Question

您好，我只是想问为什么我无法启动我的程序，当我开始使用MessageBox或AlertBox时，它始终显示错误。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，他就无法进入学习者阶段，当他点击学习者时，它将显示一个他需要回答10+的消息框能够进入下一个回答的答案是我的代码

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

您对super的致电必须是您做的第一件事。

@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：

我想我现在看到了这个错误。您正在尝试在调用onCreate之前调用View.OnClickListener：

public class Menu extends Activity 
{

    ...

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

    }

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

}

相反，你应该在onCreate中进行调用：

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，你可能是applying this onClickListener to the Activity：

public class Menu extends Activity 
{

    ...

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

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

}

引导我的提示是堆栈跟踪线：

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

您可能要求哪些系统服务？ View.OnClickListener！

作为一般规则，当涉及到Java时，您不希望实例化任何方法或构造函数之外的项。例如：

class A {
    B b = new B();

    A(){

    }
}

您可能希望在构造函数中实例化项目：

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，您在调用onCreate之前创建了对de活动上下文的引用，因此上下文不可用。点netinept在类中声明属性然后在constuctor / methode中实例化它们也是一件好事。在Android中，您通常不必为活动或片段使用构造函数，因为存在被调用的生命周期方法。有关详细信息，请参阅Android文档Activity Lifecycle。

如果在创建activity / fragment之前确实需要上下文，则需要在Application类包装器中创建一个返回Application上下文的静态方法。如果没有正确使用，这可能会导致泄漏，所以只有在你知道自己在做什么的情况下才会这样做。

下面一个应该更好的代码示例。

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;
       }
    }
}

MessageBox错误

2 个答案:

编辑：