陷入空指针异常

Question

我被困住了。我试图用 SQLite 表的字段填充列表视图。我对 Android 编程还很陌生。关于为什么我收到此错误的任何建议：

10-24 10:24:31.154: ERROR/AndroidRuntime(1298): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dvd.clase/com.dvd.clase.Lista_Classes}: java.lang.NullPointerException
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2596)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2621)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread.access$2200(ActivityThread.java:126)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1932)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.os.Looper.loop(Looper.java:123)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread.main(ActivityThread.java:4595)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at java.lang.reflect.Method.invokeNative(Native Method)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at java.lang.reflect.Method.invoke(Method.java:521)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at dalvik.system.NativeStart.main(Native Method)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298): Caused by: java.lang.NullPointerException
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at com.dvd.clase.Lista_Classes.onCreate(Lista_Classes.java:72)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2544)
10-24 10:24:31.154: ERROR/AndroidRuntime(1298):     ... 11 more

使用此代码（我认为问题在这里而不是在 SQL 查询中，但我可能是错的）：

public class Lista_Classes extends ListActivity{
BaseDatosHelper miBBDDHelper;
private class ClasseAdapter extends ArrayAdapter<Classe> {         
        ArrayList<Classe> items;

        public ClasseAdapter(Context context, int textViewResourceId, ArrayList<Classe> items) 
        {
            super(context, textViewResourceId, items);
            this.items = items;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View v = convertView;
            if (v == null) {
                LayoutInflater vi = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                v = vi.inflate(R.layout.lista_item, null);
            }

            Classe classe = items.get(position);

            //if (classe != null) {
                TextView tnClasse = (TextView) v.findViewById(R.id.numClasse);
                TextView tColor = (TextView) v.findViewById(R.id.Color);
                if (tnClasse != null) {
                    tnClasse.setText(classe.GetNumClase());
                }
                if (tColor != null) {
                    tColor.setText(classe.GetColor());
                }
            //}
            return v;
        }
    }



    public void crearBBDD() {
        miBBDDHelper = new BaseDatosHelper(this);
        try {
            miBBDDHelper.crearDataBase();
        } catch (IOException ioe) {
            throw new Error("Unable to create database");
        }
    }


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        crearBBDD();

        ArrayList<Classe> classes = getItems();
        setListAdapter(new ClasseAdapter(this, R.layout.lista_item, classes));

        Button home= (Button) this.findViewById(R.id.boton_home);
        home.setOnClickListener(new View.OnClickListener() {
        public void onClick(View view){
             Intent myIntent = new Intent(view.getContext(), main.class);
              startActivityForResult(myIntent, 0);
        }

    });
    }
    public ArrayList<Classe> getItems() {
        miBBDDHelper.abrirBaseDatos();
        ArrayList<Classe> listaClasses = miBBDDHelper.GetLlistaClasses();
        miBBDDHelper.close();
        return listaClasses;
    }
}

感谢您的建议。

Answer 1

您正在调用 findViewById 但您从未设置内容视图。调用 super 后，您必须将 setContentView(R.layout.yourlayout) 放入 onCreate 方法中。这就是为什么你的主页按钮为空。

Answer 2

根据您的日志，您应该查看文件的第 72 行，位于 onCreate() 中的某个位置——我无法从您提供的代码中分辨出行号。我建议使用调试器单步执行 onCreate，这样应该很快就能发现问题。我的猜测是您的 getItems() 返回 null。