java - Android slider 布局和 NullPointerException

Question

好的，所以我有一个带有按钮和 ListView 的主布局，我想做的是当点击按钮时，其他一些东西会显示在它的顶部(其他一些按钮和 TextView .. .)，所以从技术上讲，按钮将使 Listview 及其本身向下滑动，以便显示其他元素。 所以我创建了一个名为 Slider 的类，它扩展了 LinearLayout 类，我在其中编写了制作幻灯片的方法，并且我在 XML 文件中使用了这个类来显示布局。

首先，这是代码:

slider .java:

public class Slider extends LinearLayout{

    //The layout that'll show up when the slide is done: 

    public RelativeLayout Show= (RelativeLayout)findViewById(R.id.Show);

    public Slider(Context context) {
        super(context);
    }

    //Method that'll make the slide
    public void toggle(){
        TranslateAnimation animation;
        animation = new TranslateAnimation(0.0f, 0.0f, 0.0f, -Show.getHeight());

        animation.setDuration(2000);
        animation.setInterpolator(new AccelerateInterpolator());
        startAnimation(animation);
    }
    public String toString(){
        return "THE TOSTRING....";
    }

}

slider .xml:

<com.example.andtestbdd3.Slider
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/slider"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal"
    android:padding="5dip">

    <RelativeLayout
        android:id="@+id/Show"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/bouton1">

        <EditText
            android:id="@+id/Edit1"
            android:layout_height="wrap_content"
            android:layout_width="130dp"
            android:hint="Nom"
            android:paddingTop="1dp"
            android:paddingBottom="1dp"
            android:background="#C2BCBC"
            android:maxLength="25"
            android:layout_marginRight="20dp"
            android:layout_marginBottom="3dp"/>

        <EditText
            android:id="@+id/Edit2"
            android:layout_height="wrap_content"
            android:layout_width="130dp"
            android:hint="Prenom"
            android:paddingTop="1dp"
            android:paddingBottom="1dp"
            android:layout_toRightOf="@id/Edit1"
            android:background="#C2BCBC"
            android:maxLength="25"/>

    </RelativeLayout>

</com.example.andtestbdd3.Slider>

维护 Activity .java:

public class MainActivity extends Activity {

    Slider slider = null;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
           //Getting the slider from the XML file which uses the Slider class (Slider.java)
        slider = (Slider)findViewById(R.id.slider);
        //everything is ok till here, When I try to use some stuff from the Slider,                      //the NullPointerEXCEPTION 
        //Will show up, for example; if I call the toString methode:
    Toast.makeText(this,"Le text: "+slider.toString(), Toast.LENGTH_LONG).show();
    }

}

日志:

02-12 09:07:20.870: D/AndroidRuntime(933): Shutting down VM
02-12 09:07:20.870: W/dalvikvm(933): threadid=1: thread exiting with uncaught exception (group=0xb4aaaba8)
02-12 09:07:20.890: E/AndroidRuntime(933): FATAL EXCEPTION: main
02-12 09:07:20.890: E/AndroidRuntime(933): Process: com.example.andtestbdd3, PID: 933
02-12 09:07:20.890: E/AndroidRuntime(933): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.andtestbdd3/com.example.andtestbdd3.MainActivity}: java.lang.NullPointerException
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.os.Handler.dispatchMessage(Handler.java:102)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.os.Looper.loop(Looper.java:136)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread.main(ActivityThread.java:5017)
02-12 09:07:20.890: E/AndroidRuntime(933):  at java.lang.reflect.Method.invokeNative(Native Method)
02-12 09:07:20.890: E/AndroidRuntime(933):  at java.lang.reflect.Method.invoke(Method.java:515)
02-12 09:07:20.890: E/AndroidRuntime(933):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
02-12 09:07:20.890: E/AndroidRuntime(933):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
02-12 09:07:20.890: E/AndroidRuntime(933):  at dalvik.system.NativeStart.main(Native Method)
02-12 09:07:20.890: E/AndroidRuntime(933): Caused by: java.lang.NullPointerException
02-12 09:07:20.890: E/AndroidRuntime(933):  at com.example.andtestbdd3.MainActivity.onCreate(MainActivity.java:53)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.Activity.performCreate(Activity.java:5231)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
02-12 09:07:20.890: E/AndroidRuntime(933):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
02-12 09:07:20.890: E/AndroidRuntime(933):  ... 11 more

Activity_main.xml:

<RelativeLayout 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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:background="#000000"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/bouton1"
        android:layout_width="wrap_content"
        android:layout_height="30dp"
        android:background="#595959"
        android:gravity="center"
        android:paddingBottom="2dp"
        android:text="Ajouter" />



<LinearLayout
    android:id="@+id/layout"
    android:layout_width="250dp" 
    android:layout_height="wrap_content"
    android:layout_marginTop="70dp"
    android:layout_marginLeft="17dp"
    android:padding="2dp"
    android:background="#595959">

    <ListView
        android:id="@+id/lista"
        android:layout_width="250dp"
        android:layout_height="fill_parent"
        android:background="#7E7E7E"
        android:choiceMode="singleChoice"
        android:divider="#6F6F6F"
        android:dividerHeight="2dp"
        android:paddingLeft="15dp"
        android:paddingRight="15dp"
        />

</LinearLayout>

</RelativeLayout>

所以当我从 xml 文件中获取 Slider 时，没问题。 但是当我尝试使用类文件中的一些方法时，我得到了 NullPointerException ! 我希望我的问题很清楚。如果您需要更多信息，请告诉我，我会尽快更新。 谢谢

Answer 1

在我看来，当您尝试使用 slider.toString() 时，您的 Slider 对象未找到并且为 null 从您的 logcat 行可以看出:

com.example.andtestbdd3.MainActivity.onCreate(MainActivity.java:53)