java.lang.String不能轉換爲java.util.HashMap錯誤

Question

我試圖檢索Firebase值的代碼如下。我的數據庫結構如下：

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) { 

       } 
      }); 
      } 

我已經嘗試了不同的方法來做到這一點，但我不只是得到任何運氣here..All我得到的是這種錯誤java.lang.ClassCastException：JAVA。 lang.String無法轉換爲java.util.HashMap at com.example.moses.farm.Mystatement $ 1.onDataChange（Mystatement.java:39）指向此HashMap值=（HashMap）child.getValue（） ;一行代碼。我一直在這裏堅持了近一個星期，任何人如何得到一個想法，這將真正幫助我

Answer 1

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 2

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) { 

      } 
     }); 
     } 

使用此代碼

Answer 3

做是先創建正確的方法數據模型：

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; 
    } 
} 

然後加入，而不是ChildEventListener：

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 4

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

嘗試這一個

Answer 5

改變這一行value.get（ 「量」）的toString（）;

TO

將String.valueOf（value.get（ 「量」））; 它解決了我的問題。