Control the back "button" in android

Question

Hi, I want to get the Text data value from Sub-Activity back. And everything is ok. But when the Sub-activity was opened, then I just click back "button" on the phone, it throw an exception error. I found on the android NotePad life-cycle control toturial but It's hard to understand. Can someone help me? This is my code:

public class SBooks extends ListActivity {
private String title_raw;
private SBooksDbAdapter mDbHelper; 
private static final int ACTIVITY_SEARCH = 0; 

private static final int SEARCH_ID = Menu.FIRST;  

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.sbooks_list);       
    mDbHelper = new SBooksDbAdapter(this);
    mDbHelper.open();                    

}

// Create Menu Option
@Override
public boolean onCreateOptionsMenu(Menu menu) {        
    boolean result = super.onCreateOptionsMenu(menu);
    menu.add(0, SEARCH_ID, 0, R.string.menu_search);         
    return result;      
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch(item.getItemId()){
    case SEARCH_ID:
        Intent intent = new Intent(this, SBooksSearch.class);
        startActivityForResult(intent, ACTIVITY_SEARCH);         

    return super.onOptionsItemSelected(item);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent intent){
 super.onActivityResult(requestCode, resultCode, intent);
  switch(requestCode){
  case ACTIVITY_SEARCH:
   Bundle bundle = intent.getExtras();
   title_raw = bundle.getString(SBooksDbAdapter.KEY_TITLE_RAW);
   if(title_raw!=null){
    Cursor cursor = mDbHelper.searchData(title_raw);
    String[] from = new String[]{ SBooksDbAdapter.KEY_ROWID,
      SBooksDbAdapter.KEY_TITLE, SBooksDbAdapter.KEY_LYRICS };
    int[] to = new int[]{ R.id.id, R.id.title, R.id.lyrics };
    SimpleCursorAdapter adapter = 
     new SimpleCursorAdapter(this, R.layout.sbooks_row, cursor, from, to );
    setListAdapter(adapter);

  }

 } 
}

}

This is my Sub-Activity:

public class SBooksSearch extends Activity {
private EditText mTextSearch;
private Button searchButton;
private SBooksDbAdapter mDbHelper;

@Override
protected void onCreate(Bundle savedInstanceState) {
 // TODO Auto-generated method stub
 super.onCreate(savedInstanceState);  
 setContentView(R.layout.sbooks_search); 

 mTextSearch = (EditText)findViewById(R.id.text_search);  
 searchButton = (Button)findViewById(R.id.btn_search);  

 searchButton.setOnClickListener(new View.OnClickListener(){
  public void onClick(View v){    
   Intent intent = new Intent();
   intent.putExtra(SBooksDbAdapter.KEY_TITLE_RAW, mTextSearch.getText().toString());    
   setResult(RESULT_OK, intent);
   finish();

  }
 });
} 

@Override
protected void onSaveInstanceState(Bundle outState){
 super.onSaveInstanceState(outState);

}
@Override
protected void onPause(){
 super.onPause();
 checkState();

}
@Override
protected void onResume(){
 super.onResume();  
}


private void checkState() {  

} }

This is Logcat:

