NullPointerException安卓开发错误

Question

我在这条线上得到了一个NullPointerException，如果你能帮我弄清楚，那就太好了！

double radius = Double.parseDouble(radiusEditText.getText().toString());

以下是整个代码：

    public class areacircle extends Activity {
    TextView radiusTextView;
    TextView areaTextView;
    EditText radiusEditText;
    Button areaButton;
    EditText answerEditText;

    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
         setContentView(R.layout.areacircle);

         areaButton = (Button) findViewById(R.id.areaButton);
         answerEditText = (EditText) findViewById(R.id.answerEditText);
         radiusEditText = (EditText) findViewById(R.id.radiusEditText);


    }
    double pi = 3.14;
    double radius = Double.parseDouble(radiusEditText.getText().toString());
    double finalRadius = radius * radius *pi;
    double area = Double.parseDouble(answerEditText.getText().toString());

    public void findarea(){

        answerEditText.setText(String.valueOf(finalRadius));


    }
}

areacircle.xml代码：

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/radiusTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/acirclefirstnumbtn"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:textSize="30sp" />

    <EditText
        android:id="@+id/radiusEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="@string/radius"
        android:inputType="numberDecimal"
        android:textSize="30sp" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/areaButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:onClick="findarea"
        android:text="@string/findarea"
        android:textSize="50sp" />

    <TextView
        android:id="@+id/areaTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="30dp"
        android:text="@string/area"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:textSize="40sp" />

    <EditText
        android:id="@+id/answerEditText"
        android:layout_width="83dp"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:ems="10"
        android:hint="@string/areatextview"
        android:inputType="numberDecimal"
        android:text="@string/areaanswer" />

</LinearLayout>

Answer 1

这些行是：

double pi = 3.14;
double radius = Double.parseDouble(radiusEditText.getText().toString());
double finalRadius = radius * radius *pi;
double area = Double.parseDouble(answerEditText.getText().toString());

真的在任何方法之外吗？如果是这样的话，它们将在对象构造函数期间执行-即在运行onCreate()之前。这意味着radiusEditText尚未被分配，并将解释空指针异常。

为了解决这个问题，要么把这些放在findarea()中(可能是由回调方法调用)，要么放到其他回调方法中，这样它们就会在onCreate()之后运行。例如：

// Calculate PI beforehand
final double pi = Math.PI;

public void findarea(){
    // Calculate the value of answerEditText
    double radius = Double.parseDouble(radiusEditText.getText().toString());
    double finalRadius = radius * radius *pi;
    answerEditText.setText(String.valueOf(finalRadius));

    double area = Double.parseDouble(answerEditText.getText().toString());
    // do something with area?
}