How to serialize a List of custom objects and recover it onRestoreInstanceState in Android?

Question

Hi everyone I have a custom object MasterWithValue (an object with a value and a list of Detail objects I have made) which extends a class Master (which has only a name property).

This is the class MasterWithValue:

public class MasterWithValue extends Master {

private String value;
private List<Detail> detailList;

public MasterWithValue(String masterName, String masterValue) {
    super(masterName);
    this.value = masterValue;
    this.detailList = new ArrayList<Detail>();
}

@Override 
public int getViewType() {
    return super.getViewType();
}

@Override
public View getView(LayoutInflater inflater, View convertView) {
    View view;
    if (convertView == null) {
        view = inflater.inflate(R.layout.statistics_rowlist_master, null);
    }
    else {
        view = convertView;
    }

    TextView MasterEntryName = (TextView) view.findViewById(R.id.statistics_master_name);
    TextView MasterEntryValue = (TextView) view.findViewById(R.id.statistics_master_value);

    MasterEntryName.setText(super.name);
    MasterEntryValue.setText(this.value);

    return view;
}

public String getMasterValue() {
    return value;
}

public List<Detail> getDetailList() {
    return this.detailList;
}

public void addDetailToMaster(Detail detail) {
    this.detailList.add(detail);
}

}

In the onSaveInstanceState() method I have this code:

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putSerializable("master_detail_list", (ArrayList<MasterWithValue>)   
      MasterAndDetailStatistics);
}

Where MasterAndDetailStatistics is a List.

Now in onRestoreInstanceState I have tried this code:

@Override
protected void onRestoreInstanceState(Bundle savedState) {
    super.onRestoreInstanceState(savedState);

    MasterAndDetailStatistics = (List<MasterWithValue>) savedState.getSerializable("master_detail_list");

}

By I get a type-safety warning: Type safety: Unchecked cast from Serializable to List How can I check it? I read that I should implement the Parcable interface, but I am new to android and I have no idea on how to do that. What should I do?

Answer 1

There are many examples of how to implement parcelable. Here is the canonical one. Here is an answer from SO.

Basically, you implement a method public void writeToParcel(Parcel out, int flags) where you write all the fields you want persisted into the out value.

Then implement an anonymous implementation of a Parcelable.Creator parameterized with your parcelable class. This anonymous implementation must be named CREATOR .

Finally, make a private constructor like this: private MyParcelable(Parcel in) . Where you read all the values you wrote in the out back into the fields of your object in the in .

Then you need to make Detail parcelable too! Oh joy!

It needs to look like this:

public class MasterWithDetails implements Parcelable {
    private String value;
    private List<Detail> detailList;

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel out, int flags) {
        out.writeString(value);
        out.writeString(getName());
        out.writeTypedList(detailList); //don't forget to make Detail parcelable too!
    }

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

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

    private MasterWithDetails(Parcel in) {
        value = in.readString();
        setName(in.readString());
        detailList = in.createTypedArrayList(Detail.CREATOR);
    }
}