StartActivityForResult

SubActivity.java

public class SubActivity extends AppCompatActivity {

    private EditText et_comeback;
    private Button btn_close;

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

        et_comeback = findViewById(R.id.et_comeback);
        btn_close = findViewById(R.id.btn_close);

        btn_close.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent();
                intent.putExtra("comeback", et_comeback.getText().toString()); // 입력폼에 적은 값 담아주기
                setResult(RESULT_OK, intent); // 결과 값 설정
                finish(); // 현재 액티비티 종료
            }
        });
    }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SubActivity">

    <EditText
        android:id="@+id/et_comeback"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Main으로 보낼 값 입력해주세요."/>

    <Button
        android:id="@+id/btn_close"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="종료"/>

</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private TextView tv_comeback;
    private Button btn_go;

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

        tv_comeback = findViewById(R.id.tv_comeback);
        btn_go = findViewById(R.id.btn_go);

        btn_go.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(getApplicationContext(), SubActivity.class);
                startActivityForResult(intent,REQUEST_CODE);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(resultCode == RESULT_OK) {
            Toast.makeText(getApplicationContext(), "수신 성공",Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(getApplicationContext(), "수신 실패", Toast.LENGTH_SHORT).show();
        }


        if(requestCode == REQUEST_CODE) {
            String resultTxt = data.getStringExtra("comeback");
            tv_comeback.setText(resultTxt);
        }

    }

}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_comeback"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="30sp"
        android:text="값을 가져와주세요!"/>

    <Button
        android:id="@+id/btn_go"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="GO"/>

</LinearLayout>

안드로이드 앱 만들기 #26 StartActivityForResult
https://www.youtube.com/watch?v=YxwYmTyh7gg