package com.example.ui;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
  private TextView textView;
  private Handler handler = new Handler(){
    @Override
    public void handleMessage(Message msg) {
      super.handleMessage(msg);
    }
  };
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    textView = (TextView) findViewById(R.id.textView);
    new Thread(){
      @Override
      public void run() {
        super.run();
        try {
          Thread.sleep(2000);
          handler.post(new Runnable() {
            @Override
            public void run() {
             textView.setText("ok");
            }
          });
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }.start();
  }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  tools:context="com.example.ui.MainActivity">
  <TextView
    android:textSize="40sp"
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Hello World!" />
</RelativeLayout>