自定义ArrayAdapter：为什么会出现NullPointerException？ [英] Custom ArrayAdapter: why do I get a NullPointerException?
问题描述
很抱歉打扰到如此精通的人，但是我是Android的新手，我正在尝试学习...
Sorry for bothering people so proficient, but I am new to Android and I am trying to learn...
无论如何，我正在尝试通过一个自定义适配器。
Anyway, I am trying to build a Gallery with a custom adapter.
但是一旦程序尝试调用适配器，我就会收到NullPointerException。
But I get NullPointerException as soon as the program tries to invoke the adapter.
任何
public class MainActivity extends Activity {
private Gallery gal;
private List<Elementi> el;
@Override
protected void onCreate(Bundle savedInstanceState) {
List<Elementi> el = new ArrayList<Elementi>();
int[] tabDrawables = new int[] {
R.drawable.img1,R.drawable.img2,R.drawable.img3,R.drawable.img1,R.drawable.img4,R.drawable. img5,R.drawable.img6,R.drawable.img7,R.drawable.img8,R.drawable.img9,R.drawable.img10};
String[] descrizione = {"cani","boffo","gigo","belo","fyyfy","bogin","boginetti","zippe","ninne","cestino","cagnin o","cucciolo",};
for (int numero =1; numero <= 10;numero++) {
ImageView imgm = new ImageView(this);
imgm.setImageResource(tabDrawables[numero]);
el.add(new Elementi(imgm,descrizione[numero]));
};
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Gallery gal=(Gallery)findViewById(R.id.galleria);
gal.setAdapter(new MyAdapter());
}
public class MyAdapter extends ArrayAdapter<Elementi> {
public MyAdapter() {
super(MainActivity.this, R.layout.row, el);
}
public View getView(int position, View convertView,ViewGroup parent) {
View row= convertView;
TextView txt =(TextView)row.findViewById(R.id.testorow);
txt.setText(el.get(position).tx) ;
ImageView ima =(ImageView)row.findViewById(R.id.immagine);
ima.setImageResource(R.drawable.img1);
Toast.makeText(getApplicationContext(), Inte ger.toString(position), Toast.LENGTH_LONG).show();
return row;};
}
public class Elementi {
public ImageView im;
public String tx;
public Elementi(ImageView img,String txt){
ImageView im =img;
String tx = txt;
}
}
}
我的LogCat：
11-29 20:26:28.327: E/AndroidRuntime(786): FATAL EXCEPTION: main
11-29 20:26:28.327: E/AndroidRuntime(786): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gallery/com.example.gallery.MainActivity}: java.lang.NullPointerException
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.os.Handler.dispatchMessage(Handler.java:99)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.os.Looper.loop(Looper.java:137)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread.main(ActivityThread.java:5039)
11-29 20:26:28.327: E/AndroidRuntime(786): at java.lang.reflect.Method.invokeNative(Native Method)
11-29 20:26:28.327: E/AndroidRuntime(786): at java.lang.reflect.Method.invoke(Method.java:511)
11-29 20:26:28.327: E/AndroidRuntime(786): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
11-29 20:26:28.327: E/AndroidRuntime(786): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
11-29 20:26:28.327: E/AndroidRuntime(786): at dalvik.system.NativeStart.main(Native Method)
11-29 20:26:28.327: E/AndroidRuntime(786): Caused by: java.lang.NullPointerException
11-29 20:26:28.327: E/AndroidRuntime(786): at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.widget.AbsSpinner.setAdapter(AbsSpinner.java:114)
11-29 20:26:28.327: E/AndroidRuntime(786): at com.example.gallery.MainActivity.onCreate(MainActivity.java:52)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.Activity.performCreate(Activity.java:5104)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
11-29 20:26:28.327: E/AndroidRuntime(786): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
11-29 20:26:28.327: E/AndroidRuntime(786): ... 11 more
11-29 20:26:28.547: D/dalvikvm(786): GC_CONCURRENT freed 387K, 17% free 2496K/3000K, paused 13ms+24ms, total 191ms
推荐答案
您需要检查 if（convertView == null），当需要扩展新视图时，或者可以让超类处理以下问题：
You need to check if(convertView == null) and when it is you need to inflate a new View, or you can let the super class handle this:
public View getView(int position, View convertView,ViewGroup parent) {
if(convertView == null)
convertView = super.getView(position, convertView, parent);
...
}
演示文稿：对您的用户界面进行Turbo收费或 ListView的世界将帮助您了解如何使用 getView（）而不会降低您的应用程序速度。
The presentations: Turbo Charge Your UI or World of ListView by lead Android programmers will help you understand how to work with getView() without slowing down your app.
添加
来自LogCat：
Addition
From your LogCat:
Caused by: java.lang.NullPointerException
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:330)
这意味着您传递给ArrayAdapter的列表是 null ...
This means the list you passed to the ArrayAdapter is null...
这是因为您有两个不同的 el ：
private List<Elementi> el; // This one is null and is what's passed to your adapter
@Override
protected void onCreate(Bundle savedInstanceState) {
List<Elementi> el = new ArrayList<Elementi>(); // This one only exists in onCreate!
我真的不喜欢Java如何让您在没有警告的情况下执行此操作...只需更改最后一行：
I don't really like how Java let's you do this without a warning... Simply change the last line to:
protected void onCreate(Bundle savedInstanceState) {
el = new ArrayList<Elementi>();
（但是，如果您不处理 getView，则会遇到另一个NullPointerException ）如上所述。）
(However you will hit another NullPointerException if you don't address getView() as stated above.)
这篇关于自定义ArrayAdapter：为什么会出现NullPointerException？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
