CLASSPATH中的值顺序重要吗？

问题描述

我有2个Java程序分别位于
中，一个位于c：\test中，另一个位于c：\test\new

我可以编译它们都没有任何错误\javac

但是当我尝试执行文件\java
时，它显示出这样的错误

 线程 AWT-EventQueue-0中的异常java.lang.NullPointerException 
在ButtonFrame.makeButton（ButtonTest3.java:42） < init>（ButtonTest3.java:29）
在ButtonTest $ 1.run（ButtonTest.java:17）
在java.awt.event.InvocationEvent.dispatch（未知来源）java.awt.EventQueue.dispatchEvent（未知来源）
（java.awt.EventDispatchThread.pumpOneEventForFilters（未知来源）java.awt.EventDispatchThread.pumpEventsForFilter（未知来源）
 
在java.awt.EventDispatchThread.pumpEventsForHierarchy（未知来源）
在java.awt.EventDispatchThread.pumpEvents（未知来源）java.awt.EventDispatchThread.pumpE在
 java.awt.EventDispatchThread.run上的vents（未知源）
（未知源）
  

我把它放在我的类路径中

但是如果我将CLASSPATH中的值顺序更改为此

错误根本消失了

为什么？
仅顺序重要吗？

解决方案

您在两个文件夹中都有一个具有相同名称的类。在 C：\test 中，有一个 ButtonTest3 类的版本，其中包含导致此<$ c $的编程错误。 c> NullPointerException 。在 C：\test\new 中，有一个不同版本的 ButtonTest3 类，其中不包含此错误。 ，或者可能有一个 ButtonTest 类，它的作用与 C：\test 中的完全不同。

清理您的类路径。在类路径中重复具有相同签名的不同版本的类是不好的。如果您的意图是 new 应该是程序包标识符，则需要将其远离类路径。但是，这样的程序包名称会导致编译错误，所以不能这样。

---

该错误， NullPointerException 对于确定和修复相对而言是微不足道的。首先看一下堆栈跟踪的第一行：

 在ButtonFrame.makeButton（ButtonTest3.java:42）
  

这表明它已发生在 ButtonTest3 类的第42行中，在 makeButton（）方法中。现在转到 ButtonTest3.java 的第42行，它将类似于：

  someObject.doSomething（）; 
  

在哪里找到点运算符。用于调用方法或访问某些对象的字段。 NullPointerException 表示 someObject 在特定时刻为 null 。没有实例！

这是一个简单的解决方法：只需确保它不是 null 目前正在调用/访问它：

  someObject = new SomeObject（）; 
 // ... 
 someObject.doSomething（）; 
  

I have 2 java program located seperatelyOne in c:\test and the other in c:\test\new

I can compile both of it without any error \javac

But when i try to execute the file \javait shows the error like this

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at ButtonFrame.makeButton(ButtonTest3.java:42)
    at ButtonFrame.<init>(ButtonTest3.java:29)
    at ButtonTest$1.run(ButtonTest.java:17)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

i put this in my classpath

but if i change the order of the value in CLASSPATH to this

the error is simply gone

Why?? this could happeningOnly the order matters?

解决方案

You've a class with the same name in the both folders. In C:\test there's a version of the ButtonTest3 class which contains a programming bug causing this NullPointerException. In C:\test\new there's a different version of the ButtonTest3 class which doesn't contain this bug, or probably there's a ButtonTest class which does entirely different things than the one in C:\test.

Cleanup your classpath. It's not good to have duplicate different versioned classes with the same signature in the classpath. If your intent is that new is supposed to be a package identifier, then you need to leave it away from the classpath. However, such a package name would have resulted in a compilation error, so that can't be it.

---

As to the bug, a NullPointerException is relatively trivial to naildown and fix. First look at the first line of the stacktrace:

at ButtonFrame.makeButton(ButtonTest3.java:42)

It's telling that it has occurred in line 42 of ButtonTest3 class, inside the makeButton() method. Now go to line 42 of ButtonTest3.java, it'll look something like:

someObject.doSomething();

Look there where a dot operator . is been used to invoke a method or access a field of some object. The NullPointerException means that someObject is null at the particular moment. There is no instance!

It's an easy fix: just ensure that it is not null at the moment you're invoking/accessing it:

someObject = new SomeObject();
// ...
someObject.doSomething();

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