StartActivityForResult

<?xml version="1.0" encoding="utf-8"?>
<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"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context="com.codinginflow.startactivityforresultexample.MainActivity">

    <EditText
        android:id="@+id/edit_text_number1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Number 1"
        android:inputType="number" />

    <EditText
        android:id="@+id/edit_text_number2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Number 2"
        android:inputType="number" />

    <TextView
        android:id="@+id/text_view_result"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Result"
        android:textSize="30sp" />

    <Button
        android:id="@+id/button_open_activity2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="open activity 2" />

</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<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"
    android:gravity="center"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context="com.codinginflow.startactivityforresultexample.Activity2">

    <TextView
        android:id="@+id/text_view_numbers"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Numbers: "
        android:textSize="30sp" />

    <Button
        android:id="@+id/button_add"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="add" />

    <Button
        android:id="@+id/button_subtract"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="subtract" />

</LinearLayout>

package com.codinginflow.startactivityforresultexample;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private TextView mTextViewResult;
    private EditText mEditTextNumber1;
    private EditText mEditTextNumber2;

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

        mTextViewResult = findViewById(R.id.text_view_result);
        mEditTextNumber1 = findViewById(R.id.edit_text_number1);
        mEditTextNumber2 = findViewById(R.id.edit_text_number2);

        Button buttonOpenActivity2 = findViewById(R.id.button_open_activity2);
        buttonOpenActivity2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (mEditTextNumber1.getText().toString().equals("")
                        || mEditTextNumber2.getText().toString().equals("")) {
                    Toast.makeText(MainActivity.this, "Please insert numbers", Toast.LENGTH_SHORT).show();
                } else {

                    int number1 = Integer.parseInt(mEditTextNumber1.getText().toString());
                    int number2 = Integer.parseInt(mEditTextNumber2.getText().toString());

                    Intent intent = new Intent(MainActivity.this, Activity2.class);
                    intent.putExtra("number1", number1);
                    intent.putExtra("number2", number2);
                    startActivityForResult(intent, 1);
                }
            }
        });
    }

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

        if (requestCode == 1) {
            if (resultCode == RESULT_OK) {
                int result = data.getIntExtra("result", 0);
                mTextViewResult.setText("" + result);
            }
            if (resultCode == RESULT_CANCELED) {
                mTextViewResult.setText("Nothing selected");
            }
        }
    }
}

package com.codinginflow.startactivityforresultexample;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class Activity2 extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_2);

        setTitle("Activity 2");

        Intent intent = getIntent();
        final int number1 = intent.getIntExtra("number1", 0);
        final int number2 = intent.getIntExtra("number2", 0);

        TextView textViewNumbers = findViewById(R.id.text_view_numbers);
        textViewNumbers.setText("Numbers: " + number1 + ", " + number2);

        Button buttonAdd = findViewById(R.id.button_add);
        Button buttonSubtract = findViewById(R.id.button_subtract);

        buttonAdd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int result = number1 + number2;

                Intent resultIntent = new Intent();
                resultIntent.putExtra("result", result);

                setResult(RESULT_OK, resultIntent);
                finish();
            }
        });

        buttonSubtract.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int result = number1 - number2;

                Intent resultIntent = new Intent();
                resultIntent.putExtra("result", result);

                setResult(RESULT_OK, resultIntent);
                finish();
            }
        });
    }
}