public class MainActivity extends Activity {

    // Use a unique request code for each use case 
    private static final int REQUEST_CODE_EXAMPLE = 0x9345; 

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

        // Create a new instance of Intent to start DetailActivity
        final Intent intent = new Intent(this, DetailActivity.class);

        // Start DetailActivity with the request code
        startActivityForResult(intent, REQUEST_CODE_EXAMPLE);
    }

    // onActivityResult only get called 
    // when the other Activity previously started using startActivityForResult
    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        // First we need to check if the requestCode matches the one we used.
        if(requestCode == REQUEST_CODE_EXAMPLE) {

            // The resultCode is set by the DetailActivity
            // By convention RESULT_OK means that whatever
            // DetailActivity did was executed successfully
            if(resultCode == Activity.RESULT_OK) {
                // Get the result from the returned Intent
                final String result = data.getStringExtra(DetailActivity.EXTRA_DATA);

// Use the data - in this case, display it in a Toast.
Toast.makeText(this, "Result: " + result, Toast.LENGTH_LONG).show();
} else {
// setResult wasn't successfully executed by DetailActivity 
// Due to some error or flow of control. No data to retrieve.
}
}
}
}

public class DetailActivity extends Activity {

    // Constant used to identify data sent between Activities.
    public static final String EXTRA_DATA = "EXTRA_DATA";

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

        final Button button = (Button) findViewById(R.id.button);
        // When this button is clicked we want to return a result
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // Create a new Intent object as container for the result
                final Intent data = new Intent();

                // Add the required data to be returned to the MainActivity
                data.putExtra(EXTRA_DATA, "Some interesting data!");
                
                // Set the resultCode as Activity.RESULT_OK to 
                // indicate a success and attach the Intent
                // which contains our result data
                setResult(Activity.RESULT_OK, data); 

                // With finish() we close the DetailActivity to 
                // return back to MainActivity
                finish();
            }
        });
    }

    @Override
    public void onBackPressed() {
        // When the user hits the back button set the resultCode 
        // as Activity.RESULT_CANCELED to indicate a failure
        setResult(Activity.RESULT_CANCELED);
        super.onBackPressed();
    }
}