Part of a not-so-short tutorial on inertial sensors sampling in Android

AccelerometerSamplesWriter.java

package com.s0lver.samplingtutorial.simpleaccelerometersampling;

import android.os.Environment;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

public class AccelerometerSamplesWriter {
    private final String fileName;

    public AccelerometerSamplesWriter(String fileName) {
        this.fileName = fileName;
    }

    public void writeSamples(List<SensorAccess.AccelerometerSample> samples) {
        String filepath = Environment.getExternalStorageDirectory().toString() + "/" + fileName;
        try {
            PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter(filepath, false)));
            pw.println("x,y,z,t");
            for (SensorAccess.AccelerometerSample sample : samples) {
                pw.println(sample.toString());
            }
            pw.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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">

    <Button
        android:id="@+id/btnStartSampling"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickStartSampling"
        android:text="Start sampling"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btnStopSampling"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onClickStopSampling"
        android:text="Stop sampling"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.s0lver.samplingtutorial.simpleaccelerometersampling">

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.s0lver.samplingtutorial.simpleaccelerometersampling;

import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    private SensorAccess sensorAccess;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        this.sensorAccess = new SensorAccess(this);
    }

    public void onClickStartSampling(View view) {
        if (!sensorAccess.startSampling()) {
            Toast.makeText(this, "Could not start sampling (I am already sampling!)", Toast.LENGTH_SHORT).show();
        }
    }

    public void onClickStopSampling(View view) {
        if (!sensorAccess.stopSampling()) {
            Toast.makeText(this, "Could not stop sampling (I am not sampling)", Toast.LENGTH_SHORT).show();
        }
    }
}

SensorAccess.java

package com.s0lver.samplingtutorial.simpleaccelerometersampling;

import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.util.Log;

import androidx.annotation.NonNull;

import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;

public class SensorAccess {
    private final Context context;
    private final SensorManager sensorManager;
    private final Sensor accelerometer;
    private final AccelerometerListener accelerometerListener;
    private List<AccelerometerSample> samples;
    private boolean isSampling;
    private Timer samplingTimer;

    public SensorAccess(Context context) {
        this.context = context;
        this.sensorManager = (SensorManager) context.getSystemService(Context.SENSOR_SERVICE);
        this.accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        this.accelerometerListener = new AccelerometerListener();
        this.isSampling = false;
    }

    public boolean startSampling() {
        if (isSampling) return false;

        samples = new ArrayList<>();
        long delayInMilliseconds = 5 * 1000;
        Log.i("SensorAccess", String.format("I will start reading in %s seconds", delayInMilliseconds / 1000));
        samplingTimer = new Timer();
        samplingTimer.schedule(new TimerTask() {
            @Override
            public void run() {
                Log.i("SensorAccess", "Sampling started");
                sensorManager.registerListener(
                        accelerometerListener,
                        accelerometer,
                        SensorManager.SENSOR_DELAY_FASTEST);
            }
        }, delayInMilliseconds);
        isSampling = true;
        return true;
    }

    public boolean stopSampling() {
        if (!isSampling) return false;

        samplingTimer.cancel();
        Log.i("SensorAccess", "Sampling stopped");
        this.sensorManager.unregisterListener(this.accelerometerListener);
        AccelerometerSamplesWriter writer = new AccelerometerSamplesWriter("samples.csv");
        writer.writeSamples(samples);
        isSampling = false;
        return true;
    }

    class AccelerometerListener implements SensorEventListener {
        @Override
        public void onSensorChanged(SensorEvent sensorEvent) {
            float[] acc = sensorEvent.values;
            long timestampMilliseconds = sensorEvent.timestamp / 1000;
            Log.i("SensorAccess", String.format("ACC %s,%s,%s @%s", acc[0], acc[1], acc[2], timestampMilliseconds));
            samples.add(new AccelerometerSample(acc[0], acc[1], acc[2], timestampMilliseconds));
        }

        @Override
        public void onAccuracyChanged(Sensor sensor, int i) {

        }
    }

    class AccelerometerSample {
        float x, y, z;
        long timestampMilliseconds;

        AccelerometerSample(float x, float y, float z, long timestampMilliseconds) {
            this.x = x;
            this.y = y;
            this.z = z;
            this.timestampMilliseconds = timestampMilliseconds;
        }

        @NonNull
        @Override
        public String toString() {
            return String.format("%s,%s,%s,%s", x, y, z, timestampMilliseconds);
        }
    }
}