java - Android Firebase : Hashmap can't be cast to a given Object type

Question

我正在尝试查询 Firebase 数据库以检查用户是否存在，以便我可以登录他们。但是，一个奇怪的问题完全阻止了我:

查询代码

String email = emailEditText.getText().toString();
String password = emailEditText.getText().toString();

fbUsers.orderByChild("email").equalTo(email).limitToFirst(1).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        HashMap<String, User> usersHashMap = (HashMap<String, User>) dataSnapshot.getValue();

        Map.Entry<String, User> firstEntry = usersHashMap.entrySet().iterator().next();
        User foundUser = firstEntry.getValue();

        Log.d("Login: ", foundUser.getEmail());
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

用户类别

public class User {
    private String email;
    private String password;

    User() {}

    public User(String email, String password) {
        this.email = email;
        this.password = password;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

奇怪的问题是编译器说ALL OK，但是在运行时环境中，这个异常是在Log.d()处抛出的

java.lang.ClassCastException: java.util.HashMap cannot be cast to com.example.project.User

Answer 1

此代码使用 getChildren().iterator().next() 获取单个查询结果的快照。

fbUsers.orderByChild("email")
        .equalTo(email).limitToFirst(1)
        .addListenerForSingleValueEvent(new ValueEventListener() {

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        Iterator<DataSnapshot> iter = dataSnapshot.getChildren().iterator();

        if (iter.hasNext()) {
            User foundUser = iter.next().getValue(User.class);
            Log.d("Login: ", foundUser.getEmail());
        } else {
            Log.w("Login: ", "No match: " + email);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        if (databaseError != null) {
            Log.e("Login: ", "onCancelled: " + databaseError.getMessage());
        }
    }
});