java - 致命异常尝试对空对象引用调用虚拟方法 'java.lang.String java.lang.Object.toString()'

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

使用 Firebase 数据库在 RecyclerView 中显示数据。当他们运行这行代码时应用程序崩溃

itemGroup.setHeaderTitle(groupSnapShot.child("headerTitle").getValue(true).toString());

我收到此错误:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference

private void getFirebaseData(){
    dialog.show();
    myData.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            List<ItemGroup> itemGroups = new ArrayList<>();
            for(DataSnapshot groupSnapShot:dataSnapshot.getChildren())
            {
                ItemGroup itemGroup = new ItemGroup();


   itemGroup.setHeaderTitle(groupSnapShot.child("headerTitle").getValue(true).String());
   GenericTypeIndicator<ArrayList<ItemData>> genericTypeIndicator = new GenericTypeIndicator<ArrayList<ItemData>>(){};

   itemGroup.setListItem(groupSnapShot.child("listItem").getValue (genericTypeIndicator));
   itemGroups.add(itemGroup);

            }
            iFirebaseLoadListener.onFirebaseLoadSuccess(itemGroups);
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

 iFirebaseLoadListener.onFirebaseLoadFailed(databaseError.getMessage());

        }
    });

}

当我注释掉该行代码时，我得到这个 E/RecyclerView: No adaptor Attached;从我的 logcat 中跳过布局...我不确定这是否会影响 RecyclerView 适配器...

Answer 1

尝试这样做

private void getFirebaseData(){
        dialog.show();
        myData.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                List<ItemGroup> itemGroups = new ArrayList<>();
                for(DataSnapshot groupSnapShot:dataSnapshot.getChildren())
                {
                    ItemGroup itemGroup = groupSnapShot.getValue(ItemGroup.class);


       itemGroup.setHeaderTitle(itemGroup.getHeaderTitle());
       GenericTypeIndicator<ArrayList<ItemData>> genericTypeIndicator = new GenericTypeIndicator<ArrayList<ItemData>>(){};

       itemGroup.setListItem(groupSnapShot.child("listItem").getValue (genericTypeIndicator));
       itemGroups.add(itemGroup);

                }
                iFirebaseLoadListener.onFirebaseLoadSuccess(itemGroups);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

     iFirebaseLoadListener.onFirebaseLoadFailed(databaseError.getMessage());

            }
        });

    }

如果您需要更多帮助，请发布您的数据库结构和 ItemGroup.class