Mobile Fitness Tracker Java, Android

```java
// MainActivity.java (or your main activity file)
package com.example.mobilefitnesstracker;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements SensorEventListener {

    private TextView stepsTextView;
    private SensorManager sensorManager;
    private Sensor stepSensor;
    private boolean isStepSensorPresent = false;
    private int stepCount = 0;
    private static final int SENSOR_PERMISSION_REQUEST_CODE = 100;


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

        stepsTextView = findViewById(R.id.stepsTextView);

        sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);

        if (sensorManager.getDefaultSensor(Sensor.TYPE_STEP_COUNTER) != null) {
            stepSensor = sensorManager.getDefaultSensor(Sensor.TYPE_STEP_COUNTER);
            isStepSensorPresent = true;
        } else {
            stepsTextView.setText("Step counter sensor is not present.");
            isStepSensorPresent = false;
        }

        // Check and request permission
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACTIVITY_RECOGNITION) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACTIVITY_RECOGNITION}, SENSOR_PERMISSION_REQUEST_CODE);
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        if (isStepSensorPresent) {
             if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACTIVITY_RECOGNITION) == PackageManager.PERMISSION_GRANTED) {
                sensorManager.registerListener(this, stepSensor, SensorManager.SENSOR_DELAY_NORMAL);
             } else {
                // Permission not granted, handle accordingly (e.g., show a message to the user)
                Toast.makeText(this, "Activity recognition permission is required to use the step counter.", Toast.LENGTH_SHORT).show();
            }


        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (isStepSensorPresent) {
            sensorManager.unregisterListener(this, stepSensor);
        }
    }


    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {
        if (sensorEvent.sensor == stepSensor) {
            stepCount = (int) sensorEvent.values[0];
            stepsTextView.setText("Steps: " + stepCount);
        }
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {
        // Not needed for step counter.  Can leave empty.
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == SENSOR_PERMISSION_REQUEST_CODE) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                // Permission granted, register the sensor listener
                 if (isStepSensorPresent) {
                     sensorManager.registerListener(this, stepSensor, SensorManager.SENSOR_DELAY_NORMAL);
                 }


            } else {
                // Permission denied, handle accordingly
                Toast.makeText(this, "Activity recognition permission denied.", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

// activity_main.xml
// Make sure to add the permission to the manifest
//<uses-permission android:name="android.permission.ACTIVITY_RECOGNITION"/>

```

```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">

    <TextView
        android:id="@+id/stepsTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Steps: 0"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>
```

```xml
<!-- AndroidManifest.xml -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mobilefitnesstracker">

    <uses-permission android:name="android.permission.ACTIVITY_RECOGNITION"/>

    <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/Theme.MobileFitnessTracker">
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>
```