Ich bekomme eine NullPointerException, wenn ich auf eine Schaltfläche klicke, aber nicht weiß, wo ich es beheben kann

Question

Ich bekomme eine Nullzeiger-Ausnahme, wenn ich auf die Schaltfläche 'create' auf meiner Schnittstelle klicke, die auf eine andere Aktivität verweist, und dies stürzt meine ab app sobald ich klicke. Ich weiß nur nicht, wo das Problem liegt. Ich habe nicht viel Erfahrung in der Java-Programmierung, also könnte das ein bisschen dumm aussehen, aber könnte jemand darauf hinweisen, wo das Problem ist und wie man es beheben kann?

Vielen Dank im Voraus.

Hier ist mein Code:

MainActivity.java:

package iftikhar.danayal.uninotes; 

import android.app.ListActivity; 
import android.content.Intent; 
import android.os.Bundle; 
import android.provider.ContactsContract; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.view.View; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.widget.ArrayAdapter; 
import android.widget.Button; 
import android.widget.ListView; 

import java.util.List; 

import iftikhar.danayal.uninotes.data.NoteItem; 
import iftikhar.danayal.uninotes.data.NotesData; 

public class MainActivity extends ListActivity { 

    public static final int EDITOR_ACTIVITY_REQUEST = 1001; 
    private NotesData datasource; 
    List<NoteItem> notesList; 
    Button action_create; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     //setSupportActionBar(toolbar); 

     action_create = (Button)findViewById(R.id.action_create); 

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

//  FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
//  fab.setOnClickListener(new View.OnClickListener() { 
//   @Override 
//   public void onClick(View view) { 
//    Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
//      .setAction("Action", null).show(); 
//   } 
//  }); 

     datasource = new NotesData(this); 

     refreshDisplay(); 




    } 



    private void refreshDisplay() { 
     notesList = datasource.findAll(); 
     ArrayAdapter<NoteItem> adapt = new ArrayAdapter<NoteItem>(this, R.layout.list_item_layout, notesList); 
     setListAdapter(adapt); 
    } 

    private void setListAdapter(ArrayAdapter<NoteItem> adapt) { 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     createNote(); 



     return super.onOptionsItemSelected(item); 
    } 


    private void createNote() { 
     NoteItem note = NoteItem.getNew(); 
     Intent intent = new Intent(this, NoteEditorActivity.class); 
     intent.putExtra("key", note.getKey()); 
     intent.putExtra("text", note.getText()); 
     startActivityForResult(intent, EDITOR_ACTIVITY_REQUEST); 
    } 



    protected void onListItemClick(ListView l, View v, int position, long id) { 
     NoteItem note = notesList.get(position); 
     Intent intent = new Intent(this, NoteEditorActivity.class); 
     intent.putExtra("key", note.getKey()); 
     intent.putExtra("text", note.getText()); 
     startActivityForResult(intent, EDITOR_ACTIVITY_REQUEST); 
    } 

    @Override 
    protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
     if (requestCode == EDITOR_ACTIVITY_REQUEST && resultCode == RESULT_OK) { 
      NoteItem note = new NoteItem(); 
      note.setKey(data.getStringExtra("key")); 
      note.setText(data.getStringExtra("text")); 
      datasource.update(note); 
      refreshDisplay(); 
     } 
    } 
} 

NoteEditorActivity.java:

package iftikhar.danayal.uninotes; 

/** 
* Created by danayal_94 on 01/04/16. 
*/ 

import android.app.Activity; 
import android.content.Intent; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.MenuItem; 
import android.widget.EditText; 

import iftikhar.danayal.uninotes.data.NoteItem; 


public class NoteEditorActivity extends Activity{ 

    private NoteItem note; 

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

     Intent intent = this.getIntent(); 
     note = new NoteItem(); 
     note.setKey(intent.getStringExtra("key")); 
     note.setText(intent.getStringExtra("text")); 

