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);
}
}
3、采用Handler与Timer、TimerTask结合的方法利用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);
}
}
三、Android单例模式应用及优点
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;
}
}