android - MediaPlayer play music from SD card

activity_main.xml

<?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:id="@+id/root_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:orientation="vertical"
    android:background="#adeab8"
    >
    <Button
        android:id="@+id/btn_do_task"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Play Music From SD Card"
        />
</LinearLayout>

MainActivity.java

package com.cfsuman.me.androidcodesnippets;

import android.Manifest;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.pm.PackageManager;
import android.media.MediaPlayer;
import android.net.Uri;
import android.os.Build;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Toast;

import java.io.File;


public class MainActivity extends AppCompatActivity {
    private Context mContext;
    private Activity mActivity;

    private LinearLayout mRootLayout;
    private Button mBtnDoTask;

    private MediaPlayer mPlayer;

    private static final int MY_PERMISSION_REQUEST_CODE = 123;

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

        // Get the application context
        mContext = getApplicationContext();
        mActivity = MainActivity.this;

        // Get the widget reference from xml layout
        mRootLayout = findViewById(R.id.root_layout);
        mBtnDoTask = findViewById(R.id.btn_do_task);

        // Set a click listener for the button
        mBtnDoTask.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M){
                    checkPermission();
                }else {
                    playMusic();
                }
            }
        });
    }

    protected void playMusic(){
        File file = new File("/storage/0F14-0713/sd_card_song.mp3");

        if(file.exists()){
            Uri uri = Uri.fromFile(file);

            // Initialize the media player
            mPlayer = MediaPlayer.create(mContext,uri);

            // Start playing music
            mPlayer.start();

            Toast.makeText(mContext,"Music playing.",Toast.LENGTH_SHORT).show();
        }
    }

    protected void checkPermission(){
        if(Build.VERSION.SDK_INT>=Build.VERSION_CODES.M){
            if(checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED){
                if(shouldShowRequestPermissionRationale(Manifest.permission.READ_EXTERNAL_STORAGE)){
                    // show an alert dialog
                    AlertDialog.Builder builder = new AlertDialog.Builder(mActivity);
                    builder.setMessage("Need Read external storage permission.");
                    builder.setTitle("Please grant permission");
                    builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {
                            ActivityCompat.requestPermissions(
                                    mActivity,
                                    new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                                    MY_PERMISSION_REQUEST_CODE
                            );
                        }
                    });
                    builder.setNeutralButton("Cancel",null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                }else {
                    // Request permission
                    ActivityCompat.requestPermissions(
                            mActivity,
                            new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                            MY_PERMISSION_REQUEST_CODE
                    );
                }
            }else {
                // Permission already granted
                playMusic();
            }
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults){
        switch(requestCode){
            case MY_PERMISSION_REQUEST_CODE:{
                if(grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                    // Permission granted
                    playMusic();
                }else {
                    // Permission denied
                }
            }
        }
    }
}

AndroidManifest.xml [Permission]

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