从一个活动向另一个活动发送和接收arraylist [英] Sending and receiving arraylist from one activity to another

问题描述

我正在尝试从NoteActivity发送一个ArrayList到MainActivity.

I am trying to send an ArrayList from NoteActivity to MainActivity.

ArrayList<String> data = new ArrayList<String>();

@Override
protected void onCreate(Bundle savedInstanceState) 
{
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_note_pad);

  final FloatingActionButton button = 
     (FloatingActionButton)findViewById(R.id.save_button);

  button.setOnClickListener(new View.OnClickListener() 
   {

     @Override
     public void onClick(View v) 
     {
       SaveData();
     }
   });
}

public void SaveData()
{
   FileOutputStream outputStream;

   try 
   {
     EditText editText = findViewById(R.id.textView3);

     final String str = editText.getText().toString();

     String content = findViewById(R.id.textView).toString();
     Log.d("Test","inside try block");

     outputStream = openFileOutput(str, Context.MODE_PRIVATE);
     outputStream.write(content.getBytes());

     Toast.makeText(getApplicationContext(),"Note 
     saved",Toast.LENGTH_SHORT).show();

     Log.d("Test","save done");

     final String str1 = editText.getText().toString();
     data.add(str1);

     SendData();
   }
   catch (FileNotFoundException e)
   {
       e.printStackTrace();
   }
   catch (IOException e)
   {
       e.printStackTrace();
   }
}

   public void SendData()
   {
      EditText editText = findViewById(R.id.textView3);

      Log.d("Check","str:"+data);
      Intent intent = new Intent(NotePad.this,MainActivity.class);
      intent.putExtra(EXTRA_MESSAGE, data);
      startActivity(intent);
   }
}

My Main Activity

public class MainActivity extends AppCompatActivity 
{
   private RecyclerView recyclerView;
   private RecyclerView.Adapter mAdapter;
   private RecyclerView.LayoutManager layoutManager;
   ArrayList<String> notes = new ArrayList<String>();




   @Override
   protected void onCreate(Bundle savedInstanceState)
   {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);


      Intent intent = getIntent();
      notes = intent.getStringArrayListExtra(NotePad.EXTRA_MESSAGE);

      Log.d("Check","title added: "+notes);


      final FloatingActionButton button = 
       (FloatingActionButton)findViewById(R.id.add_notebutton);


      button.setOnClickListener(new View.OnClickListener() 
      {

         @Override
         public void onClick(View v) 
         {
             Intent intent = new Intent(MainActivity.this,NotePad.class);
             startActivity(intent);
         }
      });

      recyclerView = (RecyclerView)findViewById(R.id.recycle_list);


      layoutManager = new LinearLayoutManager(this);
      recyclerView.setLayoutManager(layoutManager);
      mAdapter = new MyAdapter(notes);
      recyclerView.setAdapter(mAdapter);

   }

}

my Adapter code

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> 
{

  ArrayList<String> Notes_list = new ArrayList<String>();

   public MyAdapter(ArrayList<String> notes)
   {
      Notes_list = notes;
   }

   @NonNull
   @Override
     public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int 
     viewType) 
   {
     LayoutInflater inflater = LayoutInflater.from(parent.getContext());
      View view = inflater.inflate(R.layout.list_items,parent,false);
      return new MyViewHolder(view);
   }

   @Override
   public void onBindViewHolder(@NonNull MyViewHolder holder, int position) 
   {

       holder.mTextView.setText(Notes_list.get(position));
   }

   @Override
   public int getItemCount()  
   {
      return Notes_list.size();
   }

   public class MyViewHolder extends RecyclerView.ViewHolder 
   {
       TextView mTextView;
       public MyViewHolder(@NonNull View itemView)
       {
          super(itemView);
          mTextView = (TextView) itemView.findViewById(R.id.view_holder);
       }
   }
}

我在笔记(arrayList)中未收到任何数据.当我运行代码

I dont receive any data in my notes (arrayList). When I run the code

该应用程序立即崩溃，并出现空指针异常.

The app instantly crashes with a null pointer exception.

错误:java.lang.NullPointerException:尝试调用虚拟方法'int java.util.ArrayList.size()'上的空对象引用 com.example.quicknote.MyAdapter.getItemCount(MyAdapter.java:39)

Error: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference at com.example.quicknote.MyAdapter.getItemCount(MyAdapter.java:39)

推荐答案

您正尝试传递ArrayList，并使用getStringArrayListExtra方法从Intent中读取数据.您必须像这样放置数据:

You are trying to pass ArrayList and you read data from intent with getStringArrayListExtra method. You have to put data like this:

intent.putStringArrayListExtra(EXTRA_MESSAGE, data);

编辑:检查意图是否包含键:

Edit: Check if intent has key:

if (intent.hasExtra(NotePad.EXTRA_MESSAGE)) {
    notes = intent.getStringArrayListExtra(NotePad.EXTRA_MESSAGE);
}

注意:如果这只是一个游乐场，那就可以了.但是，如果您尝试 建立东西，那么你的架构是错误的.这不是一个好主意 在两个活动之间每次传递相同的数组.您可以使用 startActivityForResult方法并在onActivityResult方法上获得新注释 MainActivity.

Note: If this is just a playground it is OK. But if you trying to build something then your architecture is wrong. It is not a good idea to pass same array every time between two activity. You can use startActivityForResult method and get new note onActivityResult method of MainActivity.

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