Question

我正在创建一个简单的应用程序，其中有两个NumberPickers用于选择一定的分钟数和秒数。代码不多，所以我会在这里发布：

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

    }
}
}

我感觉崩溃与我在任何方法之外声明两个NumberPickers相关（这会自动使它们变为静态？）。最初我在onCreate（）中有这两行代码，但由于我在内部类和其他方法中需要它们，所以我把它移到了外面。这是导致崩溃的原因吗？如果是这样，我该如何正确地执行此操作并仍然可以访问我的类（包括内部类）的numberPicker1和2？

谢谢！

Answer 1

你不能像这样发现对象

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

在onCreate之后，只有布局加载，所以你没有对象，但尚未初始化。您可以在

之后在onCreate中初始化对象

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

您应该将findViewById置于onCreate方法内并创建局部变量：

// 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);

}

然后，您可以在其他方法中调用这些变量。根据{{3}}：

非静态嵌套类（ InnerClasses ）可以访问封闭类的其他成员，即使它们被声明为私有。静态嵌套类无权访问封闭类的其他成员。 [...] InnerClass 可以直接访问其封闭实例的方法和字段。

应用程序在启动之前崩溃

2 个答案: