Read & writing arrays of Parcelable objects

Question

I have following class which reads and writes an array of objects from/to a parcel:

class ClassABC extends Parcelable {
    MyClass[] mObjList;

    private void readFromParcel(Parcel in) {
        mObjList = (MyClass[]) in.readParcelableArray(
                com.myApp.MyClass.class.getClassLoader()));
    }

    public void writeToParcel(Parcel out, int arg1) {
        out.writeParcelableArray(mObjList, 0);
    }

    private ClassABC(Parcel in) {
        readFromParcel(in);
    }

    public int describeContents() {
        return 0;
    }

    public static final Parcelable.Creator<ClassABC> CREATOR =
            new Parcelable.Creator<ClassABC>() {

        public ClassABC createFromParcel(Parcel in) {
            return new ClassABC(in);
        }

        public ClassABC[] newArray(int size) {
            return new ClassABC[size];
        }
    };
}

In above code I get a ClassCastException when reading readParcelableArray:

ERROR/AndroidRuntime(5880): Caused by: java.lang.ClassCastException: [Landroid.os.Parcelable;

What is wrong in above code? While writing the object array, should I first convert the array to an ArrayList?

UPDATE:

Is it OK to convert an Object array to an ArrayList and add it to parcel? For instance, when writing:

    ArrayList<MyClass> tmpArrya = new ArrayList<MyClass>(mObjList.length);
    for (int loopIndex=0;loopIndex != mObjList.length;loopIndex++) {
        tmpArrya.add(mObjList[loopIndex]);
    }
    out.writeArray(tmpArrya.toArray());

When reading:

    final ArrayList<MyClass> tmpList = 
            in.readArrayList(com.myApp.MyClass.class.getClassLoader());
    mObjList= new MyClass[tmpList.size()];
    for (int loopIndex=0;loopIndex != tmpList.size();loopIndex++) {
        mObjList[loopIndex] = tmpList.get(loopIndex);
    }

But now I get a NullPointerException. Is above approach is correct? Why it is throwing an NPE?

Answer 1

You need to write the array using the Parcel.writeTypedArray() method and read it back with the Parcel.createTypedArray() method, like so:

MyClass[] mObjList;

public void writeToParcel(Parcel out) {
    out.writeTypedArray(mObjList, 0);
}

private void readFromParcel(Parcel in) {
    mObjList = in.createTypedArray(MyClass.CREATOR);
}

The reason why you shouldn't use the readParcelableArray()/writeParcelableArray() methods is that readParcelableArray() really creates a Parcelable[] as a result. This means you cannot cast the result of the method to MyClass[]. Instead you have to create a MyClass array of the same length as the result and copy every element from the result array to the MyClass array.

Parcelable[] parcelableArray =
        parcel.readParcelableArray(MyClass.class.getClassLoader());
MyClass[] resultArray = null;
if (parcelableArray != null) {
    resultArray = Arrays.copyOf(parcelableArray, parcelableArray.length, MyClass[].class);
}

Answer 2

ERROR/AndroidRuntime(5880): Caused by: java.lang.ClassCastException: [Landroid.os.Parcelable;

According to the API, readParcelableArray method returns Parcelable array (Parcelable[]), which can not be simply casted to MyClass array (MyClass[]).

But now i get Null Pointer Exception.

It is hard to tell the exact cause without the detailed exception stack trace.

---

Suppose you have made MyClass implements Parcelable properly, this is how we usually do for serialize/deserialize a array of parcelable objects:

public class ClassABC implements Parcelable {

  private List<MyClass> mObjList; // MyClass should implement Parcelable properly

  // ==================== Parcelable ====================
  public int describeContents() {
    return 0;
  }

  public void writeToParcel(Parcel out, int flags) {
    out.writeList(mObjList);
  }

  private ClassABC(Parcel in) {
    mObjList = new ArrayList<MyClass>();
    in.readList(mObjList, getClass().getClassLoader());
   }

  public static final Parcelable.Creator<ClassABC> CREATOR = new Parcelable.Creator<ClassABC>() {
    public ClassABC createFromParcel(Parcel in) {
      return new ClassABC(in);
    }
    public ClassABC[] newArray(int size) {
      return new ClassABC[size];
    }
  };

}

Hope this helps.

You can also use the following methods:

  public void writeToParcel(Parcel out, int flags) {
      out.writeTypedList(mObjList);
  }

  private ClassABC(Parcel in) {
      mObjList = new ArrayList<ClassABC>();
      in.readTypedList(mObjList, ClassABC.CREATOR);
  }

Answer 3

I had a similar problem, and solved it this way. I defined a helper method in MyClass, for converting an array of Parcelable to an array of MyClass objects:

public static MyClass[] toMyObjects(Parcelable[] parcelables) {
    MyClass[] objects = new MyClass[parcelables.length];
    System.arraycopy(parcelables, 0, objects, 0, parcelables.length);
    return objects;
}

Whenever I need to read a parcelable array of MyClass objects, e.g. from an intent:

MyClass[] objects = MyClass.toMyObjects(getIntent().getParcelableArrayExtra("objects"));

EDIT: Here is an updated version of the same function, that I am using more recently, to avoid compile warnings:

public static MyClass[] toMyObjects(Parcelable[] parcelables) {
    if (parcelables == null)
        return null;
    return Arrays.copyOf(parcelables, parcelables.length, MyClass[].class);
}

Answer 4

You need to write the array using the Parcel.writeTypedArray() method and read it back with the Parcel.readTypedArray() method, like so:

MyClass[] mObjArray;

public void writeToParcel(Parcel out, int flags) {
    out.writeInt(mObjArray.length);
    out.writeTypedArray(mObjArray, flags);
}

protected MyClass(Parcel in) {
    int size = in.readInt();
    mObjArray = new MyClass[size];
    in.readTypedArray(mObjArray, MyClass.CREATOR);
}

For lists, you can do the following:

  ArrayList<MyClass> mObjList;

  public void writeToParcel(Parcel out, int flags) {
      out.writeTypedList(mObjList);
  }

  protected MyClass(Parcel in) {
      mObjList = new ArrayList<>(); //non-null reference is required
      in.readTypedList(mObjList, MyClass.CREATOR);
  }