Activity

       Activity可以理解为应用程序里可以与用户交互的组件；系统给每个Activity一个单独的窗口用于绘制它的UI，窗口通常是填满屏幕，但有时候某些窗口会比屏幕小并能漂浮在其他窗口上。应用程序是由多个Activity组成的，但应用程序有一个特殊的“main”Activity，该Activity是用户第一次启动应用程序时最先见到的Activity。

     创建一个Activity，你需要创建一个Activity的子类（或从Activity的某个子类继承），并实现与Activity生命周期相关的一些回调方法。

public class ExampleActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // The activity is being created.
    }
    @Override
    protected void onStart() {
        super.onStart();
        // The activity is about to become visible.
    }
    @Override
    protected void onResume() {
        super.onResume();
        // The activity has become visible (it is now "resumed").
    }
    @Override
    protected void onPause() {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
    }
    @Override
    protected void onStop() {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        // The activity is about to be destroyed.
    }
}