从第二活动的第一个发送数据不起作用的Android

Question

我目前工作的一个应用程序，允许你做的笔记列表：

• 名单中的第一个活动
可见
• 新的笔记按钮导致与编辑文本和几个文本格式按钮（粗体，斜体，更大的文字，更小）的第二个活动
• 该应用程序允许您长时间点击编辑或删除列表中的笔记。

问题是，当我从一个活动的数据发送到另一个我得到这个错误的日志：

显示java.lang.NullPointerException：试图调用虚拟方法 “java.lang.String中java.lang.Object.toString（）”上一个空对象 参考

，当我尝试将数据发送回第一个活动只会发生。当我尝试从第二个活动中的按钮内部保存时，采用相同的方法：

getText().toString() 

工作得很好。以下是在活动和保存方法之间发送/接收数据的代码。

public void sendMessage(View view) 
{ 
    editText = (EditText) findViewById(R.id.add_text); 
    String message = editText.getText().toString(); 
    Intent intent = new Intent(this, MainActivity.class); 
    intent.putExtra(key_Message, message); 
    startActivity(intent); 
} 


@Override 
protected void onResume() { 
    super.onResume(); 
    Intent intent = getIntent(); 
    String message = intent.getStringExtra(key_Message); 
    list.add(message); 
    adapter.notifyDataSetChanged(); 
} 

这里是保存文本内部（工作正常）的方法。请注意，我在的WriteMessage添加SendMessage函数（但评论它）

public void writeMessage(View view) 
{ 
    String Message = editText.getText().toString(); 
    String file_name = "Notes_file"; 
    try { 
     FileOutputStream fileOutputStream = openFileOutput(file_name, MODE_PRIVATE); 
     fileOutputStream.write(Message.getBytes()); 
     fileOutputStream.close(); 
     Toast.makeText(getApplicationContext(),"Message has been saved", Toast.LENGTH_SHORT).show(); 
     //String message = editText.getText().toString(); 
     //Intent intent = new Intent(this, MainActivity.class); 
     //intent.putExtra(key_Message, message); 
     //startActivity(intent); 

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

} 

也是另一个小问题我不能换我的头周围之一：

public void shrinkText(View view) 
{ float modifySize = editText.getTextSize(); 
    modifySize = modifySize - 3; 
    editText.setTextSize(modifySize); 
} 

是认为小法在用户单击按钮时从编辑文本中缩小文本。它总是增加文字。一些帮助将不胜感激。

public void writeMessage(View view) 
{ 
    String Message = editText.getText().toString(); 
    String file_name = "Notes_file"; 
    try { 
     FileOutputStream fileOutputStream = openFileOutput(file_name, MODE_PRIVATE); 
     fileOutputStream.write(Message.getBytes()); 
     fileOutputStream.close(); 
     Toast.makeText(getApplicationContext(), "Message has been saved", Toast.LENGTH_SHORT).show(); 
     String message = editText.getText().toString(); 
     Intent intent = new Intent(); 
     intent.putExtra(key_edit, message); 
     setResult(RESULT_OK, intent); 
     finish(); 


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

} 

@Override 
protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
    super.onActivityResult(requestCode, resultCode, data); 
    if(requestCode == 1){ 
     if(resultCode == RESULT_OK){ 
      String item_tobeadded1; 
      item_tobeadded1 = data.getStringExtra(key_edit); 
      list.add(item_tobeadded1); 
      //adapter.notifyDataSetChanged(); 
      adapter = new ArrayAdapter<String>(this,android.R.layout.simple_expandable_list_item_1, list); 
      list_view.setAdapter(adapter); 
     } 
    } 
} 

Answer 1

正如已经建议你可以做的是开始你的第二个活动与您，当您编辑只是点击listview项目，或当你箱子一个新的没有通过任何数据。

通过这种方式，您可以编辑以及创建新的Note，而无需对代码进行太多更改。

int RESPONSE_CODE = 123; // from this code will will differentiate if 
         // the activity which we started actually did the task or not 

