Question

这是我的firebase结构。

我想基于子类别节点获取内容节点。意味着所有带有有趣子类别的内容都应显示在列表视图中，并且为此我要进行过滤，但是当我获取内容节点时，我会得到NullPointerException。

这是我的代码：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_content);
    imageView = findViewById(R.id.backbtncntent);
    sharebtn = findViewById(R.id.sharebtn);
    textView = findViewById(R.id.contenttext);
    listView = findViewById(R.id.listview);
    final ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(ContentActivity.this, android.R.layout.simple_list_item_1,contentname);
    listView.setAdapter(arrayAdapter);
    String classname = getIntent().getStringExtra("classname");
    if(classname.equals("A")){
       content = getIntent().getStringExtra("jokecontent");
       funnyJokesQuery = FirebaseDatabase.getInstance().getReference("ContentJokes").orderByChild("subcategory")
               .equalTo(content);
       funnyJokesQuery.addChildEventListener(new ChildEventListener() {
           @Override
           public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
               for(DataSnapshot item : dataSnapshot.getChildren()) {
                   String contentdisplay = item.child("content").getValue().toString();   //in this line i am getting error
                   contentname.add(contentdisplay);
                   arrayAdapter.notifyDataSetChanged();

               }
           }

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


           }

           @Override
           public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

           }

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

           }

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

           }
       });

    }else if(classname.equals("B")){
        content = getIntent().getStringExtra("shayaricontent");
        textView.setText(content);
    }
    imageView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(ContentActivity.this, HomeActivity.class);
            startActivity(i);
        }
    });
    sharebtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(android.content.Intent.ACTION_SEND);
            intent.setType("text/plain");
            intent.putExtra(android.content.Intent.EXTRA_SUBJECT, "Hey Read Some Interesting Content");
            intent.putExtra(android.content.Intent.EXTRA_TEXT, content);
            startActivity(Intent.createChooser(intent, "Share Via"));
        }
    });
}

}

我收到此错误：

2020-06-24 11:05:59.155 27216-27216/example.example1.shayariapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: example.example1.shayariapp, PID: 27216
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
    at example.example1.shayariapp.Acitivties.ContentActivity$1.onChildAdded(ContentActivity.java:52)
    at com.google.firebase.database.core.ChildEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.4:79)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.4:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.4:55)
    at android.os.Handler.handleCallback(Handler.java:790)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:202)
    at android.app.ActivityThread.main(ActivityThread.java:6962)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:528)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:888)

请告诉我如何解决此错误。这将是很大的帮助。

Answer 1

由于您正在侦听子节点，因此您的account name balance clasif_1 clasif_2 clasif_3 clasif_4 level last_level 0 1.00.00.00 Total Assets 1000 Total Assets 1 False 1 1.01.00.00 Cash 200 Total Assets Cash 2 True 2 1.02.00.00 Inventory 800 Total Assets Inventory 2 False 3 1.02.01.00 Goods in process 300 Total Assets Inventory Goods in process 3 True 4 1.02.02.00 Goods for sale 500 Total Assets Inventory Goods for sale 3 True 5 2.00.00.00 Liabilities 750 Liabilities 1 False 6 2.01.00.00 Commercial Liabilities 700 Liabilities Commercial Liabilities 2 True 7 2.02.00.00 Other liabilities 50 Liabilities Other liabilities 2 True 8 3.00.00.00 Net equity 250 Net equity 1 True方法将为每个子节点调用onChild...。因此，无需遍历其中的DataSnapshot。

snapshot.getChildren()

如果您将funnyJokesQuery = FirebaseDatabase.getInstance().getReference("ContentJokes") .orderByChild("subcategory") .equalTo(content); funnyJokesQuery.addChildEventListener(new ChildEventListener() { @Override public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) { String contentdisplay = item.child("content").getValue(String.class); contentname.add(contentdisplay); arrayAdapter.notifyDataSetChanged(); }附加到相同的查询/引用，则需要循环，因为在这种情况下，ValueEventListener会被所有孩子使用一个onDataChange来调用。 >

进行过滤后无法从Firebase获取数据

1 个答案: