Android：ViewPager 和 PagerAdapter 发生 NullPointerException

Question

我正在用 NumberPickers 编写一个应用程序。当没有使用任何方法时，数字选择器很好，但是当我调用 NumberPicker.setMaxValue 方法时，会发生 NullPointer 异常，并且应用程序无法启动。如果不使用 set max ot set min 方法，不会发生错误。 更新：我认为这个错误可能是由我使用 PagerAdapter 和 ViewPager 引起的，因为有问题的 NumberPicker 不在第一页上。 这是我的 onCreate 方法：

protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 //All the pager stuff
 MyPagerAdapter adapter = new MyPagerAdapter();
 ViewPager myPager = (ViewPager) findViewById(R.id.pager);
 myPager.setAdapter(adapter);
 myPager.setCurrentItem(0);
 //Start of real code.
 final NumberPicker LGmade = (NumberPicker) findViewById(R.id.LGmade);
 //Code responsible for error follows
 LGmade.setMaxValue(3);
 LGmade.setMinValue(0);
 }

这是 XML：

<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent" android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:background="@drawable/bc"
    android:orientation="horizontal">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Low Goals"
        android:id="@+id/textView"
        android:layout_row="0"
        android:layout_column="0"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="20dp" />

    <NumberPicker
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/LGmade"
        android:layout_row="3"
        android:layout_column="1"
        android:layout_marginTop="10dp"
        android:showDividers="middle"
        android:orientation="vertical"/>

    <NumberPicker
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/LGmissed"
        android:layout_row="3"
        android:layout_column="2"
        android:layout_marginTop="10dp"
        android:showDividers="middle"
        android:orientation="vertical"/>

</GridLayout>

这里是日志：

03-12 00:24:56.944    1334-1334/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lmrobotics.frcgamescout.app/com.lmrobotics.frcgamescout.app.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.lmrobotics.frcgamescout.app.MainActivity.onCreate(MainActivity.java:55)
            at android.app.Activity.performCreate(Activity.java:5104)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

嘿，您尝试在 onCreate 方法中初始化最终变量 NumberPicker LGmade，这是不允许的。好吧，您可以简单地删除 final 关键字，或者您可以在构造函数中对其进行初始化。

NOTE: 
A final variable can only be initialized once, either via an 
      initializer or an assignment statement. 
It does not need to be initialized at the point of declaration: this is called a 
      "blank final" variable. 
A blank final instance variable of a class must be definitely assigned in every 
      constructor of class in which it is declared

下面改

final NumberPicker LGmade = (NumberPicker) findViewById(R.id.LGmade);

到

NumberPicker LGmade = (NumberPicker) findViewById(R.id.LGmade);

Answer 2

根据提供的信息，我会说

R.id.LGmade

可能不是正确的 id，因此 var LGMade 为空。