java-Android NumberPicker NullPointer为什么？

概述我试图实现一个numberPicker以选择分钟值.但是我在这一行收到一个NullPointer异常：minutesPicker=(NumberPicker)findViewById(R.id.minuten_picker);以下代码：publicclassMainActivityextendsActivity{NumberPickerminutePicker;@OverrideprotectedvoidonCreate

我试图实现一个numberPicker以选择分钟值.

但是我在这一行收到一个NullPointer异常：

minutesPicker =(NumberPicker)findVIEwByID(R.ID.minuten_picker);

以下代码：

public class MainActivity extends Activity {NumberPicker minutePicker;@OverrIDeprotected 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.access0(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.access0(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)

解决方法:

minutePicker = new NumberPicker(MainActivity.this);minutePicker = (NumberPicker) findVIEwByID(R.ID.minuten_picker);

您正在以编程方式创建NumberPicker,然后用findVIEwByID()返回的值覆盖引用.如果您的activity_main布局不包含minutesn_picker,则返回null.

>仅选择另一个：以编程方式创建它,或者从您夸大的视图层次结构中找到它.
>如果您选择以编程方式new NumberPicker(),请记住将其添加到活动视图层次结构中的某些布局,例如与setContentVIEw()
>如果选择充气方式,请确保在XML布局文件中具有视图.

我猜您正在看到的NPE实际上在以下行中,您正在尝试在minutePicker上调用一个方法,该方法为null.

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