Java if语句中的NullPointerException

最近我一直在使用java和javafx开发一个计算器。我一直在为我的事件设置条件，但我得到了一个NullPointerException。下面是这些值：

Boolean eventTest;
String stringX, stringY;

运行此命令时，stringX已被定义

使用调试器后，问题归结为以下代码行：

if(eventTest == false & stringY == null & stringX != null||eventTest == false & stringY != null & stringX != null)

以下是堆栈跟踪：

java.lang.NullPointerException
at calculator$3.handle(calculator.java:263)
at calculator$3.handle(calculator.java:1)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
at com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
at javafx.event.Event.fireEvent(Unknown Source)
at javafx.scene.Node.fireEvent(Unknown Source)
at javafx.scene.control.Button.fire(Unknown Source)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(Unknown Source)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(Unknown Source)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source)
at com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source)
at com.sun.javafx.event.EventUtil.fireEvent(Unknown Source)
at javafx.event.Event.fireEvent(Unknown Source)
at javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.scene.Scene$MouseHandler.access$1500(Unknown Source)
at javafx.scene.Scene.impl_processMouseEvent(Unknown Source)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(Unknown Source)
at com.sun.glass.ui.View.handleMouseEvent(Unknown Source)
at com.sun.glass.ui.View.notifyMouse(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$141(Unknown Source)
at com.sun.glass.ui.win.WinApplication$$Lambda$37/1109371569.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

问题是为什么我会从这行代码中得到一个NullPointerException，以及如何修复它

谢谢。

检查eventTest是否为空。因为布尔值允许空值。

您的布尔值（大写）是一个包装类，变量的默认值为空。您可能只需要一个基本布尔值。如果有特殊原因需要包装器，请初始化它

这里

 eventTest == false // default value of a Java object is null

eventTest是布尔对象类型，其默认值为null

所以eventTest为null，并且无法将null取消装箱为boolean，所以您将得到NullPointerException

也许能帮助你了解这里发生了什么

在这里，从布尔类型到布尔类型的取消装箱就像r是布尔类型的引用一样发生，然后取消装箱转换将r转换为r.booleanValue

---

当r为null时。您将获得NullPointerException。

这行代码让我害怕：

if(eventTest == false 
  & stringY == null & stringX != null ||
  eventTest == false & stringY != null & stringX != null)

你们有并没有意识到在这两种情况下，你们做的验证是一样的

无论如何，这不是重点

---

eventTest是一个布尔值，因此它是一个对象，这意味着如果未初始化它就为null，因此在检查false/true值之前如果未验证它就可以抛出NPE。

eventTest==false将导致eventTest的失败，如果它为null，则

---

获取空指针异常。

如果您使用对象的值进行比较，则使用equals方法比使用==。以下代码可以解决您的问题：

if((eventTest != null && eventTest == false) & stringY == null & stringX != null || (eventTest != null && eventTest == false) & stringY != null & stringX != null)

在检查空eventTest之后，eventTest==false最终可以写入bol.equalsfalse或bol.booleanValue==false！=空

如果您对上述代码不满意，请将eventTest声明为原始布尔值：

---

布尔事件检验

您的意思是执行按位&运算，还是改为&&and？@MadProgrammer布尔型上的单字符运算符不是按位的。他们只是不短路。有人投了一票就结束了，给理由一个有趣的理由。。。我认为如果你不理解这个问题，最好留下评论或远离它。是的，这绝对是一个明确的问题。微妙的问题，但需要的一切都在那里。