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>