08-12 18:44:39.211: WARN/InputManagerService(581): Starting input on non-focused client com.android.internal.view.IInputMethodClient$Stub$Proxy@43719140 (uid=10004 pid=623)
08-12 18:44:39.510: INFO/ActivityManager(581): Displayed activity com.original.sbooks/.SBooks: 4934 ms
08-12 18:44:40.821: WARN/KeyCharacterMap(730): No keyboard for id 0
08-12 18:44:40.821: WARN/KeyCharacterMap(730): Using default keymap: /system/usr/keychars/qwerty.kcm.bin
08-12 18:44:41.412: INFO/ARMAssembler(581): generated scanline__00000177:03515104_00001A01_00000000 [ 64 ipp] (89 ins) at [0x297608:0x29776c] in 952924 ns
08-12 18:44:41.520: INFO/ARMAssembler(581): generated scanline__00000177:03515104_00000A01_00000000 [ 46 ipp] (70 ins) at [0x297798:0x2978b0] in 517061 ns
08-12 18:44:42.171: INFO/ActivityManager(581): Starting activity: Intent { comp={com.original.sbooks/com.original.sbooks.SBooksSearch} }
08-12 18:44:52.196: WARN/ActivityManager(581): Launch timeout has expired, giving up wake lock!
08-12 18:44:52.279: WARN/ActivityManager(581): Activity idle timeout for HistoryRecord{436bfc80 {com.original.sbooks/com.original.sbooks.SBooksSearch}}
08-12 18:44:57.350: DEBUG/dalvikvm(620): GC freed 4103 objects / 231808 bytes in 75ms
08-12 18:45:05.130: DEBUG/dalvikvm(649): GC freed 2604 objects / 150112 bytes in 88ms
08-12 18:45:10.120: DEBUG/dalvikvm(623): GC freed 2750 objects / 149592 bytes in 71ms
08-12 18:45:11.920: INFO/ActivityManager(581): Displayed activity com.original.sbooks/.SBooksSearch: 29748 ms
08-12 18:45:17.263: WARN/ActivityManager(581): Activity pause timeout for HistoryRecord{436bfc80 {com.original.sbooks/com.original.sbooks.SBooksSearch}}
08-12 18:45:26.762: WARN/ActivityManager(581): Launch timeout has expired, giving up wake lock!
08-12 18:45:27.272: WARN/ActivityManager(581): Activity idle timeout for HistoryRecord{435a7760 {com.original.sbooks/com.original.sbooks.SBooks}}
08-12 18:46:04.905: DEBUG/AndroidRuntime(730): Shutting down VM
08-12 18:46:04.905: WARN/dalvikvm(730): threadid=3: thread exiting with uncaught exception (group=0x4000fe70)
08-12 18:46:04.905: ERROR/AndroidRuntime(730): Uncaught handler: thread main exiting due to uncaught exception
08-12 18:46:04.991: ERROR/AndroidRuntime(730): java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=0, result=0, data=null} to activity {com.original.sbooks/com.original.sbooks.SBooks}: java.lang.NullPointerException
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.deliverResults(ActivityThread.java:3005)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.handleSendResult(ActivityThread.java:3047)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.access$2300(ActivityThread.java:112)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1721)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.os.Looper.loop(Looper.java:123)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.main(ActivityThread.java:3948)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at java.lang.reflect.Method.invoke(Method.java:521)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:782)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:540)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at dalvik.system.NativeStart.main(Native Method)
08-12 18:46:04.991: ERROR/AndroidRuntime(730): Caused by: java.lang.NullPointerException
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at com.original.sbooks.SBooks.onActivityResult(SBooks.java:111)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.Activity.dispatchActivityResult(Activity.java:3595)
08-12 18:46:04.991: ERROR/AndroidRuntime(730):     at android.app.ActivityThread.deliverResults(ActivityThread.java:3001)

Answer 1

Try putting in some logging into the onActivityResult(...) method of your main activity. The "back" button from the subactivity may be giving you a result code of RESULT_CANCELED, and the intent can be null in that case. So you'll get an NPE at

Bundle bundle = intent.getExtras();

Answer 2

I think Nate is correct about the NPE at intent.getExtras().

You should be checking the value of your result code before you do anything. You also might want to check to make sure intent.hasExtra(SBooksDbAdapter.KEY_TITLE_RAW) before you get that value. So, something like this:

switch(requestCode){
case ACTIVITY_SEARCH:
    if (resultCode == RESULT_OK && intent.hasExtra(SBooksDbAdapter.KEY_TITLE_RAW)) {
        title_raw = intent.getStringExtra(SBooksDbAdapter.KEY_TITLE_RAW);
        ... do all that other stuff ...
    }
}