当在侦听器中调用getValue（）时，android firebase无法实例化抽象类

Question

我有一个服装类，其中一个抽象类是其属性之一。当我侦听数据库中的更改时，出现此错误java.lang.RuntimeException: java.lang.InstantiationException: Can't instantiate abstract class com.example.recouture.Item

 DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
    Query query = reference.child(FirebaseAuth.getInstance().getCurrentUser().getUid()).child("Outfits");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot singleSnapshot : dataSnapshot.getChildren()) {
                outfits.add(singleSnapshot.getValue(Outfit.class)); //error is here
            }

是否有可能避免这种情况，因为将Item设为非抽象会在我的其他代码部分中引起设计问题。 Item是抽象的，因为它可以包含从Item类扩展的衬衫，裤子等。

Outfit.java

public class Outfit implements Parcelable{

private List<Item> itemList = new ArrayList<>();
private String mName;
private String mImageUrl;


public Outfit() {}


public Outfit(String mName, String mImageUrl,List<Item> items) {
    this.mName = mName;
    this.mImageUrl = mImageUrl;
    this.itemList = items;
}

public List<Item> getItemList() {
    return itemList;
}

protected Outfit(Parcel in) {
    in.readList(itemList,Item.class.getClassLoader());
    mName = in.readString();
    mImageUrl = in.readString();
}

public static final Creator<Outfit> CREATOR = new Creator<Outfit>() {
    @Override
    public Outfit createFromParcel(Parcel in) {
        return new Outfit(in);
    }

    @Override
    public Outfit[] newArray(int size) {
        return new Outfit[size];
    }
};

public String getmName() {
    return mName;
}

public String getmImageUrl() {
    return mImageUrl;
}

@Override
public String toString() {
    return mName;
}

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

@Override
public void writeToParcel(Parcel parcel, int i) {
    parcel.writeList(itemList);
    parcel.writeString(mName);
    parcel.writeString(mImageUrl);
}

public void setItemList(List<Item> itemList) {
    this.itemList = itemList;
}

}

public abstract class Item implements Parcelable {

private String mName;

private String mColor;

private String mImageUrl;

private List<TagHolder> tags = new ArrayList<>();

private String mKey;




public Item(){

}



public Item(String mName, String mColor, String mImageUrl) {
    this.mName = mName;
    this.mColor = mColor;
    this.mImageUrl = mImageUrl;
}


protected Item(Parcel in) {
    mName = in.readString();
    mColor = in.readString();
    mImageUrl = in.readString();
    tags = in.createTypedArrayList(TagHolder.CREATOR);
    mKey = in.readString();
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(mName);
    dest.writeString(mColor);
    dest.writeString(mImageUrl);
    dest.writeTypedList(tags);
    dest.writeString(mKey);
}

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


public void setmName(String mName) {
    this.mName = mName;
}

public void setmImageUrl(String mImageUrl) {
    this.mImageUrl = mImageUrl;
}

public List<TagHolder> getTags() {
    return tags;
}

public String getmColor() {
    return mColor;
}

public String getmImageUrl() {
    return mImageUrl;
}

public void setmColor(String mColor) {
    this.mColor = mColor;
}

public String getmName() {
    return mName;
}

public void setTags(List<TagHolder> tags) {
    this.tags = tags;
}

@Exclude // dont need this in our firebase database
public String getKey() {
    return mKey;
}

@Exclude
public void setMkey(String key) {
    mKey = key;
}


public abstract String getCategory();

}

firebase数据库结构：

userid : {
    outfits : {
        uniqueKey1 : {
            itemList : {
                0: {..}
                1: {...}
            }
                mName : {...}
                mImageUrl : {...}

         } // uniquekey1
         uniqueKey2 : {
            itemList : {
                0: {..}
                1: {...}
            }
                mName : {...}
                mImageUrl : {...}
            } // end unique key 2 
         }
     } //end outfits
} // end user id 

完整堆栈跟踪：

08-02 00:04:28.191 6353-6353/com.example.recouture E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.recouture, PID: 6353
java.lang.RuntimeException: java.lang.InstantiationException: Can't instantiate abstract class com.example.recouture.Item
    at com.google.android.gms.internal.zzelx.zze(Unknown Source:305)
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:196)
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:301)
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:60)
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:0)
    at com.google.android.gms.internal.zzelx.zze(Unknown Source:125)
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:196)
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:0)
    at com.google.firebase.database.DataSnapshot.getValue(Unknown Source:10)
    at com.example.recouture.Outfit.ViewOutfits$2.onDataChange(ViewOutfits.java:74) // line of my error 
    at com.google.firebase.database.zzp.onDataChange(Unknown Source:7)
    at com.google.android.gms.internal.zzegf.zza(Unknown Source:13)
    at com.google.android.gms.internal.zzeia.zzbyc(Unknown Source:2)
    at com.google.android.gms.internal.zzeig.run(Unknown Source:71)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.InstantiationException: Can't instantiate abstract class com.example.recouture.Item
    at java.lang.reflect.Constructor.newInstance0(Native Method)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
    at com.google.android.gms.internal.zzelx.zze(Unknown Source:53)
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:196) 
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:301) 
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:60) 
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:0) 
    at com.google.android.gms.internal.zzelx.zze(Unknown Source:125) 
    at com.google.android.gms.internal.zzelw.zzb(Unknown Source:196) 
    at com.google.android.gms.internal.zzelw.zza(Unknown Source:0) 
    at com.google.firebase.database.DataSnapshot.getValue(Unknown Source:10) 
    at com.example.recouture.Outfit.ViewOutfits$2.onDataChange(ViewOutfits.java:74) 
    at com.google.firebase.database.zzp.onDataChange(Unknown Source:7) 
    at com.google.android.gms.internal.zzegf.zza(Unknown Source:13) 
    at com.google.android.gms.internal.zzeia.zzbyc(Unknown Source:2) 
    at com.google.android.gms.internal.zzeig.run(Unknown Source:71) 
    at android.os.Handler.handleCallback(Handler.java:873) 
    at android.os.Handler.dispatchMessage(Handler.java:99) 
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:6669) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

Answer 1

要解决此问题，请更改以下代码行：

public abstract class Item implements Parcelable

到

public class Item implements Parcelable

您不能实例化抽象类，只能在Java中实例化具体类。因此，您的模型类不应为abstract，以便可以实例化。

Answer 2

您的具体Outfit类具有定义Item的字段和属性：

public class Outfit implements Parcelable{
    private List<Item> itemList = new ArrayList<>();

    ...    
    public List<Item> getItemList() 

这意味着Firebase SDK将尝试实例化Item，因为它是抽象的，所以不可能。您最可能希望Firebase在读取数据时实例化Item的正确（具体）子类型，但无法知道它是什么子类型。

您要么必须将Item标记为非抽象，要么必须将itemList标记为具有特定的具体子类型Item。