Android中的四大组件：Activity、Service、BroadcastReceiver、Content Provider
Activity是一个应用程序组件，提供用户与程序交互的界面。
Activity运行机制

图片来源The Android Developer’s Guide
创建使用Activity

1. 继承Android的Activity类
2. 重写方法
3. 设置显示布局
4. 在AndroidManifest文件中，注册Activity

Activity的生命周期

1. onCreate();创建
2. onStart();运行
3. onResume();获取焦点
4. onPause();失去焦点
5. onStop();暂停
6. onDestroy();销毁

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.
    }
}

Activity的四种状态

1.活动状态（Active/Running ）Activity处于界面最顶端获取焦点
2.暂停状态（Paused）Activity失去焦点，但对用户可见
3.停止信息（Stopped）Activity被完全遮挡，但保留所有状态和成员信息
4.非活动状态（Killed）Activity被停止

附Android的四种数据存储方式
—SharedPreferences
—SQLite
—File
—Content Provider