火力地堡无法转换java.lang.String类型的对象键入

Question

我创建一个简单的库应用程序可加入书，可以被看作一个书单。

这里是我的书类

public class Book {

private int _id;
private String _bookTitle;
private String _bookAuthor;
private int _bookISBN;

private Map<String, Object> mp;

public Book() {

}

public Book(String _bookTitle, String _bookAuthor, int _bookISBN) {
    this._id = _id;
    this._bookTitle = _bookTitle;
    this._bookAuthor = _bookAuthor;
    this._bookISBN = _bookISBN;

}

public int get_id() {
    return _id;
}

public void set_id(int _id) {
    this._id = _id;
}

public String get_bookTitle() {
    return _bookTitle;
}

public void set_bookTitle(String _bookTitle) {
    this._bookTitle = _bookTitle;
}

public String get_bookAuthor() {
    return _bookAuthor;
}

public void set_bookAuthor(String _bookAuthor) {
    this._bookAuthor = _bookAuthor;
}

public int get_bookISBN() {
    return _bookISBN;
}

public void set_bookISBN(int _bookISBN) {
    this._bookISBN = _bookISBN;
}


public Map<String, Object> toMap() {
    mp = new HashMap<>();
    mp.put("bookTitle", this._bookTitle);
    mp.put("bookAuthor", this._bookAuthor);
    mp.put("bookISBN", this._bookISBN);


    return mp;
} }

而这里就是我想从已经写入火力值阅读

public class ViewBook extends AppCompatActivity {


ArrayList<String> myArrayList = new ArrayList<>();

final FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference ref = database.getReference();
//  DatabaseReference mPostReference = ref.child("books");
DatabaseReference temp = ref.child("books");

ArrayAdapter<String> adapter;

@Override
public void onStart(){
    super.onStart();


    ValueEventListener postListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Book book = postSnapshot.getValue(Book.class);

                myArrayList.add(book.get_bookTitle() + "\n" + book.get_bookAuthor()  + "\n" + book.get_bookISBN());


            }

            ListView lv = (ListView)findViewById(R.id.list);
            adapter = new ArrayAdapter<String>(ViewBook.this, android.R.layout.simple_list_item_1, myArrayList);
            lv.setAdapter(adapter);
        }


        @Override
        public void onCancelled(DatabaseError databaseError) {
            //Getting Post failed, log a message
            Log.w("VIEWBOOKS", "loadPost:onCancelled", databaseError.toException());

        }
    };
    temp.addValueEventListener(postListener);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_book);
}}

我收到错误的是

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.taylorcampos.androidfirebasedatabase, PID: 25339
              com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.taylorcampos.androidfirebasedatabase.Book
                  at com.google.android.gms.internal.zzbtg.zze(Unknown Source)
                  at com.google.android.gms.internal.zzbtg.zzb(Unknown Source)
                  at com.google.android.gms.internal.zzbtg.zza(Unknown Source)
                  at com.google.firebase.database.DataSnapshot.getValue(Unknown Source)
                  at com.example.taylorcampos.androidfirebasedatabase.ViewBook$1.onDataChange(ViewBook.java:38)
                  at com.google.android.gms.internal.zzbpx.zza(Unknown Source)
                  at com.google.android.gms.internal.zzbqx.zzZT(Unknown Source)
                  at com.google.android.gms.internal.zzbra$1.run(Unknown Source)
                  at android.os.Handler.handleCallback(Handler.java:751)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6119)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

有人请解释为什么发生这种情况，我怎么能拿列表视图中列出这些书籍已投入火力点。

Answer 1

dataSnapshot.getChildren()是要回报你的属性列表一本书，而不是一个Book对象本身。其中之一是一个字符串，因此错误

你需要的数据的另一个层面的数据库，或者如果你要存储是一个书单，删除child("books")并使用产生的唯一密钥，而不是字符串"books"的

此外，火力地堡是要尝试设置使用camelCase方法和变量（如那些在数据库中）的领域，而不是强调像那些在你的Java类。换句话说，他们应该匹配

Answer 2

您的代码：

DatabaseReference ref = database.getReference();
DatabaseReference temp = ref.child("books");

更新代码修正来解决错误：

DatabaseReference ref = database.getReference();
DatabaseReference temp = ref.child("books").toString();