Calling activity's methods from a separate class.

package me.johnweland.androidrtp;

import android.app.Activity;
import android.os.Build;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (Build.VERSION.SDK_INT >= 23) {
            // Marshmallow+
            PermissionChecker permissions = PermissionChecker.getInstance(this);
            permissions.permissionsCheck();
        } else {
            // Pre-Marshmallow
            demo();
        }

    }

    protected void demo() {
        Toast.makeText(this, "Demo toast", Toast.LENGTH_LONG).show();
    }
}

package me.johnweland.androidrtp;

import android.Manifest;
import android.annotation.TargetApi;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.pm.PackageManager;
import android.os.Build;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Created by jweland on 12/10/2015.
 */

public class PermissionChecker {
    private static final String TAG = PermissionChecker.class.getSimpleName();
    final private int REQUEST_CODE_ASK_MULTIPLE_PERMISSIONS = 0;

    private Activity mainActivity;
    private static PermissionChecker instance = null;
    private PermissionChecker(Activity activity) {
        this.mainActivity = activity;
    }

    static public PermissionChecker getInstance(Activity activity) {
        if (instance == null) {
            instance = new PermissionChecker(activity);
            return instance;
        } else {
            return instance;
        }
    }

    protected static boolean hasPermissions;

    @TargetApi(Build.VERSION_CODES.M)
    protected void permissionsCheck(){
        List<String> permissionsNeeded = new ArrayList<String>();

        final List<String> permissionsList = new ArrayList<String>();
        // Add permission check for any permission that is not NORMAL_PERMISSIONS
        if(!addPermission(permissionsList, Manifest.permission.RECORD_AUDIO))
            permissionsNeeded.add(mainActivity.getString(R.string.permission_microphone));
        if(!addPermission(permissionsList, Manifest.permission.CAMERA))
            permissionsNeeded.add(mainActivity.getString(R.string.permission_camera));

        if(permissionsList.size() > 0) {
            if(permissionsNeeded.size() > 0) {
                // Need Rationale
                String message = mainActivity.getString(R.string.permission_grant_message) + permissionsNeeded.get(0);
                for (int i = 1; i < permissionsNeeded.size(); i++)
                    message = message + "\n" +permissionsNeeded.get(i);
                showMessageOKCancel(message,
                        new DialogInterface.OnClickListener() {
                            @TargetApi(Build.VERSION_CODES.M)
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                mainActivity.requestPermissions(permissionsList.toArray(new String[permissionsList.size()]),
                                        REQUEST_CODE_ASK_MULTIPLE_PERMISSIONS);
                            }
                        });
                return;
            }
            mainActivity.requestPermissions(permissionsList.toArray(new String[permissionsList.size()]),
                    REQUEST_CODE_ASK_MULTIPLE_PERMISSIONS);
            return;
        }
        // launch method demo();
    }

    private void showMessageOKCancel(String message, DialogInterface.OnClickListener okListener) {
        new AlertDialog.Builder(mainActivity)
                .setMessage(message)
                .setPositiveButton(R.string.dialog_ok_button_text, okListener)
                .setNegativeButton(R.string.dialog_cancel_button_text, null)
                .create()
                .show();
    }

    @TargetApi(Build.VERSION_CODES.M)
    private boolean addPermission(List<String> permissionsList, String permission) {
        if (mainActivity.checkSelfPermission(permission) != PackageManager.PERMISSION_GRANTED) {
            permissionsList.add(permission);
            // Check for Rationale Option
            if (!mainActivity.shouldShowRequestPermissionRationale(permission))
                return false;
        }
        return true;
    }


    @TargetApi(Build.VERSION_CODES.M)
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        switch (requestCode) {
            case REQUEST_CODE_ASK_MULTIPLE_PERMISSIONS:
            {
                Map<String, Integer> perms = new HashMap<String, Integer>();
                // Initial
                perms.put(Manifest.permission.RECORD_AUDIO, PackageManager.PERMISSION_GRANTED);
                perms.put(Manifest.permission.CAMERA, PackageManager.PERMISSION_GRANTED);
                // Fill with results
                for (int i = 0; i < permissions.length; i++)
                    perms.put(permissions[i], grantResults[i]);
                // Check for RECORD_AUDIO
                if (perms.get(Manifest.permission.RECORD_AUDIO) == PackageManager.PERMISSION_GRANTED
                        && perms.get(Manifest.permission.CAMERA) == PackageManager.PERMISSION_GRANTED) {
                    // All Permissions Granted
                    // launch method demo();
                }
                else {
                    // Permission Denied
                    Toast.makeText(mainActivity, R.string.permission_denied_message, Toast.LENGTH_SHORT).show();
                }
            }
            break;
            default:
                mainActivity.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
}