Android - 在特定时间后更改背景颜色

Question

我在更改背景颜色时遇到问题。我想在活动开始后三秒钟更改它。我创建了一个计时器和一个处理程序+ runnable.It在三秒钟后想要更改特殊的EditText时正常工作。但是当我添加行layout_interact.setBackgroundColor(Color.GREEN);并在虚拟设备中启动应用程序时，它会在启动包含计时器和处理程序的活动时崩溃。 我也尝试过三秒钟后开始一项新活动，但最终发生同样的错误。 这里是我的代码：

public class ActivityInteractProcess extends Activity{ 
final Handler handler_interact=new Handler();//not defined as final variable. may cause problem 
View layout_interact=(View) findViewById(R.id.layoutintprocess); 

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


    //creating timer 
    Timer timer_interact=new Timer(); 
    timer_interact.schedule(new TimerTask() { 
      @Override 
     public void run() {UpdateGUI();} 
     }, 3000); 
    } 
    private void UpdateGUI() { 
    handler_interact.post(runnable_interact);   
    } 
    //creating runnable 
    final Runnable runnable_interact = new Runnable() { 
     public void run() { 
     layout_interact.setBackgroundColor(Color.GREEN); //this is the line which makes the app Force close. 
     } 
    }; 
} 

我还没忘了添加下面一行到活动中的XML布局：

android:id="@+id/layoutintprocess" 

下面是我在logcat中得到了错误：

AndroidRuntime(571): FATAL EXCEPTION: main 
AndroidRuntime(571): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{aiLab.tests.autism/aiLab.tests.autism.ActivityInteractProcess}: java.lang.NullPointerException 
AndroidRuntime(571): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739) 
AndroidRuntime(571): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831) 
AndroidRuntime(571): at android.app.ActivityThread.access$500(ActivityThread.java:122) 
AndroidRuntime(571): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024) 
AndroidRuntime(571): at android.os.Handler.dispatchMessage(Handler.java:99) 
... 

Answer 1

将代码更改为：

public class ActivityInteractProcess extends Activity{ 
final Handler handler_interact=new Handler();//not defined as final variable. may cause  problem 
View layout_interact; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
// TODO Auto-generated method stub 
super.onCreate(savedInstanceState); 
setContentView(R.layout.page_interact_process); 
layout_interact =(View) findViewById(R.id.layoutintprocess); 


} 


@Override 
public void onWindowFocusChanged(boolean hasFocus) { 
    super.onWindowFocusChanged(hasFocus); 

    //creating timer 
    Timer timer_interact=new Timer(); 
    timer_interact.schedule(new TimerTask() { 
     @Override 
     public void run() {UpdateGUI();} 
    }, 3000); 
} 
private void UpdateGUI() { 
    handler_interact.post(runnable_interact); 
} 
//creating runnable 
final Runnable runnable_interact = new Runnable() { 
    public void run() { 
     layout_interact.setBackgroundColor(Color.GREEN); 
    } 
}; 
}