1.Intent intent = new Intent(this, MainMenu.class); 
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP); 
startActivity(intent); 

上述情况是指 如果你有多个activivty，如果俺back建要一丛丛返回，上面的实现了只返回到主界面

2.

private static final int MEDIA_IMAGE_REQUEST_CODE = 203948; // This can be any unique number you like 
 
Intent getImageFromGalleryIntent =  
  new Intent(Intent.ACTION_PICK, MediaStore.Images.Media.INTERNAL_CONTENT_URI); 
startActivityForResult(getImageFromGalleryIntent, MEDIA_IMAGE_REQUEST_CODE); 

protected final void onActivityResult(final int requestCode, final int resultCode, final Intent i) { 
  super.onActivityResult(requestCode, resultCode, i); 
  if(resultCode == RESULT_OK) { 
    switch(requestCode) { 
      case MEDIA_IMAGE_REQUEST_CODE: 
 
        // Get the chosen images Uri 
        Uri imageUri = i.getData(); 
 
        // Load the bitmap data from the Uri  
        // It is probably best to do this in an AsyncTask to avoid clogging up the Main Thread 
      break; 
    } 
  } 

public class ListViewDemoActivity extends Activity {
 private String[] Countries;
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Countries = getResources().getStringArray(R.array.countries);
        Arrays.sort(Countries);
       
  ListView list = (ListView)findViewById(R.id.list);
  ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.listitem, Countries);
  list.setAdapter(adapter);
  registerForContextMenu(list);
    }
   
    @Override
    public void onCreateContextMenu(ContextMenu menu, View v,
      ContextMenuInfo menuInfo) {
     if (v.getId()==R.id.list) {
         AdapterView.AdapterContextMenuInfo info = (AdapterView.AdapterContextMenuInfo)menuInfo;
      menu.setHeaderTitle(Countries[info.position]);
      String[] menuItems = getResources().getStringArray(R.array.menu);
      for (int i = 0; i<menuItems.length; i++) {
       menu.add(Menu.NONE, i, i, menuItems[i]);
   }
     }
    }
   
    @Override
    public boolean onContextItemSelected(MenuItem item) {
     AdapterView.AdapterContextMenuInfo info = (AdapterView.AdapterContextMenuInfo)item.getMenuInfo();
     int menuItemIndex = item.getItemId();
  String[] menuItems = getResources().getStringArray(R.array.menu);
  String menuItemName = menuItems[menuItemIndex];
     String listItemName = Countries[info.position];
    
     TextView text = (TextView)findViewById(R.id.footer);
     text.setText(String.format("Selected %s for item %s", menuItemName, listItemName));
     return true;
    }
}