英文:
How should I instantiate List<List<String>> in Java
问题
我有以下的代码：
List<List<String>> allData = getData()
if (allData == null)
allData = new ArrayList<ArrayList<String>>();
// 在下面填充 allData
现在我想要初始化 allData 但是我得到了 Type mismatch: cannot convert from ArrayList<ArrayList<String>> to List<List<String>> 错误。我应该如何正确地初始化它？
从 getData() 返回 ArrayList<ArrayList<String>> 是不可能的。
谢谢！
英文:
I have the following code:
List<List<String>> allData= getData()
if (allData== null)
allData= new ArrayList<ArrayList<String>>();
// populate allData below
Now I want to initialize allData but I get Type mismatch: cannot convert from ArrayList<ArrayList<String>> to List<List<String>>. What is the correct way I can initialize this?
It is not possible to return ArrayList<ArrayList<String>> from getData()
Thanks!
答案1
得分: 9
你可以很简单地这样做：
allData = new ArrayList<>();
然后你可以向allData中添加新的列表：
```java
List innerList = new ArrayList<>();
innerList.add("一些字符串");
// .... 等等 ....
allData.add(innerList);
英文:
You do it very simply:
allData = new ArrayList<>();
Then you can add new lists to allData:
List innerList = new ArrayList<>();
innerList.add("some string");
// .... etc ...
allData.add(innerList);
答案2
得分: 3
你不能在实例化具体实现时重新定义引用的泛型类型。引用是List<List<String>>，因此分配的List必须能够接受任何List<String>作为元素。当你实例化实例时，你试图将其限制为ArrayList<String>。
显式的解决方案是：
allData = new ArrayList<List<String>>();
或者更简单地写为：
allData = new ArrayList<>();
英文:
You cannot redefine the generic type of the reference when you instantiate the concrete implementation. The reference is List<List<String>> so the assigned List must be capable of accepting any List<String> as an element. When you instantiated your instance, you attempted to limit this to ArrayList<String>.
The explicit solution is:
allData = new ArrayList<List<String>>();
or more simply as:
allData = new ArrayList<>();
答案3
得分: 1
一个简单的getData示例可能如下所示：
public static List<List<String>> getData(String fileName){
List<List<String>> content = null;
try (Stream<String> lines = Files.lines(Paths.get(fileName ))) {
content = lines
.map(l -> l.split(" "))
.map(Arrays::asList)
.collect(Collectors.toList());
} catch (IOException e) {
e.printStackTrace();
}
return content;
}
英文:
A simple example of getData might be as below -
public static List<List<String>> getData(String fileName){
List<List<String>> content = null;
try (Stream<String> lines = Files.lines(Paths.get(fileName ))) {
content = lines
.map(l -> l.split(" "))
.map(Arrays::asList)
.collect(Collectors.toList());
} catch (IOException e) {
e.printStackTrace();
}
return content;
}
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