MessageBox错误

Question

嗨，我只想问一问，为什么我不能启动我的程序--当我用MessageBox或AlertBox启动时总是会出现错误。我想要做的是，如果玩家在初学者阶段没有回答超过15个问题，他就不能在学习阶段继续，当他点击学习者时，它会显示一个消息框，他需要回答10+答案才能进入下一个圆环--这是我的代码

public class Menu extends Activity 
{

private Integer beginnerAns = 0;


Button beginner, learner;

@Override
protected void onCreate(Bundle MenuButtons) 
{
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    super.onCreate(MenuButtons);
    setContentView(R.layout.menu);

    beginner = (Button) findViewById(R.id.btnBeginner);
    learner = (Button) findViewById(R.id.btnLearner);

    beginner.setOnClickListener(myOnlyhandler);
    learner.setOnClickListener(myOnlyhandler);

}

View.OnClickListener myOnlyhandler = new View.OnClickListener(){
public AlertDialog.Builder dlgAlert  = new AlertDialog.Builder(Menu.this);

@Override
public void onClick(View v) 
{
    switch(v.getId()) {
    case R.id.btnBeginner:
        Intent openBeginner = new Intent("com.thesis.logipic.BEGINNER");
        startActivity(openBeginner);
    break;
    case R.id.btnLearner:
        if (beginnerAns < 15)
        {
            dlgAlert.setMessage("wrong password or username");
            dlgAlert.setTitle("Error Message...");
            dlgAlert.setPositiveButton("OK", null);
            dlgAlert.setCancelable(true);
            dlgAlert.create().show();
        }
        else
        {
        Intent openLearner = new Intent("com.thesis.logipic.GAMEPLAY");
        startActivity(openLearner);
        }
    break;
    }
}
};

}

和logcat

10-06 21:00:28.673: D/AndroidRuntime(561): Shutting down VM
10-06 21:00:28.692: W/dalvikvm(561): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
10-06 21:00:28.712: E/AndroidRuntime(561): FATAL EXCEPTION: main
10-06 21:00:28.712: E/AndroidRuntime(561): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.thesis.logipic/com.thesis.logipic.Menu}: java.lang.IllegalStateException: System services not available to Activities before onCreate()
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.os.Looper.loop(Looper.java:123)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread.main(ActivityThread.java:4627)
10-06 21:00:28.712: E/AndroidRuntime(561):  at java.lang.reflect.Method.invokeNative(Native Method)
10-06 21:00:28.712: E/AndroidRuntime(561):  at java.lang.reflect.Method.invoke(Method.java:521)
10-06 21:00:28.712: E/AndroidRuntime(561):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
10-06 21:00:28.712: E/AndroidRuntime(561):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
10-06 21:00:28.712: E/AndroidRuntime(561):  at dalvik.system.NativeStart.main(Native Method)
10-06 21:00:28.712: E/AndroidRuntime(561): Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate()
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.Activity.getSystemService(Activity.java:3526)
10-06 21:00:28.712: E/AndroidRuntime(561):  at com.android.internal.app.AlertController$AlertParams.<init>(AlertController.java:743)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.AlertDialog$Builder.<init>(AlertDialog.java:273)
10-06 21:00:28.712: E/AndroidRuntime(561):  at com.thesis.logipic.Menu$1.<init>(Menu.java:38)
10-06 21:00:28.712: E/AndroidRuntime(561):  at com.thesis.logipic.Menu.<init>(Menu.java:37)
10-06 21:00:28.712: E/AndroidRuntime(561):  at java.lang.Class.newInstanceImpl(Native Method)
10-06 21:00:28.712: E/AndroidRuntime(561):  at java.lang.Class.newInstance(Class.java:1429)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
10-06 21:00:28.712: E/AndroidRuntime(561):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
10-06 21:00:28.712: E/AndroidRuntime(561):  ... 11 more

Answer 1

你打电话给super必须是你做的第一件事。

@Override
protected void onCreate(Bundle MenuButtons) 
{
    super.onCreate(MenuButtons);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    setContentView(R.layout.menu);

    beginner = (Button) findViewById(R.id.btnBeginner);
    learner = (Button) findViewById(R.id.btnLearner);

    beginner.setOnClickListener(myOnlyhandler);
    learner.setOnClickListener(myOnlyhandler);

}

编辑：

我想我现在看到了错误。您正在尝试在调用View.OnClickListener之前给onCreate打个电话：

public class Menu extends Activity 
{

    ...

    @Override
    protected void onCreate(Bundle MenuButtons) 
    {
        super.onCreate(MenuButtons);
        ...

    }

    View.OnClickListener myOnlyhandler = new View.OnClickListener(){
         ...
    };

}

相反，您应该在onCreate中调用：

public class Menu extends Activity 
{

    ...

    @Override
    protected void onCreate(Bundle MenuButtons) 
    {
        super.onCreate(MenuButtons);
        ...

        View.OnClickListener myOnlyhandler = new View.OnClickListener(){
             ...
        };

    }


}

这允许在尝试设置侦听器之前创建活动。

更好的是，你可以成为将此onClickListener应用于活动

public class Menu extends Activity 
{

    ...

    @Override
    protected void onCreate(Bundle MenuButtons) 
    {
        super.onCreate(MenuButtons);
        ...
    }

    @Override
    public void onClick(View v)
    {
        ...
    };

}

导致我找到这一点的提示是堆栈跟踪线：

Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate()

您可能请求哪些系统服务？View.OnClickListener！

通常情况下，当涉及到Java时，您不希望在任何方法或构造函数之外实例化项。例如：

class A {
    B b = new B();

    A(){

    }
}

您可能希望在构造函数中实例化项：

class A {
    B b;

    A(){
        b = new B();
    }
}

Answer 2

为什么不在活动上实现onClickListener。在您的代码中，在调用onCreate之前，您正在创建对de活动上下文的引用，因此上下文不可用。netinept在类中声明属性并在构造器/methode中实例化属性的要点也是一件好事。在Android中，您通常不必为活动或片段使用构造函数，因为有一些被调用的生命周期方法。有关更多信息，请参见android，活动生命周期。

如果在创建活动/片段之前确实需要一个上下文，则需要在应用程序类包装器中创建一个返回应用程序上下文的静态方法。这可能导致泄漏，如果使用不当，所以只有当你知道你在做什么。

下面是一个应该工作得更好的代码示例。

public class Menu extends Activity implements onClickListener {
   public AlertDialog.Builder dlgAlert;

   public void onCreat() {
       super.onCreate(MenuButtons);
       ....
       beginner.setOnClickListener(this);
       learner.setOnClickListener(this);
       dlgAlert  = new AlertDialog.Builder(this);
   }

   @Override
   public void onClick(View v) {
       switch(v.getId()) {
       case R.id.btnBeginner:
           Intent openBeginner = new Intent("com.thesis.logipic.BEGINNER");
           startActivity(openBeginner);
       break;
       case R.id.btnLearner:
           if (beginnerAns < 15) {
               dlgAlert.setMessage("wrong password or username");
               dlgAlert.setTitle("Error Message...");
               dlgAlert.setPositiveButton("OK", null);
               dlgAlert.setCancelable(true);
               dlgAlert.create().show();
           } else {
               Intent openLearner = new Intent("com.thesis.logipic.GAMEPLAY");
               startActivity(openLearner);
           }
       break;
       }
    }
}