Type safety: Unchecked cast from Object to ArrayLi

问题:

Here is a part of a program that sends an ArrayList from a server to a client. I want to remove the warning about the last line in this code:

Client code:

Socket s;
(...)
// A server is sending a list from the other side of the link.
ois = new ObjectInputStream(s.getInputStream());
MyList = (ArrayList<MyVariable>) ois.readObject();

MyVariable is a Java class with some attributes. The server is creating an ArrayList and filling it with MyVariable variables as items. Then it sends the complete list to the client.

I would like to know why do I have a warning there and how to code perfectly in order to have 0 warnings. If it is possible I would like to avoid using "@SuppressWarnings("unchecked")". ;)

Thank you,

Luis

回答1:

Try this

Object obj = ois.readObject();
// Check it's an ArrayList
if (obj instanceof ArrayList<?>) {
  // Get the List.
  ArrayList<?> al = (ArrayList<?>) obj;
  if (al.size() > 0) {
    // Iterate.
    for (int i = 0; i < al.size(); i++) {
      // Still not enough for a type.
      Object o = al.get(i);
      if (o instanceof MyVariable) {
        // Here we go!
        MyVariable v = (MyVariable) o;
        // use v.
      }
    }
  }
}

回答2:

It's impossible to avoid this warning. readObject() returns an Object. You need to cast it. And casting to a generic type will always generate such a warning.

If you want to make your code as clean as possible, which is a good idea, you should respect the Java naming conventions though, and make variable names start with a lowercase letter.

回答3:

I don't like that, but you can have a container (sort of an alias or typedef):

// add "implements Serializable" in your case
private static class MyVariableList {
    public List<MyVariable> value;
}

And work with MyVariableList instead. That way you explicitly provide enough information to the compiler to do type checking in runtime.

回答4:

I was running into a similar problem as OP and found a good solution with a combination of comment from @VGR and Java 1.8 method for Arrays.

I will provide my answer in terms of OP's question, so it is generic and hopefully helps others:

1. Instead of returning a collection (list), return an array from the server. Covert collection to an array using the following on the server side code:

   myVariableList.toArray(new MyVariable[0]);

   If performance is an issue with above, following could be used, so that array does not need to be resized:

   myVariableList.toArray(myVariableList.size());

2. On client side convert array of object, to an array of MyVariable class.

   This is specific to JAVA 8.

   MyVariable[] myVarArr = Arrays.stream(ois.readObject()).toArray(MyVariable[]::new);

3. Then, finally convert Array to a Collection (list).

   List<MyVariable> myList = Arrays.asList(myVarArr);

Thanks.