自定义 ArrayAdapter：为什么会出现 NullPointerException？

Question

很抱歉打扰了这么精通的人，但我是Android新手，我正在努力学习......

无论如何，我正在尝试使用自定义适配器构建画廊。

但是一旦程序尝试调用适配器，我就会得到 NullPointerException。

非常感谢任何帮助！！！！

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

Answer 1

你需要检查if(convertView == null)，什么时候你需要膨胀一个新的视图，或者你可以让超类处理这个：

public View getView(int position, View convertView,ViewGroup parent) {
    if(convertView == null)
        convertView = super.getView(position, convertView, parent);
    ...
} 

主要 Android 程序员的演示文稿：Turbo Charge Your UI 或 World of ListView 将帮助您了解如何使用 getView() 而不会降低您的应用速度。

---

加法
从您的 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...

这是因为您有 两个 el 的不同副本：

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!

我不太喜欢 Java 如何让您在没有警告的情况下执行此操作...只需将最后一行更改为：

protected void onCreate(Bundle savedInstanceState) {
    el = new ArrayList<Elementi>();

（但是，如果您不按上述方式处理getView()，您将遇到另一个 NullPointerException。）

Answer 2

没有看到 logcat，我认为这是你的错误。 getView() 方法的convertView 参数在第一次调用该方法时为空。你的工作是在convertView == null 时为视图充气：

public View getView(int position, View convertView,ViewGroup parent) {
   View row= convertView;  
   if(row==null){
        LayoutInflater inflater = (LayoutInflater) context.getSystemService( Context.LAYOUT_INFLATER_SERVICE );
        row = inflater.inflate(R.layout.mylayout, parent, false);    
   }

   TextView txt =(TextView)row.findViewById(R.id.testorow);
   //(...) more code
}

当再次调用getView() 时，它会将这个View 用作convertView，因此convertView 可能不会是null。

Answer 3

我认为在您的自定义适配器中 getView() View row 是 null。

所以这行代码txt.setText(el.get(position).tx); 给你NullPointerException。

其实你要检查convertView是NULL还是NOT。

如果convertView 是NULL，那么您必须再次扩充 XML 布局。

类似的，

View row = convertView;  
if(row == null)
{
 LayoutInflater inflater = (LayoutInflater) MainActivity.this.getSystemService( Context.LAYOUT_INFLATER_SERVICE );
 row = inflater.inflate(R.layout.row);    
}