應用程序在開始之前崩潰

Question

我使用兩個NumberPickers創建了一個簡單的應用程序，用於選擇一定數量的分鐘和秒鐘。沒有太多的代碼，所以我將它張貼在這裏：

public class MainActivity extends Activity implements OnClickListener { 

private static final String TAG = "Interval Trainer"; 
private CountDownTimer countDownTimer; 
private boolean timerHasStarted = false; 
private Button startButton; 
public TextView text; 
private final long interval = 1 * 1000; 

//Create NumberPickers 
NumberPicker numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1); 
NumberPicker numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    Log.i(TAG,"Entering onCreate()"); 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    startButton = (Button) this.findViewById(R.id.button); 
    startButton.setOnClickListener(this); 
    text = (TextView) this.findViewById(R.id.timer); 

    //Set min and max values for NumberPickers 
    numberPicker1.setMaxValue(100); 
    numberPicker1.setMinValue(0); 
    numberPicker2.setMaxValue(59); //This is the seconds picker 
    numberPicker2.setMinValue(0); 

    Log.i(TAG,"Exiting onCreate()"); 
} 

@Override 
public void onClick(View v) { 
    //Calculate total time from NumberPickers in seconds 
    long startTime = (numberPicker1.getValue() * 60) + numberPicker2.getValue(); 

    //Create CountDownTimer with values from NumberPickers 
    countDownTimer = new MyCountDownTimer(startTime, interval); 
    text.setText(text.getText() + String.valueOf(startTime/1000)); //should be removed 

    if(!timerHasStarted) { 
     countDownTimer.start(); 
     timerHasStarted = true; 
     startButton.setText("STOP"); 
    } else { 
     countDownTimer.cancel(); 
     timerHasStarted = false; 
     startButton.setText("RESTART"); 
    } 

    //Disable the NumberPickers after 'Start' is pressed 
    numberPicker1.setEnabled(false); 
    numberPicker2.setEnabled(false); 
} 

public class MyCountDownTimer extends CountDownTimer { 
    public MyCountDownTimer(long startTime, long interval) { 
     super(startTime, interval); 
    } 

    @Override 
    public void onFinish() { 
     text.setText("Time's up!"); 
     //re-enable the NumberPickers once countdown is done 
     numberPicker1.setEnabled(true); 
     numberPicker2.setEnabled(true); 
    } 

    @Override 
    public void onTick(long millisUntilFinished) { 
     text.setText("" + millisUntilFinished/1000); 

     //Changes the value of the NumberPickers after each tick 

    } 
} 
} 

我有一種感覺崩潰有關，我宣佈的任何方法外兩個NumberPickers（這會自動將它們靜態？）。最初我在我的onCreate（）中有這兩行代碼，但因爲我需要他們在我的內部類和其他方法我把它移到外面。這是什麼導致了崩潰？如果是這樣，我該如何正確執行此操作，並仍然可以訪問我的課程（包括內部課程）周圍的numberPicker1和2？

謝謝！

Answer 1

你不能intiate對象這樣

//Create NumberPickers 
NumberPicker numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1); 
NumberPicker numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2); 

的onCreate後

只有你佈置的負載，所以你不要有你對象尚未初始化。 您可以initlize你的onCreate對象後

setContentView(R.layout.activity_main); 

例如

// init variable 
NumberPicker numberPicker1 = null; 
NumberPicker numberPicker2 = null; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    setContentView(R.layout.activity_main); 
// After here only your Activity gets the layout objects 

    numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1); 
    numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2); 

} 

Answer 2

你應該把你的findViewById內onCreate方法，並創建局部變量：

// init variable 
NumberPicker numberPicker1, numberPicker2; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    // ... 
    numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1); 
    numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2); 

} 

然後，您將能夠調用內其他方法這些變量。據this reference：

非靜態內部類（InnerClasses）可以訪問外部類中的其他成員，即使它們被聲明爲private。靜態嵌套類不能訪問封閉類的其他成員。 [...] InnerClass可以直接訪問其封閉實例的方法和字段。