android实现倒计时

<?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">

    <TextView
        android:id="@+id/count_down"
        android:layout_width="60dp"
        android:layout_height="60dp"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:layout_marginRight="16dp"
        android:layout_marginTop="16dp"
        android:gravity="center"
        android:textSize="32sp"
        android:textColor="#50000000"
        android:background="@drawable/count_down_background"
        tools:text="2"
        />
</RelativeLayout>

package com.example.counttimer;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.view.Menu;
import android.view.MenuItem;
import android.view.Window;
import android.widget.TextView;

public class WelcomeActivity extends Activity {
    private final static int COUNT = 1;
    private TextView countDown;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.activity_welcome);
        initView();
    }
   //sehedule的第而个参数是第一次启动延时的时间，第三个是每隔多长时间执行一次。单位都是ms。
　　//因此这里是每一秒发送一次消息给handler更新UI。
　　//然后三秒后时间到了，在timer的第二个sehedule中进行跳转到另外一个界面
    private void initView() {
        countDown =  (TextView) findViewById(R.id.count_down);
        final Timer timer = new Timer();
        final long end = System.currentTimeMillis() + 1000*3;
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                handler.sendEmptyMessage(COUNT);
                
            }
        }, 0, 1000);
        //这里的schedule的第二个参数意义是到了这个时间尽快运行run里面的方法
        timer.schedule(new TimerTask() {
            
            @Override
            public void run() {
                 Intent i = new Intent(WelcomeActivity.this, SecondActivity.class);
                 i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                 startActivity(i);
                 finish();
                 timer.cancel();
            }
        }, new Date(end));
        
    }
    
    private Handler handler = new Handler(){
        int num = 2;
        public void handleMessage(android.os.Message msg) {
            
            switch (msg.what) {
            case COUNT:
                countDown.setText(String.valueOf(num));
                num--;
                break;

            default:
                break;
            }
        };
    };
        
}

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <corners
        android:radius="8dp"/>

    <solid
        android:color="#1e000000"/>
</shape>

package com.example.counttimer;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.CountDownTimer;
import android.view.Window;
import android.widget.TextView;

public class WelcomeActivity extends Activity {
    private final static int COUNT = 1;
    private TextView countDown;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.activity_welcome);
        initView();
    }

    private void initView() {
        countDown =  (TextView) findViewById(R.id.count_down);
　　　　//CountDownTimer构造器的两个参数分别是第一个参数表示总时间，第二个参数表示间隔时间。
　　　　//意思就是每隔xxx会回调一次方法onTick，然后xxx之后会回调onFinish方法。
        CountDownTimer timer = new CountDownTimer(3200,1000) {
            int num = 2;
            @Override
            public void onTick(long millisUntilFinished) {
                countDown.setText(String.valueOf(num));
                num--;
            }
            
            @Override
            public void onFinish() {
　　　　　　　　　　//计时完成调用
                 Intent i = new Intent(WelcomeActivity.this, SecondActivity.class);
                 i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                 startActivity(i);
                 finish();
            }
        };
        timer.start();
    }
        
}