Android：NullPointerException与ViewPager和PagerAdapter一起发生

Question

我正在用NumberPickers编写应用程序。 当没有使用任何方法时，数字选择器会很好，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，则不会发生任何错误。 更新：我认为此错误可能是由于我使用PagerAdapter和ViewPager引起的，因为所涉及的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>

这是logcat：

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

有了提供的信息，我会说

R.id.LGmade

可能不是正确的ID，因此var LGMade为null。

Answer 2

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，也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

在下面更改

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

至

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