     EditText edit = (EditText) findViewById(R.id.noteText); 
     edit.setText(note.getText()); 
     edit.setSelection(note.getText().length()); 
    } 

    private void save() { 
     EditText edit = (EditText) findViewById(R.id.noteText); 
     String noteText = edit.getText().toString(); 
     Intent intent = new Intent(); 
     intent.putExtra("key", note.getKey()); 
     intent.putExtra("text", noteText); 
     setResult(RESULT_OK, intent); 
     finish(); 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     if (item.getItemId() == android.R.id.home) { 
      save(); 
     } 
     return false; 
    } 

    @Override 
    public void onBackPressed() { 
     this.save(); 
    } 
} 

NoteItem.java:

package iftikhar.danayal.uninotes.data; 

import android.widget.ListView; 

import java.text.SimpleDateFormat; 
import java.util.Date; 
import java.util.Locale; 

/** 
* Created by danayal_94 on 24/03/16. 
*/ 
public class NoteItem { 
    private String text; 
    private String key; 


    public String getKey() { 
     return key; 
    } 

    public void setKey(String key) { 
     this.key = key; 
    } 

    public String getText() { 
     return text; 
    } 

    public void setText(String text) { 
     this.text = text; 
    } 


    public static NoteItem getNew() { 
     Locale locale = new Locale("en_UK"); 
     Locale.setDefault(locale); 
     String format = "dd-MM-yyyy HH:mm:ss Z"; 
     SimpleDateFormat formatter = new SimpleDateFormat(format); 
     String key = formatter.format(new Date()); 

     NoteItem note = new NoteItem(); 
     note.setKey(key); 
     note.setText(""); 
     return note; 
    } 

    @Override 
    public String toString() { 
     return this.getText(); 
    } 
} 

activity_main.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" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:fitsSystemWindows="true" 
    tools:context="iftikhar.danayal.uninotes.MainActivity"> 

    <android.support.design.widget.AppBarLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:theme="@style/AppTheme.AppBarOverlay" 
     android:id="@+id/listView"> 


     <android.support.v7.widget.Toolbar 
      android:id="@+id/toolbar" 
      android:layout_width="match_parent" 
      android:layout_height="?attr/actionBarSize" 
      android:background="?attr/colorPrimary" 
      app:popupTheme="@style/AppTheme.PopupOverlay"> 

     </android.support.v7.widget.Toolbar> 





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

    <ListView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:id="@android:id/list" /> 

    <Button 
     style="?android:attr/buttonStyleSmall" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="@string/action_create" 
     android:id="@+id/action_create" 
     android:layout_below="@+id/listView" 
     android:layout_alignParentLeft="true" 
     android:layout_alignParentStart="true" 
     android:layout_marginLeft="44dp" 
     android:layout_marginStart="44dp" 
     android:layout_marginTop="50dp" /> 






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

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="iftikhar.danayal.uninotes"> 

    <application 
     android:allowBackup="true" 
     android:icon="@mipmap/ic_launcher" 
     android:label="@string/app_name" 
     android:supportsRtl="true" 
     android:theme="@style/AppTheme"> 
     <activity 
      android:name=".MainActivity" 
      android:label="@string/app_name" 
      android:theme="@style/AppTheme.NoActionBar"> 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 
     <activity android:name=".NoteEditorActivity"/> 
    </application> 

</manifest> 

Answer 1

In NoteEditor.onCreate() Sie haben dies:

note.setKey(intent.getStringExtra("key")); 
note.setText(intent.getStringExtra("text")); 

EditText edit = (EditText) findViewById(R.id.noteText); 
edit.setText(note.getText()); 
edit.setSelection(note.getText().length()); 

Da Sie jedoch beim Erstellen der Intent die "zusätzlichen" Informationen nicht angeben, gibt getStringExtra("text") den Wert null zurück. Wenn Sie also note.getText().length() tun, werden Sie einen NullPointerException auslösen, weil note.getText()null sein wird.

Sie müssen diese „extra“ Strings zur Verfügung zu stellen, wenn Sie Ihre Absicht erstellen, wie folgt aus:

public void onClick(View v) { 
    Intent intent = new Intent(MainActivity.this, NoteEditorActivity.class); 
    intent.putExtra("key", "my key"); 
    intent.putExtra("text", "my text"); 
    startActivity(intent); 
}