Sending variable between activities

Question

This question is simple. There are for example 2 activities Main Activity and Main Activity 2. How can I send string(for example) from Main Activity to Main Activity 2. Let's say if Main Activity 2 gets string. It calls a function to change that string. And then how to send that changed string back to Main Activity?

Answer 1

The Standard Way:

If you want to send a primitive data from Activity1 to Activity2, you need to use Intent. For example if you just want to send a String:

public class Activity1 extends Activity {
   public static final String EXTRA_STRING = "extra_string";

   private void sendString(String s) {
        Intent intent = new Intent(this, Activity2.class);
        intent.putExtra(EXTRA_STRING, s);
        startActivity(intent);
   }
}

Suppose you change that string in Activity2 and also want the result back to Activity1, you need to use startActivityForResult() in Activity1 and setResult() in Activty2.

For example:

public class Activity1 extends Activity {
   public static final String EXTRA_STRING = "extra_string";
   private static final int REQUEST_CODE = 1;

   @Override
   protected void onActivityResult(int requestCode, int resultCode, Intent data) {    
       if (requestCode == REQUEST_CODE && resultCode == RESULT_OK) {
           // Get the string back
           String changedString = data.getStringExtra(Activity2.EXTRA_STRING);
       }
   }


   private void sendString(String s) {
        Intent intent = new Intent(this, Activity2.class);
        intent.putExtra(EXTRA_STRING, s);
        startActivityForResult(intent, REQUEST_CODE);
   }
}

In Activity2:

public class Activity2 extends Activity {
    public static final String EXTRA_STRING = "string_extra";
    private String myString;

    @Override
    public void onCreate(Bundle bundle) {
         // receive the string from activity1
         if (getIntent() != null) {
              myString = getIntent().getStringExtra(Activity1.EXTRA_STRING);
         }  
    }

    // Send the string back to activity1
    private void sendBack() {
        Intent data = new Intent();
        data.putExtra(EXTRA_STRING, myString);
        setResult(RESULT_OK, data);
        finish();
    }
}

The downside of the method is you can only send primitive variable. If you want to send an object, it must implements Parcelable or Serializable.

The Quick and Easy Way:

You can send variable of any kind, any complicated object between activity, service or whatever you want using EventBus. Please take a look at the Documentation to know how to use it. It's really quick and easy.

Answer 2

From Main activity 1 send the string:

                Intent intent = new Intent(this, MainActivity2);
                intent.putExtra("string", stringVal);
                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                startActivity(intent);

Then in Main Activity 2, receive the string:

String str = getIntent().getExtras().getString("string");

Then just change the value and send it back to Main Activity 1, you could get the intent within the onResume if you want, but check for nulls.