Null-Zeiger-Ausnahme bei Text bearbeiten Android

Question

Ich schreibe ein Programm, um Benutzereingaben aus dem Bearbeitungstext zu speichern und den Text in eine Listview einzufügen. Ich bekomme diese Null-Ausnahme, obwohl ich den Text bereits bearbeitet habe.

public class AddEditAlbum extends AppCompatActivity { 

/** 
* These keys are to send back and forth information between the bundles and intents 
*/ 
public static final String ALBUM_INDEX = "albumIndex"; 
public static final String ALBUM_NAME = "albumName"; 
EditText input; 
Button save, cancel; 
int albumIndex; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    save = (Button) findViewById(R.id.save); 
    cancel = (Button) findViewById(R.id.cancel); 
    input = (EditText) findViewById(R.id.add); 

    // see if info was passed in to populate field 
    Bundle bundle = getIntent().getExtras(); 
    if (bundle != null) { 
     albumIndex = bundle.getInt(ALBUM_INDEX); 
     input.setText(bundle.getString(ALBUM_NAME)); 
    } 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.add_album); 
} 

public void Cancel(View view) { 
    setResult(RESULT_CANCELED); 
    finish(); //Returns to previous page on call stack 
} 

public void addAlbum(View view){ 

    String name = input.getText().toString(); //Fix this, goes to null pointer 

    //Checks to see if input is null and returns 
    if(name == null || name.length()==0){ 
     Toast.makeText(AddEditAlbum.this, "Enter valid album name", Toast.LENGTH_SHORT).show(); 
     Bundle bundle = new Bundle(); 
     bundle.putString(AlbumDialog.MESSAGE_KEY, "Album Name Required"); 
     DialogFragment newFragment = new AlbumDialog(); 
     newFragment.setArguments(bundle); 
     newFragment.show(getFragmentManager(), "badfields"); 
     return; 
    } 
    //Toast.makeText(AddEditAlbum.this, "Enter valid album name", Toast.LENGTH_SHORT).show(); 
    Bundle bundle = new Bundle(); 
    bundle.putInt(ALBUM_INDEX, albumIndex); 
    bundle.putString(ALBUM_NAME, name); 

    // send back to caller 
    Intent intent = new Intent(); 
    intent.putExtras(bundle); 
    setResult(RESULT_OK,intent); 
    finish(); 
} 

}

public class MainActivity extends AppCompatActivity { 

ListView listView; 
private ArrayList<Album> albums; 
public static final int EDIT_ALBUM_CODE = 1; 
public static final int ADD_ALBUM_CODE = 2; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    listView = (ListView) findViewById(R.id.album_list); 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
} 

public void Create(View view){ 
    Intent intent = new Intent(this, AddEditAlbum.class); 
    startActivityForResult(intent, ADD_ALBUM_CODE); 
} 

protected void onActivityResult(int requestCode, int resultCode, Intent intent) { 

    if (resultCode != RESULT_OK) { 
     return; 
    } 

    Bundle bundle = intent.getExtras(); 
    if (bundle == null) { 
     return; 
    } 

    // gather all info passed back by launched activity 
    String name = bundle.getString(AddEditAlbum.ALBUM_NAME); 
    int index = bundle.getInt(AddEditAlbum.ALBUM_INDEX); 

    if (requestCode == EDIT_ALBUM_CODE){ 
     Album album = albums.get(index); 
     album.albumName = name; 
    } 
    else if (requestCode == ADD_ALBUM_CODE){ 
     ArrayList<Photo> photos = new ArrayList<>(); 
     albums.add(new Album(name, photos)); 
    } 

    // redo Adapter since source content has changed 
    //listView.setAdapter(new ArrayAdapter<Album>(this, album, albums)); 

} 

Das ist also der volle Fehler Ich erhalte,

FATAL EXCEPTION: main 
                      Process: com.example.mustu.androidphotos31, PID: 9965 
                      java.lang.IllegalStateException: Could not execute method for android:onClick 
                       at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293) 
                       at android.view.View.performClick(View.java:5610) 
                       at android.view.View$PerformClick.run(View.java:22265) 
                       at android.os.Handler.handleCallback(Handler.java:751) 
                       at android.os.Handler.dispatchMessage(Handler.java:95) 
                       at android.os.Looper.loop(Looper.java:154) 
                       at android.app.ActivityThread.main(ActivityThread.java:6077) 
                       at java.lang.reflect.Method.invoke(Native Method) 
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
                       Caused by: java.lang.reflect.InvocationTargetException 
                       at java.lang.reflect.Method.invoke(Native Method) 
                       at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                       at android.view.View.performClick(View.java:5610)  
                       at android.view.View$PerformClick.run(View.java:22265)  
                       at android.os.Handler.handleCallback(Handler.java:751)  
                       at android.os.Handler.dispatchMessage(Handler.java:95)  
                       at android.os.Looper.loop(Looper.java:154)  
                       at android.app.ActivityThread.main(ActivityThread.java:6077)  
                       at java.lang.reflect.Method.invoke(Native Method)  
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  
                       Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference 
                       at com.example.mustu.androidphotos31.AddEditAlbum.addAlbum(AddEditAlbum.java:54) 
                       at java.lang.reflect.Method.invoke(Native Method)  
                       at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)  
                       at android.view.View.performClick(View.java:5610)  
                       at android.view.View$PerformClick.run(View.java:22265)  
                       at android.os.Handler.handleCallback(Handler.java:751)  
                       at android.os.Handler.dispatchMessage(Handler.java:95)  
                       at android.os.Looper.loop(Looper.java:154)  
                       at android.app.ActivityThread.main(ActivityThread.java:6077)  
                       at java.lang.reflect.Method.invoke(Native Method)  
                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  

Answer 1

super.onCreate(savedInstanceState); 
setContentView(R.layout.add_album); 

Diese erste ausgeführt werden muss, da sonst die content nicht gesetzt ist und findViewById findet nichts, was dazu führt, dass EditText null ist.

Answer 2

@Override 
protected void onCreate(Bundle savedInstanceState) { 
super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main); 
listView = (ListView) findViewById(R.id.album_list); 

} 

auch Sie Ihre Listenansicht vor onCreate Methode aufrufen Sie wodurch sich ein NPE

Answer 3

update your onCreate() like this. 
we should call setContentView(R.layout.your_layout) for passing the layout to the java class, then only it's views can be used.. failing to do so, will lead to NPE. 

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

    save = (Button) findViewById(R.id.save); 
    cancel = (Button) findViewById(R.id.cancel); 
    input = (EditText) findViewById(R.id.add); 

    // see if info was passed in to populate field 
    Bundle bundle = getIntent().getExtras(); 
    if (bundle != null) { 
     albumIndex = bundle.getInt(ALBUM_INDEX); 
     input.setText(bundle.getString(ALBUM_NAME)); 
    } 


}