使用 POJO 对象类将 JSON 解析为 POJO 对象列表

Question

I am forced to use an interface method with the signature which I can not change:

List parse(String path, Class clazz)

It is supposed to return a list of objects with the type, passed in the clazz parameter, eg:

List<TagsRun> runs = reader.parse(path, TagsRun.class)

I need to implement a JSON parser and the question is how to set the type of clazz parameter to the result list?

I use Gson and it might be something like:

List parse(String path, Class clazz) {
    Type collectionType = new TypeToken<List<TagsRun>>(){}.getType();
    List<TagsRun> myList = new Gson.fromJson(json, collectionType);
    return myList 
}

But how to replace the particular List< TagsRun > with List< type of clazz > here?

The only working solution I did requires changing of the method signature:

<T> List<T> parse(String path, Class<T[]> clazz ) {
        T[] arr = new Gson().fromJson(new File(path).text(), clazz);
        return Arrays.asList(arr);
}

In other words, how to return the list of objects having the same type as clazz argument using the signature:

 List parse(String path, Class clazz)

Thanks in advance.

Answer 1

Since you can not change the method's signature, there is no point in returning a List with type parameter.

public List foo() {
    ArrayList<String> strings = new ArrayList<>();
    return strings;
}

public void methodUsingFoo() {
    // possible but you should check if the assumption is correct:
    List<String> uncheckedAssignment = foo(); // so don't do this

    // this is what foo *will* return:
    List actualResult = foo();
}

Instead you could do something like this:

public void methodUsingParse() {
    Class clazz = Object.class; // or whatever class you want or need

    // this is what your method *will* return:
    List actualResult = parse("/path/to/somewhere", clazz);

    // best way would be to check each element:
    for(Object element : actualResult) {
        if (clazz.equals(element.getClass())) {
            // do your stuff here
        }
    }
}

What I forgot: I don't think that it es even possible to do this like you would like to.

Answer 2

Maybe convert the list?

List<TagsRun> myList = new Gson.fromJson(json, collectionType);
List<Clazz> = Arrays.asList(myList.toArray(new Clazz[0]));

Edit: I think this could help you: https://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#checkedCollection(java.util.Collection,%20java.lang.Class) - Scroll down to the List section, if you didn't notice it

Answer 3

Solved it, finally:

@Override
List parse(String path, Class clazz) {
    List result = []
    JsonArray jsonArray = new JsonParser()
            .parse(new File(path).getText(CHARSET))
            .getAsJsonArray()

    Gson gson = new Gson()
    jsonArray.each {
        result.add(gson.fromJson(it, clazz))
    }
    return result
}

The trick is to first get an array, and then use gson.fromJson(it, clazz) for each of array items. This will convert each json element in the array to POJO and then add it to result list.