ArrayList cannot be cast to custom class extending

问题:

I have class mentioned below:

public class JsonHistoryList extends ArrayList<JsonHistory> implements Serializable{}

I wish to pass it through intent using

timerService.putExtra(TimerService.ACTIVITY_LIST_ARG, activities);

But after I receive it (way below)

JsonHistoryList temp = (JsonHistoryList) intent.getSerializableExtra(TimerService.ACTIVITY_LIST_ARG);

in my service it gives me exception:

Caused by: java.lang.ClassCastException: java.util.ArrayList cannot be cast to com.epstim.captime.jsonmodel.JsonHistoryList

I don't understand why java can't handle that operation.

I've changed my code in service to:

ArrayList<JsonHistory> temp = (ArrayList<JsonHistory>) intent.getSerializableExtra(TimerService.ACTIVITY_LIST_ARG);
activities.addAll(temp);

And it worked.

回答1:

In the internals, android puts every extra in a special Map and doesn't record how exactly you want it parcelled.

At some point android will flattern your extras into parcel, and it will do so by checking each object type (since, as I said, it doesn't remember how you want it).

Parcel supports both writeList and writeSerializable and your JsonHistoryList is also both (list of serializables and a serialisable itself)

So android parcelling goes like this:

for (Object object : extras) {
   //... check for other types here
   } else if (object instanceof List) {
     parcel.writeList(object); // This what happens in your case and will be unparcelled into arraylist
   } else if (object instanceof Serializable) {
     parcel.writeSerializable(object); // What you really want, but percelling never will get here
   }
}

If you want to preserve list you need to create a class that will be serializable and won't extend arraylist but will contain it inside.

public class SuperJsonHistory implements Serializable {
  private JsonHistoryList yourList;
  ...
}

So composition over inheritance in case you want to preserve type

回答2:

This happens if intent.getSerializableExtra(TimerService.ACTIVITY_LIST_ARG); returns an object of class ArrayList<JsonHistory> and not an object of type JSONHistoryList.

We cannot forcefully downcast a parent object.

Consider your example,say

public class JsonHistoryList extends ArrayList implements Serializable{ public int someField ;// can by anything }

For simplicity, if your getSerializableExtra(); returns, new ArrayList<JsonHistory>(), when you try to downcast this to JsonHistoryList, it cannot cast so because someField values cannot be determinied

回答3:

we cannot cast a superclass object into a sub-class object.

Cast is only possible if object which is to be casted passes the IS-A relationship.

you can have a look at this link.It will clear all doubts explicit casting from super class to subclass

回答4:

So if JsonHistoryList extends ArrayList, then a JsonHistoryList is an ArrayList but an ArrayList is not necessarily a JsonHistoryList. You expected your own subclass, but that's not what you got back.