如何从另一个活动返回到先前的活动

Question

A --> B --> C --> back to B

Where A must send an extra to B and B has a button (back button) to C. When pressing the back button from C, it must go back to B, that still has the extra from A. Basically, B and C go back and forth, where both stay the same when left, without data being destroyed.

However, in my case, it goes back to A and if I use finish() or startActivity() the app crashes, saying it's NullPointerException. I figured this is because B hasn't received any extra from A, because it was C that was calling B.

Should I pass the A's extra from B to C and back to B again? What should one do?

A

String indicate = textView.getText().toString();
Intent t = new Intent(getApplicationContext(), Details.t.putExtra("indicate", indicate);
startActivity(t);
finish();

B (onCreate())

Intent g = getIntent();
String indica = g.getExtras().getString("indicate");
TextView diseaseLabel = (TextView) findViewById(R.id.statusTxt);
diseaseLabel.setText(indica);

C

public boolean onOptionsItemSelected(MenuItem item) {
    //finish();
    //Intent i = new Intent(this.context, B.class);
    //startActivity(i);
    return super.onOptionsItemSelected(item);

}

Answer 1

One approach you can do is save your indicate to SharedPreferences thru PreferenceManager.getDefaultSharedPreferences(Context context) so you don't keep on passing things around.

However as the name suggests, this is usually used for Preferences or Settings. But you get the idea, right?

Answer 2

You need to use startActivityForResult in order to return to B activity without data being destroyed.

For example when you are on B activity start C activity with the following code

Intent intent = new Intent(B.this, C.class);
startActivityForResult(intent, REQUEST_RESULT_CODE); //suppose resultCode == 2;

and when you need to go back to B activity use the following code

public boolean onOptionsItemSelected(MenuItem item) 
{
   Intent intent = new Intent();
   setResult(RESULT_OK, intent);
   finish();
   return super.onOptionsItemSelected(item); 
}