java.lang.String 无法转换为 java.util.HashMap 错误

Question

我有以下代码试图检索 firebase 值。我的数据库结构如下：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_mystatement);
    listview = (ListView) findViewById(R.id.listview);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, list);
    listview.setAdapter(adapter);
    dref = FirebaseDatabase.getInstance().getReference();
    dref = dref.child("Dog Expenditure");

    dref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot child : dataSnapshot.getChildren()) {
                HashMap<String, Object> value = (HashMap<String, Object>) child.getValue();
                String amount = value.get("amount").toString();
                String item = value.get("item").toString();
                // This will print out amount and item
                System.out.println(amount + item);
            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

我尝试了不同的方法来做到这一点，但我在这里没有运气。我得到的只是这个错误 java.lang.ClassCastException：java.lang.String 不能转换为 java.util。哈希映射 在 com.example.moses.farm.Mystatement$1.onDataChange(Mystatement.java:39) 指向这 HashMap value = (HashMap) child.getValue(); 行代码.我在这里被困了将近一个星期，任何知道如何获得它的人都会真正帮助我

Answer 1

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_mystatement);
    listview = (ListView) findViewById(R.id.listview);
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, list);
    listview.setAdapter(adapter);
    dref = FirebaseDatabase.getInstance().getReference();
    dref = dref.child("Dog Expenditure");

    dref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot child : dataSnapshot.getChildren()) {
                String amount = child.get("amount").toString();
                String item = value.get("item").toString();
                // This will print out amount and item
                System.out.println(amount + item);
            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });
}

使用此代码

Answer 2

HashMap<String, Object> value = child.getValue();

试试这个

Answer 3

DatabaseReference dref = FirebaseDatabase.getInstance().getReference();
dref = dref.child("Dog Expenditure");

dref.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        HashMap<String, String> value = (HashMap<String, String>) dataSnapshot.getValue();
        String amount = value.get("amount");
        String item = value.get("item");
    }
    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
    }
    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
    }
    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
    }
    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
});

Answer 4

正确的做法是先创建一个数据模型：

public class DogExpenditure {
    private String amount;
    private String item;

    public String getAmount() {
        return amount;
    }

    public void setAmount(String amount) {
        this.amount = amount;
    }

    public String getItem() {
        return item;
    }

    public void setItem(String item) {
        this.item = item;
    }
}

然后添加一个 ChildEventListener 代替：

dref.addChildEventListener(new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        DogExpenditure dogExpenditure = dataSnapshot.getValue(DogExpenditure.class);
        System.out.println(dogExpenditure.getAmount()+dogExpenditure.getItem());
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {

    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {

    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
});

Answer 5

改变这一行 value.get("amount").toString();

到

String.valueOf(value.get("amount")); 它解决了我的问题。