Question

这是我的活动类，我有spinner。我需要将数据库值加载到微调器，所以使用ArrayList和ArrayAdapter，我做到了。

但是当我运行我的应用程序时，它会给出Null指针异常。

public class addexpense extends ActionBarActivity {

    DBhelper helper;
    SQLiteDatabase db;
    Spinner spinner;

    @Override
    /** Called when the activity is first created. */
    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
        setContentView(R.layout.addexpenses);


        ArrayList<category> mArrayList = helper.getCategories();
        Spinner sp =(Spinner)findViewById(R.id.spinner);
        ArrayAdapter adapter =new ArrayAdapter(this,R.layout.spinner_row,mArrayList);
       sp.setAdapter(adapter);

   }

}

这是我在DBHelper数据库类

中的方法

     public class DBhelper extends SQLiteOpenHelper{

    static final String DATABASE = "wedding9.db";
    static final int VERSION = 9;
    static final String TABLE1 = "Category";
    static final String TABLE2 = "Budget";


    static final String C_ID = "_id";
    static final String Name = "name";
    static final String B_ID = "_id";
    static final String Description = "description";
    static final String Amount = "amount";



    public DBhelper(Context context){
        super(context, DATABASE, null, VERSION);
    }

    public void onCreate(SQLiteDatabase db){

        db.execSQL("CREATE TABLE " + TABLE1+ "(" +C_ID
                + " INTEGER PRIMARY KEY AUTOINCREMENT," +Name+ " text unique not null)");

        db.execSQL("CREATE TABLE " + TABLE2+ "(" +B_ID
                + " INTEGER PRIMARY KEY AUTOINCREMENT," +Description+ " text,"
                +Amount+ " text, FOREIGN KEY ("+Description+") REFERENCES "+TABLE1+"("+Name+"));");
//        db.execSQL("CREATE TABLE " + TABLE2+ "(" +B_ID
//                + " INTEGER PRIMARY KEY AUTOINCREMENT," +Amount+ " text )");


    }

    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        db.execSQL("drop table " + TABLE1);


        onCreate(db);
    }

    public ArrayList<category> getCategories(){
        ArrayList<category> arrayList = new ArrayList<category>();
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.query(DBhelper.TABLE1, null, null, null, null, null, null);
//        c.moveToFirst();
        while (c.moveToNext()) {
            category cat = new category(c.getInt(0),c.getString(1));
            arrayList.add(cat);
        }

        return arrayList;
    }
    public boolean checkIdExist(String name) {
        SQLiteDatabase db = getReadableDatabase();
        Cursor c = db.query(DBhelper.TABLE1, null, null, null, null, null, null);
//        c.moveToFirst();
        while (c.moveToNext()) {
            if(c.getString(1).equals(name))
                return false;
        }

        return true;
    }
}

这是类别

public class category
{
    private int id;
    private String name;

    public category(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

下面是一个例外，我在运行应用程序时得到了

10-23 08：29：21.482 3075-3075 / com.example.username.weddingplanning E / AndroidRuntime：致命异常：主要 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.username.weddingplanning / com.example.username.weddingplanning.addexpense}：显示java.lang.NullPointerException 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2250）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2304）在android.app.ActivityThread.access $ 700（ActivityThread.java:152）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1284）在android.os.Handler.dispatchMessage（Handler.java:99）在android.os.Looper.loop（Looper.java:176）在android.app.ActivityThread.main（ActivityThread.java:5299） at java.lang.reflect.Method.invokeNative（Native Method）在java.lang.reflect.Method.invoke（Method.java:511）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1102）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:869） at dalvik.system.NativeStart.main（Native Method）引起：java.lang.NullPointerException 在com.example.username.weddingplanning.addexpense.onCreate（addexpense.java:43）在android.app.Activity.performCreate（Activity.java:5326）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1097）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2213）在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2304）在android.app.ActivityThread.access $ 700（ActivityThread.java:152）在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1284）在android.os.Handler.dispatchMessage（Handler.java:99）在android.os.Looper.loop（Looper.java:176）在android.app.ActivityThread.main（ActivityThread.java:5299） at java.lang.reflect.Method.invokeNative（Native Method）在java.lang.reflect.Method.invoke（Method.java:511）在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1102）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:869）在dalvik.system.NativeStart.main（本地方法）

Answer 1

首先创建助手实例问题是帮助器在helper.getCategories();处为空。

Answer 2

您必须实现Array适配器（扩展到ArrayAdapter）。并覆盖getDropDownView。

这是样本。

public class CategorySpinnerAdapter extends ArrayAdapter<Category> {

    List<Category> list = new ArrayList<>();

    private Context context;

    public CategorySpinnerAdapter(Context context, int resource, Category[] categories) {
        super(context, resource, categories);

        this.context = context;
    }

    @Override
    public View getDropDownView(int position, View convertView, ViewGroup parent) {
        return getMyView(position, convertView, parent);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        return getMyView(position, convertView, parent);
    }


    private View getMyView(int position, View convertView, ViewGroup parent){
        CSViewHolder viewHolder;
        if(convertView == null){
            convertView = LayoutInflater.from(parent.getContext()).inflate(R.layout.category_spinner_row, parent, false);
            viewHolder = new CSViewHolder(convertView);
            convertView.setTag(viewHolder);
        }else{
            viewHolder = (CSViewHolder)convertView.getTag();
        }

        viewHolder.fill(list.get(position));

        return convertView;
    }

    /**
     * Load channel list
     * @param cursor
     */
    public void setList(Cursor cursor){
        list.clear();
        Log.i(getClass().getName(), String.valueOf(cursor));
        for(cursor.moveToFirst(); !cursor.isAfterLast(); cursor.moveToNext()) {

            // The Cursor is now set to the right position
            Category category = new Category();

            category.setTitle(CursorUtil.getStringColumnFromCursor(cursor, GoftagramContract.CategoryEntry.COLUMN_TITLE));
            category.setId(CursorUtil.getStringColumnFromCursor(cursor, GoftagramContract.CategoryEntry.COLUMN_SERVER_ID));
            list.add(category);
        }
    }




    @Override
    public int getCount() {
        return list.size();
    }

    @Override
    public Category getItem(int position) {
        return list.get(position);
    }

    class CSViewHolder{
        private TextView titleView;
        private ImageView thumbnailView;

        public CSViewHolder(View itemView){

            titleView = (TextView)itemView.findViewById(R.id.category_spinner_row_title);
            thumbnailView = (ImageView)itemView.findViewById(R.id.category_spinner_row_image);

        }

        public void fill(Category category){
            titleView.setText(category.getTitle());

            if(!TextUtils.isEmpty(category.getThumbnail())){
                Glide.with(context)
                        .load(category.getThumbnail())
                        .into(thumbnailView);
            }else{
                thumbnailView.setImageResource(R.drawable.ic_discuss);
            }

        }
    }
}

使用适配器

categorySpinnerAdapter = new CategorySpinnerAdapter(this, R.layout.category_spinner_row, new Category[]{});

将项目加载到微调器

2 个答案: