java - Android NumberPicker NullPointer 为什么？

Question

我正在尝试实现一个 numberPicker 来选择分钟值。

但是我在这一行遇到了 NullPointer 异常:

minutePicker = (NumberPicker) findViewById(R.id.minuten_picker);

以下代码:

public class MainActivity extends Activity {


NumberPicker minutePicker;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Auswahl Minuten zum starten / Stoppen aller
    minutePicker = new NumberPicker(MainActivity.this);
    minutePicker = (NumberPicker) findViewById(R.id.minuten_picker);
    minutePicker.setMaxValue(30);


    minutePicker.setOnValueChangedListener(new NumberPicker.OnValueChangeListener() {
        @Override
        public void onValueChange(NumberPicker picker, int oldVal, int newVal) {
            abschaltzeit = minutePicker.getValue();
        }
    });

    minutePicker.setValue(0);
    minutePicker.setWrapSelectorWheel(false);

}
}

XML:

<NumberPicker
    android:id="@+id/minuten_picker"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_row="6"
    android:layout_column="0"
    android:paddingLeft="20dp" />

日志:

09-25 11:00:09.749  10687-10687/de.carsten.awesome.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: de.carsten.awesome.app, PID: 10687
    java.lang.RuntimeException: Unable to start activity ComponentInfo{de.carsten.awesome.app/de.carsten.awesome.app.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at de.carsten.awesome.app.MainActivity.onCreate(MainActivity.java:83)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5001)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

minutePicker = new NumberPicker(MainActivity.this);
minutePicker = (NumberPicker) findViewById(R.id.minuten_picker);

您正在以编程方式创建一个 NumberPicker，然后用 findViewById() 返回的内容覆盖该引用。如果您的 activity_main 布局不包含 minuten_picker，则返回 null。

• 仅选择另一个:以编程方式创建它或从您膨胀的 View 层次结构中找到它。

• 如果您选择编程方式 new NumberPicker()，请记住将其添加到 Activity View 层次结构中的某个布局，例如使用 setContentView()

• 如果您选择 inflation 方式，请确保您的 XML 布局文件中有该 View 。

我猜您看到的 NPE 实际上是在您尝试在 minutePicker 上调用方法的下一行，但它为 null。