    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
     @Override 
     public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 

      Intent intent = new Intent(getApplicationContext(), NoteEditActivity.class); 
      intent.putString("para1","arg1"); 
      intent.putString("para2","arg2"); 
      intent.putString("para3","arg3"); 
      intent.putString("para4","arg4"); 
      startActivityForResult(intent, RESPONSE_CODE); 
     } 
    }); 

in NoteEditActivity.class如果有东西，您可以收到您的参数。同时创建新的Note Note，para2，para3 & para4将为空。但是，如果注意已经存在，你将不得不编辑

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Intent myIntent = getIntent(); 
    String arg1 = myIntent.getStringExtra("para1"); 
    String arg2 = myIntent.getStringExtra("para2"); 
    String arg3 = myIntent.getStringExtra("para3"); 
    String arg4 = myIntent.getStringExtra("para4"); 
    // here you have to check that they or not null to differentiate 
    // if it is a new Note to be created OR you have to edit. 
    // you should probably use some another way to figure this out, i am writing this because i would not be using it :) 

} 

一旦您完成创建或编辑你的注意，相关数据附加到的意图和完成活动这样

Intent intent = new Intent(); 
intent.putExtra("para1","edited_data_1"); 
intent.putExtra("para2","edited_data_2"); 
intent.putExtra("para3","edited_data_3"); 
intent.putExtra("para4","edited_data_4"); 
setResult(RESULT_OK, intent);   
finish(); 

如果任务成功，您将设置结果RESULT_OK连接的意图和你在以前做过的事情

你必须覆盖一个名为onActivityResult()这种方法从活动哟接收数据的方法ü已经开始做一些任务，

public void onActivityResult(int requestCode, int resultCode, Intent data) { 
    super.onActivityResult(requestCode, resultCode, data); 
    if (requestCode == 1) { 
     if(resultCode == RESULT_OK){ 

      String arg1 = data.getStringExtra("para1"); 
      String arg2 = data.getStringExtra("para2"); 
      String arg3 = data.getStringExtra("para3"); 
      String arg4 = data.getStringExtra("para4"); 
      // if you receive the data here, 
      // you can add another element to List from which the - 
      //`ListView` is populated and call `notifyDataSetChanged()` on the adapter. 
      // like this `adapter.notifyDataSetChanged();` it will update the `listview` - 
      //with new data that you received from the `NoteEditActivity.class` 

      adapter.notifyDataSetChanged(); // I am assuming you would add the changed item to the List 
     }  
    } 
} 

更新，我创建了一个虚拟的代码为你和它的作品，我已经测试过它，我还上传了它在Github上，你可以检查它有

NoteListActivity.java

public class NoteListActivity extends Activity { 

    private ListView listView; 
    private ArrayList<String> strings; 
    public static final String NEW_ELEMENT = "NEW_ELEMENT"; 
    public static final String ELEMENT_VALUE = "ELEMENT_VALUE"; 
    public static final String ELEMENT_POSITION = "ELEMENT_POSITION"; 
    private ArrayAdapter<String> adapter; 

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

     listView = (ListView) findViewById(R.id.NoteListActivity_listView); 
     listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
      @Override 
      public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
       String text = (String) listView.getItemAtPosition(position); 
       Intent intent = new Intent(getApplicationContext(), NoteEditActivity.class); 
       intent.putExtra(NEW_ELEMENT, false); 
       intent.putExtra(ELEMENT_POSITION, position); 
       intent.putExtra(ELEMENT_VALUE, text); 
       startActivityForResult(intent, 123); 
      } 
     }); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.NoteListActivity_fab); 
     fab.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       Intent intent = new Intent(getApplicationContext(), NoteEditActivity.class); 
       intent.putExtra(NEW_ELEMENT, true); 
       startActivityForResult(intent, 123); 
      } 
     }); 

     setUpList(); 

     adapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.simple_list_item_1, strings); 
     listView.setAdapter(adapter); 
    } 

