new CountdownTimer(30000, 1000) {  
    public void onTick(long millisUntilFinished) {  
        mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);  
    }  
    public void onFinish() {  
        mTextField.setText("done!");  
    }  
 }.start();

package com.yydcdut.daojishi;

import android.os.Bundle;
import android.os.CountDownTimer;
import android.app.Activity;
import android.view.Menu;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity {
    private MyCount mc;  
    private TextView tv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);tv = (TextView)findViewById(R.id.show);  
        mc = new MyCount(30000, 1000);  
        mc.start();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
    
    /*定义一个倒计时的内部类*/  
    class MyCount extends CountDownTimer {     
        public MyCount(long millisInFuture, long countDownInterval) {     
            super(millisInFuture, countDownInterval);     
        }     
        @Override     
        public void onFinish() {     
            tv.setText("finish");        
        }     
        @Override     
        public void onTick(long millisUntilFinished) {     
            tv.setText("请等待30秒(" + millisUntilFinished / 1000 + ")...");     
            Toast.makeText(MainActivity.this, millisUntilFinished / 1000 + "", Toast.LENGTH_LONG).show();//toast有显示时间延迟       
        }    
    }     

}