netbeans 中的 java.lang.NullPointerException
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java.lang.NullPointerException in netbeans
提问by user2196191
i'm writing a bit of code and i keep getting this error, it's to do with where i call network.addstation
我正在编写一些代码，但不断收到此错误，这与我调用 network.addstation 的位置有关
here's the code, there are other methods in the network method but i thought i'd just show one, but it appears when you call any method. any help would be much appreaciated. thanks.
这是代码，网络方法中还有其他方法，但我以为我只会显示一个，但是当您调用任何方法时它都会出现。任何帮助都会非常感谢。谢谢。
public class MyNetworkReader implements NetworkReader {
Network network = null;
@Override
public Network read(InputStream stream) {
Scanner scan = null;
scan = new Scanner(stream);
while (scan.hasNext()) {
String fromStation = scan.next();
if (!fromStation.equalsIgnoreCase("connection:")) ;
network.addStation(fromStation)
System.out.println(fromStation);
}
}
return network;
}
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
public class MyNetwork implements Network {
HashMap<String, ArrayList> stations = new HashMap<String, ArrayList>();
public void addStation(String station) {
stations.put(station, null);
}
}
回答by TonyArra
Are you constructing a MyNetwork instance anywhere?
您是否在任何地方构建 MyNetwork 实例？
Seems like you want
好像你想要
Network network = null;
To be
成为
MyNetwork network = new MyNetwork();
Because you're trying to call a method on an uninitiated object. But your MyNetwork class needs a constructor.
因为您试图在未初始化的对象上调用方法。但是您的 MyNetwork 类需要一个构造函数。
回答by DragonLord
Note you can also get this error immediately upon running C++ code, with the default NetBeans setup, if your Project's Properties->Run->Console Type is set to Internal Terminal and not External Terminal; at least when using FLTK graphics on Ubuntu. In this case it's a NetBeans internal handshaking problem and not a source code problem.
请注意，如果您的项目的 Properties->Run->Console Type 设置为 Internal Terminal 而不是 External Terminal，则在使用默认 NetBeans 设置运行 C++ 代码时，您也可以立即收到此错误；至少在 Ubuntu 上使用 FLTK 图形时。在这种情况下，它是 NetBeans 内部握手问题而不是源代码问题。
回答by American Patriot
As DragonLord said, it is not a source code issue. You will be able to tell because that should happen whenever you run any C++ code. I had the same problem. I did something slightly different than DragonLord, though. I set the output of the program to Project Properties>Run>Console Type and set it to Standard Output.
正如 DragonLord 所说，这不是源代码问题。您将能够分辨出来，因为无论何时运行任何 C++ 代码都应该发生这种情况。我有同样的问题。不过，我做了一些与 DragonLord 略有不同的事情。我将程序的输出设置为 Project Properties>Run>Console Type 并将其设置为 Standard Output。
回答by A human being
You're yourself initializing network to null, in second line of your code, then it will obviously throw NullPointerException
您自己在代码的第二行将网络初始化为 null，然后它显然会抛出 NullPointerException