    @Override 
    protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
     super.onActivityResult(requestCode, resultCode, data); 
     Log.d("NoteListActivity1",""+data.getStringExtra(NoteListActivity.ELEMENT_VALUE)); 

     if (requestCode == 123) { 
      if (resultCode == RESULT_OK) { 
       Log.d("NoteListActivity2",""+data.getStringExtra(NoteListActivity.ELEMENT_VALUE)); 
       boolean isNewElement = data.getBooleanExtra(NoteListActivity.NEW_ELEMENT, false); 
       if (isNewElement) { 
        strings.add("" + data.getStringExtra(NoteListActivity.ELEMENT_VALUE)); 
        adapter.notifyDataSetChanged(); 
       } else { 
        strings.set(data.getIntExtra(NoteListActivity.ELEMENT_POSITION, 0), "" + data.getStringExtra(NoteListActivity.ELEMENT_VALUE)); 
        adapter.notifyDataSetChanged(); 
       } 
      } 
     } 
    } 

    private void setUpList() { 
     strings = new ArrayList<>(); 
     for (int i = 0; i < 4; i++) { 
      strings.add("Element_1_" + i); 
     } 
    } 
} 

activity_note_list.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:fitsSystemWindows="true" 
    tools:context="com.nimgade.pk.mytutorialapplication.MainActivity"> 

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
     xmlns:tools="http://schemas.android.com/tools" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent" 
     android:orientation="vertical" 
     tools:context="activities.list.first.NoteListActivity"> 


     <ListView 
      android:id="@+id/NoteListActivity_listView" 
      android:layout_width="match_parent" 
      android:layout_height="wrap_content" /> 
    </LinearLayout> 

    <android.support.design.widget.FloatingActionButton 
     android:id="@+id/NoteListActivity_fab" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="bottom|end" 
     android:layout_margin="@dimen/fab_margin" 
     android:src="@android:drawable/ic_menu_add" /> 

</android.support.design.widget.CoordinatorLayout> 

NoteEditActivity.java

public class NoteEditActivity extends Activity { 

    private EditText editText; 
    private Button button; 
    private boolean isNewElement; 
    private int position; 

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

     editText = (EditText) findViewById(R.id.NoteEditActivity_editText); 
     button = (Button) findViewById(R.id.NoteEditActivity_button); 

     Intent myIntent = getIntent(); 
     isNewElement = myIntent.getBooleanExtra(NoteListActivity.NEW_ELEMENT, false); 

     if (!isNewElement) { 
      editText.setText("" + myIntent.getStringExtra(NoteListActivity.ELEMENT_VALUE)); 
      position = myIntent.getIntExtra(NoteListActivity.ELEMENT_POSITION,0); 
     } 

     button.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       String text = editText.getText().toString(); 
       if (text != null && text.trim().length() > 0) { 
        Intent intent = new Intent(); 
        Log.d("NoteEditActivity",""+text); 
        intent.putExtra(NoteListActivity.ELEMENT_VALUE, text); 
        intent.putExtra(NoteListActivity.NEW_ELEMENT, isNewElement); 
        if (!isNewElement) { 
         intent.putExtra(NoteListActivity.ELEMENT_POSITION, position); 
        } 
        setResult(RESULT_OK, intent); 
        finish(); 
       } else { 
        Toast.makeText(getApplicationContext(), "Kindly write a valid element name", Toast.LENGTH_SHORT).show(); 
       } 
      } 
     }); 
    } 
} 

activity_note_edit.xml

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:gravity="center" 
    android:orientation="vertical" 
    tools:context="activities.list.first.NoteEditActivity"> 


    <EditText 
     android:id="@+id/NoteEditActivity_editText" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" /> 

    <Button 
     android:id="@+id/NoteEditActivity_button" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="Save" 
     android:textAllCaps="false" /> 
</LinearLayout>