知道变量不应该为null，则可以在任何更改该值的代码周围设置断点。您还可以使用条件断点，它将在值更改时通知您。我还建议使用静态分析工具，如FINDBUGS避免此类异常的最佳方法是在您自己没有创建对象时始终检查null。如果调用方传递null，但是null不是方法的有效参数，那么将异常抛出回调用方是正确的，因为这是调用方的错误。默默地忽略无效的输入，在方法中什么也不做是非常糟糕的建议，因为它隐藏了问题。我想在这篇文章中添加一条注释，解释在使用自动装箱时，即使是对原语的赋值也会导致NPE：如果b是整数，int a=b也会抛出NPE。在某些情况下，这会使调试感到困惑。是否可以从web浏览器捕获web应用程序引发的NPE？它将显示在web浏览器的查看页面源中。是，在调用对象上的方法或尝试访问该对象可能具有的变量之前，请检查该对象是否等于null。有时，结构化代码有助于避免空指针异常。例如，当用常量字符串检查输入字符串时，应该从常量字符串开始，如下所示：如果SomeString.equalInputString{}//即使inputString为null，也不会引发异常。因此，为了确保安全，您可以做很多事情。避免代码中出现NullPointerException问题的另一种方法是使用@Nullable和@NotNull注释。以下是关于这方面的更多信息。虽然这个答案是专门针对IntelliJ IDE的，但它也适用于其他工具，正如从评论中明显看到的那样。顺便说一句，我不允许直接编辑这个答案，也许作者可以添加它？在j2ee项目中，Nullpointer异常非常常见。某些情况下引用变量的值为null。因此，您应该正确检查变量初始化。在条件语句期间，您应该始终检查标志或引用是否包含null或不包含like:-ifflag=0{ur代码使用flag}值得一提的是，一些IDE（例如Eclipse）提供了基于可定制注释（例如上面列出的@Nullable）的自动空性分析，并警告潜在错误。也可以推断和生成这样的注释，例如IntelliJ可以基于现有的代码结构来完成。首先应该做的是在使用可为null的对象之前，您应该使用if obj==null检查它是否为null。如果为null，那么您应该编写代码来处理它。IMO，在可能的情况下，最好避免在方法中返回空对象，并且在不允许空输入参数时使用注释，以便根据约定减少代码中“if obj==null”的数量并提高代码的可读性。。。在您接受这些最佳实践之前，请保持简单，我喜欢这个答案，如果您考虑正确访问一个AN的未初始化属性，请添加此选项。object@Emiliano-仅访问初始化属性不会导致NPE。如果需要更多的情况，这就是您>>要做的：1使用null作为同步块的目标，2使用null作为开关的目标，并取消对null的装箱。在实例级别对未初始化对象执行非类级别的操作将导致NullPointerException。操作需要是特定于实例的。若操作在类级别，比如在未初始化的对象上调用静态方法，那个么它不会抛出NullPointerException异常。即使是原始包装类对象也会抛出NullPointerException.1。NullPointerException是RuntimeException，这意味着将在程序运行时出现，而不是在编译时出现。！：，但是大多数IDE都可以帮助您发现这一点。2.尽量减少在赋值语句中使用关键字“null”：@tomj0101我完全不清楚你为什么要发表这样的评论。。。但对于第二点，Optional之前的模式是返回null。关键词很好。知道如何防范是至关重要的。这提供了它的一种常见情况以及缓解它的方法
一个运行时异常，不建议捕获它，但应避免它。@Shomu：我甚至建议在什么时候捕获它？调用实例的方法时经常发生NullPointerException。例如，如果声明引用但不使其指向任何实例，调用其方法时将发生NullPointerException。例如：YourClass ref=null；//或ref=另一个ref；//但是另一个ref没有指向任何实例ref.someMethod；//它将抛出NullPointerException。通常是这样解决的：在调用方法之前，确定引用是否为null。例如：如果你的裁判null{yourRef.someMethod；}或使用异常捕获：例如：try{yourRef.someMethod；}捕获NullPointerException e{//TODO}空指针是指我不同意的任何地方。空指针不指向任何地方，它们指向空值将抛出NullPointerException，若要跳过此操作，可以使用try-catch块处理。非常感谢。