Why am I getting this ClassCastException?

Question

I have two very simple classes, one extends the other:

public class LocationType implements Parcelable {
    protected int    locid = -1;
    protected String desc  = "";
    protected String dir   = "";
    protected double lat   = -1000;
    protected double lng   = -1000;

    public LocationType() {}

    public int getLocid() {
        return locid;
    }

    public void setLocid(int value) {
        this.locid = value;
    }

    public String getDesc() {
        return desc;
    }

    public void setDesc(String value) {
        this.desc = value;
    }

    public String getDir() {
        return dir;
    }

    public void setDir(String value) {
        this.dir = value;
    }

    public double getLat() {
        return lat;
    }

    public void setLat(double value) {
        this.lat = value;
    }

    public double getLng() {
        return lng;
    }

    public void setLng(double value) {
        this.lng = value;
    }



    // **********************************************
    //  for implementing Parcelable
    // **********************************************

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

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt   (locid);
        dest.writeString(desc );
        dest.writeString(dir  );
        dest.writeDouble(lat  );
        dest.writeDouble(lng  );
    }

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

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

    private LocationType(Parcel dest) {
        locid = dest.readInt   ();
        desc  = dest.readString();
        dir   = dest.readString();
        lat   = dest.readDouble();
        lng   = dest.readDouble();
    }
}

and:

public class MyLocationType extends LocationType {
    private ArrayList<ArrivalType> mArrivals = new ArrayList<ArrivalType>();

    public List<ArrivalType> getArrivals() {
        return mArrivals;
    }

    public void addArrival(ArrivalType arrival) {
        mArrivals.add(arrival);
    }
}

The problem is that when I cast an instance of LocationType to MyLocationType I get a ClassCastException. Why is this?

Answer 1

Because LocationType is the superclass; it can't be cast to the subclass.

To explain a bit further: you can only cast up the inheritance tree, which is to say, an object can only be cast as the class type it was created as, any of its superclasses, or any interface that it implements. Thus, a String can be cast as either a String or an Object; a HashMap can be cast as a HashMap, an AbstractMap Map, or an Object.

In your case, a MyLocationType can be either a MyLocationType or a LocationType (or an Object), but not the other way around.

The Java docs on inheritance are pretty good, just to review here.

Answer 2

This is because LocationType is not an instance of MyLocationType it is it's parent. For example if you added new methods to MyLocationType not in LocationType and then cast LocationType to MyLocationType what would expect to happen if these new methods were called that do not exist in the base LocationType?

MyLocationType is a LocationType but LocationType is not a MyLocationType.