6. Activity life cycle

An activity can exist in essentially three states:

Resumed

The activity is in the foreground of the screen and has user focus. (This state is also sometimes referred to as "running".)

Paused

Another activity is in the foreground and has focus, but this one is still visible. That is, another activity is visible on top of this one and that activity is partially transparent or doesn't cover the entire screen. A paused activity is completely alive (the Activity object is retained in memory, it maintains all state and member information, and remains attached to the window manager), but can be killed by the system in extremely low memory situations.

Stopped

The activity is completely obscured by another activity (the activity is now in the "background"). A stopped activity is also still alive (the Activity object is retained in memory, it maintains all state and member information, but is not attached to the window manager). However, it is no longer visible to the user and it can be killed by the system when memory is needed elsewhere.

If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling itsfinish() method), or simply killing its process. When the activity is opened again (after being finished or killed), it must be created all over.

Splash.java作用相当于一个Welcome page, 当程序开始的时候显示，显示过后就不再需要切换个主界面，但是这个welcome界面状态是purse / stop. 但是还是alive, 用户可以用回退键访问这个welcome界面。所以可以使用finish()来kill这个activity, 通常可以在onPurse()中做。

package com.example.thenewboston;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;

public class Splash extends Activity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.splash);

        ScheduledExecutorService timer = Executors.newSingleThreadScheduledExecutor();
        timer.execute(new Runnable(){

            @Override
            public void run() {
                // TODO Auto-generated method stub
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }finally{
                    Intent openStartingPoint = new Intent("com.example.thenewboston.MAINACTIVITY");
                    startActivity(openStartingPoint);
                }
            }

        });
    }

    @Override
    protected void onPause() {
        // TODO Auto-generated method stub
        super.onPause();
        //Kill this acticity
        finish();
    }

}