Is it possible to create a HashMap that is Parcelable on Android?

Question

I am trying to extend HashMap as a Parcelable and I got the syntax to compile, however, at runtime it throws an exception and returns a null pointer trying to un-marshal the data.

The sender has to cast to (Parcelable) to resolve ambiguity, however, the receiver complains that is expected Parcelable but found HashMap.

Has anyone been successful with this? Is my syntax wrong? Is there a better solution?

Following is the code:

• HomeActivity.java - the sender
• ContentViewActivity.java - the receiver
• ContentItemSimple.java - as its name implies (wraps a String and Integer)
• ContentItemCollection.java - this is the HashMap

HomeActivity.java

package com.mobibob.android.studyparceable;

import java.util.HashMap;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Parcel;
import android.os.Parcelable;
import android.text.format.Time;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class HomeActivity extends Activity implements OnClickListener {
    private static final String TAG = "HomeActivity";
    private ContentItemSimple mContentItemSimple = null;
    private ContentItemContainer mContentItemContainer = null;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);

        Button click = (Button) findViewById(R.id.button_clickit);
        click.setOnClickListener(this);

        mContentItemSimple = new ContentItemSimple();
        mContentItemSimple.name = "mobibob";
        mContentItemSimple.year = 2010;

        String value = "value";
        Time nowTime = new Time();
        nowTime.setToNow();
        mContentItemContainer = new ContentItemContainer();
        mContentItemContainer.put("string", new String("baseball is great!"));
        mContentItemContainer.put("integer", new Integer(1234));
//        mContentItemContainer.put("boolean", new Boolean(true));
//        mContentItemContainer.put("date", nowTime);
//        mContentItemContainer.put("parcel", new Bundle());
        Log.d(TAG, "..... " + mContentItemContainer.toString());
    }

    @Override
    public void onClick(View v) {

        Intent i = new Intent(getBaseContext(), ContentViewActivity.class);
        i.putExtra(ContentItemSimple.EXTRA_CONTENT_DETAIL, mContentItemSimple);
        i.putExtra(ContentItemContainer.EXTRA_CONTENT_CONTAINER, (Parcelable) mContentItemContainer);
        startActivityForResult(i, 0);

    }
}

ContentViewActivity

package com.mobibob.android.studyparceable;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

import com.mobibob.android.studyparceable.ContentItemSimple;

public class ContentViewActivity extends Activity implements OnClickListener {

    private static final String TAG = "ContentViewActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.content_view);

        Button gohome = (Button) findViewById(R.id.button_gohome);

        gohome.setOnClickListener(this);

        ContentItemSimple ci = null;
        ContentItemContainer cx = null;

        try {
            ci = getIntent().getParcelableExtra(ContentItemSimple.EXTRA_CONTENT_DETAIL);
            Log.i(TAG, "ci = " + ci.toString());

            cx = getIntent().getParcelableExtra(ContentItemContainer.EXTRA_CONTENT_CONTAINER);
            Log.i(TAG, "cx = " + cx.toString());

            TextView tvName = (TextView) findViewById(R.id.ci_name);
            tvName.setText(ci.name);
            TextView tvYear = (TextView) findViewById(R.id.ci_year);
            tvYear.setText(String.format("%d", ci.year));

        } catch (Exception e) {
            Log.e(TAG, e.toString());
            e.printStackTrace();
        }
    }

    @Override
    public void onClick(View v) {
        finish();
    }

}

ContentItemSimple.java

package com.mobibob.android.studyparceable;

import android.os.Parcel;
import android.os.Parcelable;
import android.util.Log;

public class ContentItemSimple implements Parcelable {
        public static final String TAG = "ContentItem";
    public static final String EXTRA_CONTENT_DETAIL = "contentDetail";
    public String name = "name";
    public Integer year = Integer.MIN_VALUE;

    ContentItemSimple() {
        name = new String("");
        year = new Integer(Integer.MIN_VALUE);
    }

    ContentItemSimple(Parcel in) {
            try {
                name = in.readString();
                year = in.readInt();
            } catch (Exception e) {
                Log.e(TAG, e.toString());
                e.printStackTrace();
        }
    }

    @Override
    public String toString() {
        return String.format("name=%s age=%d", name, year);
    }

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

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(name);
        dest.writeInt(year);
    }

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

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

}

ContentItemContainer.java

package com.mobibob.android.studyparceable;

import java.util.HashMap;
import java.util.Iterator;

import android.os.Parcel;
import android.os.Parcelable;

public class ContentItemContainer extends HashMap<String, Object> implements Parcelable {
    /**
     * Container for wddx 'struct' elements.
     */
    private static final long serialVersionUID = 1L;
    // public String name = "?";
    // public String value = "?";
    public static final String EXTRA_CONTENT_DETAIL = "contentDetail";
    public static final String EXTRA_CONTENT_CONTAINER = "contentContainer";
    public static final String EXTRA_CONTENTDETAIL_NAME = "name";
    public static final String EXTRA_CONTENTDETAIL_VALUE = "value";

//    private static HashMap<String, Object> map = new HashMap<String, Object>();

    ContentItemContainer() {
        super();
    }

    ContentItemContainer(Parcel in) {
        in.readMap(this, ContentItemContainer.class.getClassLoader());
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder("");
        Integer x = 0;
        Iterator<HashMap.Entry<String, Object>> it = this.entrySet().iterator();
        while (it.hasNext()) {
            HashMap.Entry<String, Object> pairs = (HashMap.Entry<String, Object>) it.next();
            x++;
            sb.append("\n"+x.toString()+": ").append("name=").append(pairs.getKey()).append(" value=").append(pairs.getValue().toString());
        }
        return sb.toString();
    }

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

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeMap(this);
    }

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

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

}

Answer 1

The way your class ContentItemContainer is implemented - as extending HashMap, your writeToParcel and Parcelable.Creator will never be called.

The reason is that Map is a valid data type to be put in a Parcel, so that the class gets flattened using the logic of the HashMap class, and not yours. This is because the way Parcel is implemented, it checks whether the supplied values are descendants of certain classes in a specific order.

When unparcelling the extras, a HashMap will be created from your object's data consequently.

To work around this, you could hide the HashMap inside your class and expose getters/putters for the HashMap. This is exactly the same way that ContentValues is implemented, and parcelling/unparcelling it works without any problems.