Parcel, ClassLoader and Reading Null values

Question

I am trying to understand how to read null values from Parcel properly. Let's look my use case:

public class MyModel implements Parcelable {
    private Long id;
    private String name;

    public MyModel() {}

    public MyModel(Parcel in) {
        readFromParcel(in);
    }

    @Override public int describeContents() {
        return 0;
    }

    @Override public void writeToParcel(Parcel dest, int flags) {
        dest.writeValue(id);
        dest.writeValue(name);
    }

    public void readFromParcel(Parcel in) {
        id = (Long) in.readValue(Long.class.getClassLoader());
        name = (String) in.readValue(String.class.getClassLoader());
    }
}

For the sake of this question, let's take an example of field name which is of String type.

I understand I can use writeString instead of writeValue, but writeString will throw NullPointerException if name was null.

So now the real issue is, if name was null and i attempt to readValue and then casting null to String will throw an Exception.

What is the best approach to read null values from Parcel in that case? Do I need to check for null every time I attempt to read from Parcel? But that will be too much if you have a lot of fields in a model. Is there a difference between passing null or a ClassLoader when reading? Does a class loader returns an empty/default object if the value is null when reading Parcel?

The error I usually get is this, which happens when it is reading one of the Strings from Parcel:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.company.MyActivity}: java.lang.RuntimeException: Parcel android.os.Parcel@42b82908: Unmarshalling unknown type code 7209045 at offset 1368

writeString(null) will work, see how writeValue is implemented: http://androidxref.com/4.4_r1/xref/frameworks/base/core/java/android/os/Parcel.java#1182 it just uses writeString with the passed argument — pskink, Oct 07 '14 at 04:16
@pskink I see that writeString calls nativeWriteString, do you know where those are implemented? — SherCoder, Oct 07 '14 at 16:54
why do you want to know this? its perfectly OK to write null String, try to call writeString(null) and it will work, btw its here: http://androidxref.com/4.4_r1/xref/frameworks/base/core/jni/android_os_Parcel.cpp#641 — pskink, Oct 07 '14 at 17:07
@pskink I have a related question about writing `Long`, not primitive but the object `Long`. if it is null at time of writing, what will it be when I am reading? Because readLong returns `long` not `Long`, so will that `long` be a random value? — SherCoder, Oct 07 '14 at 17:45
then use: Long l = (Long) parcel.readValue(Long.class.getClassLoader()); — pskink, Oct 07 '14 at 18:10
@pskink but if readValue returned null then casting will fail and throw exception, right? Or does ClassLoader prevents that? I guess I don't understand ClassLoader very well. — SherCoder, Oct 07 '14 at 18:58
@pskink Actually just confirmed that writeLong will throw NullPointerException if Long is null. — SherCoder, Oct 07 '14 at 19:07
@pskink Thanks. Sorry about these extended questions. Just trying to understand the Parcel. Appreciate your time. — SherCoder, Oct 07 '14 at 19:17
What you say here is just **not true**: "writeString will throw NullPointerException if name was null" See [my answer below](http://stackoverflow.com/a/34370801/383414) and [this related question](http://stackoverflow.com/a/34370623/383414). — Richard Le Mesurier, Dec 19 '15 at 12:40
@SherCoder `writeLong(long)` expects a primitive `long` value, rather than a `Long` object. The npe you get is from Java autoboxing your `null` into a `long` before it gets passed into the method. That is why you can't do that. With a `String` object it is different, since the `writeString(String)` method expects an object reference, not a primitive. — Richard Le Mesurier, Dec 19 '15 at 12:46

score 1 · Answer 1 · answered Oct 07 '14 at 01:27

1

How about supply ClassLoader with MyModel.class. getClassLoader()?

answered Oct 07 '14 at 01:27

Feng Dai

633
5
9

An example would help improve this answer, but there's no reason to flag it. If you don't like it, down vote it. – ArtOfWarfare Oct 07 '14 at 02:14
Clicked "flag" by mistake. – Feng Dai Oct 07 '14 at 03:48
How would that help? Explanation would be helpful. Thanks. – SherCoder Oct 07 '14 at 21:08
I wrote a class the same as yours. Then I kept the ```name``` field ```null```, and passed it from ActivityA to ActivityB. When I attempted to get it out of the Intent inside ActivityB, no exception was thrown. Is there any other condition for this kind of exception to occur? – Feng Dai Oct 08 '14 at 01:56

Richard Le Mesurier · Answer 2 · 2015-12-19T12:42:21.330

What is the best approach to read null values from Parcel in that case?

You can use the string-specific methods:

Parcel.writeString(String)
Parcel.readString()

They work perfectly well with null values.

For example, assume you have a class with the following fields in it:

public class Test implements Parcelable {
    public final int id;
    private final String name;
    public final String description;
    ...

You create the Parcelable implementation like this (using Android Studio autoparcelable tool):

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeInt(id);
    dest.writeString(null); // NOTE the null value here
    dest.writeString(description);
}

protected Test(Parcel in) {
    id = in.readInt();
    name = in.readString();
    description = in.readString();
}

Your Test.name value will be deserialised as null correctly, without requiring any casting to String.

score -1 · Answer 3 · answered Oct 07 '14 at 01:24

-1

I think the best way to do this is to write the length of string name before name. If name is null, just write 0 as its length, and do not write name. When reading, if length is read and its value is zero, do not read name and just set name to null.

answered Oct 07 '14 at 01:24

Byungjoon Lee

913
6
18

This is interesting but if I have a lot of String fields in my model, then I will have to do the same for every single one of them. – SherCoder Oct 07 '14 at 21:09

Parcel, ClassLoader and Reading Null values

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