迅雷android面试题

一、android几种定时器的机制及区别。

答：在android中，经常用到的定时器主要有以下几种实现：

1、采用handler与线程的sleep(long)方法

开启一个线程，,每隔1s向Handler发送消息

public class MainActivity extends Activity {

	private Handler myhandler;
	private static int timemessage=0x01;
	private int timer=0;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		myhandler = new Handler() {
			@Override
			public void handleMessage(Message msg) {
				// TODO Auto-generated method stub
				if(msg.what==timemessage){
					System.out.println("timer="+(++timer));
				}
			}

		};
		
		new Thread(new Runnable() {
			@Override
			public void run() {
				// TODO Auto-generated method stub
				while(true){
					try {
						System.out.println("执行了");
						Thread.sleep(2000);
						myhandler.sendEmptyMessage(timemessage);
					} catch (InterruptedException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
					
				}
			}
		}).start();
	}
}

2、采用Handler自带的postDelayed(Runnable，long)方法

利用延迟几秒发送Message，嵌套postDelayed（）实现定时器。

public class MainActivity extends Activity {

	private Handler myhandler;
	private static int timemessage = 0x01;
	private int timer = 0;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		myhandler = new Handler() {
			@Override
			public void handleMessage(Message msg) {
				// TODO Auto-generated method stub
				if (msg.what == timemessage) {
					System.out.println("timer=" + (++timer));
				}

			}

		};
		myhandler.postDelayed(new Runnable() {
			@Override
			public void run() {
				// TODO Auto-generated method stub
				myhandler.sendEmptyMessage(timemessage);
				myhandler.postDelayed(this, 1000);
				if (timer == 10) {
					myhandler.removeMessages(timemessage);
				}
			}
		}, 1000);
	}
}

利用Timer类的schedule（）方法，每隔一段时间做什么事情实现定时器。

public class MainActivity extends Activity {

	private Handler myhandler;
	private static int timemessage = 0x01;
	private int timer = 0;
	private Timer timerobject;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		myhandler = new Handler() {
			@Override
			public void handleMessage(Message msg) {
				// TODO Auto-generated method stub
				if (msg.what == timemessage) {
					System.out.println("timer=" + (++timer));
				}

			}
		};
		timerobject=new Timer();
		timerobject.schedule(new TimerTask() {
//1s后，在每隔1s做什么事情
			@Override
			public void run() {
				// TODO Auto-generated method stub
				myhandler.sendEmptyMessage(timemessage);
			}
		}, 1000,1000);
	}
}

二、利用Listview显示姓名和年龄，采用SimpleAdapter实现。

public class MainActivity extends Activity {
	private ListView mylistview;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		mylistview = (ListView) findViewById(R.id.listviewid);
		List<Map<String, Object>> listdata = new ArrayList<Map<String, Object>>();
		//每行显示是一个Map对象
		for (int i = 0; i < 10; i++) {
			Map mapdata = new HashMap<String, Object>();
			mapdata.put("name", "limi");
			mapdata.put("age", "30");
			listdata.add(mapdata);
		}
		//第二个参数是ListView行布局
		//第三个参数为存放Map元素的key值
		//第四个参数将key值对应的元素在布局哪里显示
		SimpleAdapter myadpter = new SimpleAdapter(getApplicationContext(),
				listdata, R.layout.listviewdatalayout, new String[] { "name",
						"age" }, new int[] { R.id.name, R.id.age });
		mylistview.setAdapter(myadpter);
	}
}

单例模式确保某一个类只有一个实例。通过使用private的构造函数确保在应用中只产生一个实例。

由于单例模式在内存中只有一个实例，减少了内存开销。

单例模式可以避免对资源的多重占用，例如一个写文件时，由于只有一个实例存在内存中，避免了对同一个资源的同时写操作。

使用单例模式，解决的就是多线程的同时创建的问题。

 public class Person{
	private Person(){};
	private static Person zongtong;
	public static Person getPersonStance(){
		if(zongtong==null){
			synchronized(Person.class){
				if(zongtong==null){
					zongtong=new Person();
				}
			}
		}
		return zongtong;
	}	
}

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