Java 使用 HashMap 的 put 方法时出现 NullPointerException
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NullPointerException while using put method of HashMap
提问by Ankur
The following code is giving me a NullPointerException. The problem is on the following line:
下面的代码给了我一个NullPointerException. 问题出在以下几行：
...
dataMap.put(nextLine[0], nextLine[6]);
What is strange is that I have run this code without the above line and the call to nextLine[0]and nextLine[6]work exactly as expected - that is they give me back elements of a csv file. I declare and initialise the HashMapwith the code
奇怪的是，我在没有上面一行的情况下运行了这段代码，并且调用nextLine[0]并nextLine[6]完全按预期工作——也就是说，它们给了我一个 csv 文件的元素。我HashMap用代码声明并初始化
HashMap<String, String> dataMap = null;
earlier in the method
在方法的前面
String[] nextLine;
int counter=0;
while (counter<40) {
counter++;
System.out.println(counter);
nextLine = reader.readNext();
// nextLine[] is an array of values from the line
System.out.println(nextLine[0] + " - " + nextLine[6] +" - " + "etc...");
dataMap.put(nextLine[0], nextLine[6]);
}
return dataMap;
}
采纳答案by Codingscape
HashMap<String, String> dataMap = new HashMap<String,String>();
Your dataMapvariable isn't initialized at this point. You should be getting a compiler warning about that.
此时您的dataMap变量尚未初始化。您应该收到有关此的编译器警告。
回答by Brian Agnew
Where is datamap initialised ? It's always null.
数据映射在哪里初始化？它始终为空。
To clarify, you declare the variable and set it to null. But you need to instantiate a new Map, whether it's a HashMap or similar.
为了澄清起见，您声明了变量并将其设置为 null。但是你需要实例化一个新的 Map，无论是 HashMap 还是类似的。
e.g.
例如
datamap = new HashMap();
(leaving aside generics etc.)
（抛开泛型等）
回答by Sheldon
dataMap is declared but not initialized. It can be initialized with
dataMap 已声明但未初始化。它可以初始化为
datamap = new HashMap();
数据映射 = 新 HashMap();
回答by jcopenha
Well, there are three objects accessed on that line. If nextLine[0] and nextLine[6] aren't null, because the println call above worked, then that leaves dataMap. Did you do dataMap = new HashMap(); somwehere?
那么，在该行上访问了三个对象。如果 nextLine[0] 和 nextLine[6] 不为 null，因为上面的 println 调用有效，那么就剩下 dataMap。你做了 dataMap = new HashMap(); 某处？
回答by Michael Borgwardt
Um, what exactly doyou expect when you do this?
嗯，什么做你期望，当你做到这一点？
HashMap<String, String> dataMap = null;
...
dataMap.put(...)
