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dddenis
Asked: 2020-05-20 18:46:17 +0800 CST 2020-05-20 18:46:17 +0800 CST

Get the class of a generic from a list

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I'm trying to get the type Classof a List<Student>to be able to deserialize with the method Serialize#readI have mounted which works but the IDE keeps warning me that the assignment is not safe, I want it to be.

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Attempts to get the Class ofList<Student>

List<Student>.class // error de sintaxis
List.class // Es el único que me acepta y funciona.

Method to deserialize

public static <T extends Object> T read(final String fileName, final Class<T> clazz) throws IOException, ClassNotFoundException {
    ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File(fileName)));
    final T data = clazz.cast(ois.readObject());
    ois.close();
    return data;
}
java

1 Answers

  1. Best Answer

    I inform you that there is no simple way to do what you want. As far as I've seen, you have two ways to do what you request:

    1. Cast to generic and add the `@SuppressWarnings("unchecked") annotation. This just tells the compiler that the cast is safe at runtime. But we know that generics are removed at runtime, so this strategy only removes the compiler warning.

    2. Create a method to cast the list. The method is simple:

      public static <T> List<T> castList(List<?> list, Class<T> clazz) {
    List<T> newList = new ArrayList<>(list.size());
    for (Object o : list) {
        newList.add(clazz.cast(o));
    }
    return newList;
}

      Or done in Java 8 in a single line:

      public static <T> List<T> castListData(List<?> list, Class<T> clazz) {
    return list.stream()
        .map(x -> clazz.cast(x))
        .collect(Collectors.toList())
        ;
}

      And you call your method:

      return castListData(
    Serialize.read(PATH_DATA, List.class),
    Student.class);
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