can't pass custom class through parcelable if it contains HashMap field - why?

Question

I've implemented parcelable for my custom class:

public class Tag implements Parcelable {
    int id;
    int type;
    int proficiencyLevel;
    HashMap<String, String> translations;

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

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(id);
        dest.writeInt(type);
        dest.writeInt(proficiencyLevel);

        // if this part is uncommented I get a NullPointerException
        /*
        dest.writeInt(translations.size());
        for (HashMap.Entry<String, String> entry : translations.entrySet()) {
            dest.writeString(entry.getKey());
            dest.writeString(entry.getValue());
        }
        */
    }

    public static final Parcelable.Creator<Tag> CREATOR = new Parcelable.Creator<Tag>() {
        public Tag createFromParcel(Parcel in) {
            return new Tag(in);
        }

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

    private Tag(Parcel in) {
        id = in.readInt();
        type = in.readInt();
        proficiencyLevel = in.readInt();

        // I've put breakpoint here and it seems to work OK, but then I get a NullPointerException
        // deserialize `translations` field
        /*int size = in.readInt();
        for (int i = 0; i < size; i++) {
            String key = in.readString();
            String value = in.readString();
            translations.put(key, value);
        }*/
    }
}

Now I pass it like this from activity A:

Tag[] tagsList = new Gson().fromJson(tags, Tag[].class);
Intent activityTagsSelectorLauncher = new Intent(this, ActivityTagsSelector.class);
activityTagsSelectorLauncher.putParcelableArrayListExtra("tagsList", new ArrayList<>(Arrays.asList(tagsList)));
startActivityForResult(activityTagsSelectorLauncher, SELECT_TAGS_REQUEST);

And receive it like so in activity B:

ArrayList<Tag> tagsList = getIntent().getParcelableArrayListExtra("tagsList");

It's all working OK if I don't write HashMap<String, String> translations field. I get NullPointerException when activity B starts. I've put breakpoint inside constructor private Tag(Parcel in) { and there it seems that everything works fine so I don't understand what I'm doing wrong.

Answer 1

You get NPE, because you don't initialize your translations Map:

HashMap<String, String> translations

is like

HashMap<String, String> translations = null;

Change declaration and initialize the map:

HashMap<String, String> translations = new HashMap<String, String>();

After you can use it uncommenting the lines

dest.writeInt(translations.size());
for (HashMap.Entry<String, String> entry : translations.entrySet()) {
    dest.writeString(entry.getKey());
    dest.writeString(entry.getValue());
}

UPDATE: to clarify what is happening, you declare variable and do not initialize it, so JVM does, and translates into HashMap<String, String> translations = null.

Initial Values of Variables

Every variable in a program must have a value before its value is used:

Each class variable, instance variable, or array component is initialized with a default value when it is created

[...] For all reference types, the default value is null.

Take a look at this default initialization table:

Type        Default Value
---------   -------------
boolean     false
byte        (byte) 0
short       (short) 0
int         0
long        0L
char        \u0000
float       0.0f
double      0.0d
object      null         <-------------- YAY!

After, when you try to get size with translations.size() is same as null.size(), there is your NPE...

Another scenario in a bigger code, is you may not want to initialize translations, so you can safe-check your sentence by:

dest.writeInt(translations != null ? translation.size() : 0);

For further info take a look here, here, and HERE

Answer 2

null error => due to missing initialization. Please initialize your hashmap.

Assume that you are passing this parcel ActivityA to ActivityB

I will share full code for those people who are having difficulty passing hashmap

Parcelable - Tag.java

    public class Tag implements Parcelable {
    int id;
    int type;
    int proficiencyLevel;
    HashMap<String, String> translations = new HashMap<String,String>(); // initialize
...
 @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(id);
        dest.writeInt(type);
        dest.writeInt(proficiencyLevel);

      //add below 
       dest.writeInt(translations.size());
        for (String s: translations.keySet()) {
            dest.writeString(s);
            dest.writeString(records.get(s));
        }

    }

...

private Tag(Parcel in) {
        id = in.readInt();
        type = in.readInt();
        proficiencyLevel = in.readInt();

        //add below 
        int count = in.readInt();
        for (int i = 0; i < count; i++) {
            translations.put(in.readString(), in.readString());
        }

    }
...
   //same as your code
    public static final Parcelable.Creator<Tag> CREATOR = new Parcelable.Creator<Tag>() {
        public Tag createFromParcel(Parcel in) {
            return new Tag(in);
        }

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

Activity A

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;

public class ActivityA extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ParcelTest p = new ParcelTest();
        p.put("green", "go");
        p.put("yellow", "wait");
        p.put("red", "stop");

        Bundle b = new Bundle();
        b.putParcelable("com.example.trafficlight", p);

        Intent i = new Intent(this, ActivityB.class);
        i.putExtras(b);

        startActivity(i);
    }
}

Activity B

public class ActivityB extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Bundle b = getIntent().getExtras();

        ParcelTest p = b.getParcelable("com.example.trafficlight");

        String red = p.get("red");
        // ...
    }
}

Credit http://blog.cluepusher.dk/2009/10/28/writing-parcelable-classes-for-android/