Java参数化泛型类型列表创建不同类型的对象 [英] Java List of parameterized generic type creates different type of objects

问题描述

 我有一个函数需要创建一个类型的对象列表，公共静态< T>列表与LT; T> readJsonFile（final String filePath）{
 
 List< T> objectsFromFile = NULL; 
 File file = new File（System.getProperty（catalina.base）+ filePath）; 
 Gson gson = new Gson（）; 
 try（FileInputStream JSON = new FileInputStream（file））{
 BufferedReader br = new BufferedReader（new InputStreamReader（JSON））; 
 objectsFromFile = gson.fromJson（br，new TypeToken< List< T>>（）{
} .getType（））; 
} catch（Exception e）{
 e.printStackTrace（）; 
} 
返回objectsFromFile; 
} 
  

当我查看结果列表中的对象类型时，它们不是类型T（T将是我定义的不同类），但类型为 com.google.gson.internal.LinkedTreeMap 。

有谁知道为什么？我怎么能这样做，所以返回的列表是类型T？

解决方案

这是因为泛型类型的令牌不工作到期删除。

您需要注入 TypeToken< List< T>> 作为参数，具体实例化。

  public static< T>列表与LT; T> readJsonFile（
 final String filePath，final TypeToken< List< T>> typeToken）{
 // ... 
 objectsFromFile = gson.fromJson（br，typeToken.getType（））; 
 // ... 
} 
  

然后将其命名为：

  readJsonFile（/ path / to / file.json，new TypeToken< List< String>>（）{}） ; 
  

I have a function that needs to create a list of objects of a type that is passed in as a generic.

public static <T> List<T> readJsonFile(final String filePath) {

    List<T> objectsFromFile=null;
    File file = new File(System.getProperty("catalina.base") + filePath);
    Gson gson = new Gson();
    try (FileInputStream JSON = new FileInputStream(file)) {
        BufferedReader br = new BufferedReader(new InputStreamReader(JSON));
        objectsFromFile = gson.fromJson(br, new TypeToken<List<T>>() {
        }.getType());
    } catch (Exception e) {
        e.printStackTrace();
    }
    return objectsFromFile;
}

When I look at the type of objects in the resulting list they are not of type T (T will be different classes that I defined) but of type com.google.gson.internal.LinkedTreeMap.

Does anyone know why? And how could I make it so the returned list is of type T?

解决方案

It's because generic type tokens don't work due to erasure.

You need to inject the TypeToken<List<T>> as a parameter, concretely instantiated.

public static <T> List<T> readJsonFile(
    final String filePath, final TypeToken<List<T>> typeToken) {
  // ...
  objectsFromFile = gson.fromJson(br, typeToken.getType());
  // ...
}

And then call this as:

readJsonFile("/path/to/file.json", new TypeToken<List<String>>() {});

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