MainActivity.javaactivity_main.xml
package com.codinginflow.saveinstancestateexample;

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

public class MainActivity extends AppCompatActivity {
    private TextView mTextViewCount;
    private int mCount;

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

        mTextViewCount = findViewById(R.id.text_view_count);

        Button buttonDecrement = findViewById(R.id.button_decrement);
        Button buttonIncrement = findViewById(R.id.button_increment);

        buttonDecrement.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                decrement();
            }
        });

        buttonIncrement.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                increment();
            }
        });

        if (savedInstanceState != null) {
            mCount = savedInstanceState.getInt("count");
            mTextViewCount.setText(String.valueOf(mCount));
        }
    }

    private void decrement() {
        mCount--;
        mTextViewCount.setText(String.valueOf(mCount));
    }

    private void increment() {
        mCount++;
        mTextViewCount.setText(String.valueOf(mCount));
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        outState.putInt("count", mCount);
    }
}
<?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="horizontal"
    tools:context="com.codinginflow.saveinstancestateexample.MainActivity">

    <Button
        android:id="@+id/button_decrement"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="-" />

    <TextView
        android:id="@+id/text_view_count"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="16dp"
        android:text="0"
        android:textColor="@android:color/black"
        android:textSize="50sp" />

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

</LinearLayout>