java - Android how to call MainActivity method from Non Activity class -

i troubing on runtime permission method in non activity class. want call public mainactivity method helper class.

mainactivity.java method

public void a(){   //some method } 

now helper class:

class b{   private context mcontext;   public b(context context){     this.mcontext = context;     //now calling mainactivity method     mainactivity mainactivity = new mainactivity();     mainactivity.a();    } } 

calling b class cactivity

oncreate(){   b b = new b(this); } 

but gives me runtime error

update:

my method has runtime permission method isn't static . ex- requestpermissions(), onrequestpermissionsresult()

make method static can call without creating class object

eg

public static void testing() {} 

now calling

mainactivity.testing();