val navTabA = findViewById<Button>(R.id.tab_A)
    navTabA.setBackgroundResource(R.drawable.nav_button_active)
    navTabA.setOnClickListener {
        val intent = Intent(applicationContext, A_Activity::class.java)
        startActivity(intent)
    }
... 5 more ...

package com.example.app

import android.content.Intent
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.view.View
import android.widget.Button

class NavHelper
    : AppCompatActivity(), View.OnClickListener {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val navTabA = findViewById<Button>(R.id.tab_A)
        navTabA.setOnClickListener(this)
    }

    override fun onClick(v: View) {
        when(v.id) {
            R.id.tab_A -> startActivity(Intent(applicationContext, A_Activity::class.java))
            }
        }

    }

package com.example.app

import android.content.Intent
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView

class A_Activity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_A)
        setTitle(R.string.A_Activity_label)

        NavHelper()
}