How to use Activity method in another class

Question

This constructor doesn't work for me, ie:when i create a MainActivity object in the other class via this constructor, its throwing a null pointer exception

private static MainActivity instance;
public  MainActivity getInstance()
    {
     return  instance;   
    }

Answer 1

Your instance is not initialized when you call getInstance so it's default value is null. To avoid NPE, you have to give a value to your instance, you can for example call the constructor of MainActivity
I think you are trying the Singleton Pattern, do you can check the code below

private static MainActivity instance = new MainActivity();
public  MainActivity getInstance(){

     return  instance;   
}

Answer 2

class MyActivity : ExternalListener {
   val externalClass = ExternalClass(this)
   override fun printResults() {
      println("printing details from Activity: ".plus(this.javaClass.name))
   }
}

interface ExternalListener {
   fun printResults()
}

class ExternalClass(val listener: ExternalListener) {
   fun printResults() {
      listener.printResults()
   }
}

So your MyActivity implements an interface ExternalListener which you pass as an argument to your ExternalClass, in which you use MyActivity. printDetails() method.