buildscript {
  ext.kotlin_version = '1.1.2-4'
  repositories {
    jcenter()
  }
  dependencies {
    classpath 'com.android.tools.build:gradle:2.2.0'
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
  }
}

allprojects {
  repositories {
    jcenter()
  }
}

task clean(type: Delete) {
  delete rootProject.buildDir
}

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
 compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"

repositories {
  mavenCentral()
}

<?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:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  tools:context="com.yoyoyt.kotlindemo.ThreeActivity">
  <TextView
    android:id="@+id/text"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="aaaa"/>
</LinearLayout>

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.widget.TextView
import kotlinx.android.synthetic.main.activity_three.*

class ThreeActivity : AppCompatActivity() {

  private var b : TextView ?= null
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_three)

    text.text="aaa"
  }
}

import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.widget.TextView
import android.widget.Toast

class ThreeActivity : AppCompatActivity() {

  private var b : TextView ?= null
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_three)
    b = findViewById(R.id.text) as TextView
    b!!.text="shds"
    Toast.makeText(this,b!!.text.toString(),Toast.LENGTH_SHORT).show()
  }
}