在其他数组适配器中实例化数组适配器时，我得到 null

Question

我需要将列表项从 ListAdapterA onClick 方法添加到 DoneListAdapter 所以我需要实例化 DoneListAdapter 在 ListAdapterA 中调用添加列表项的方法 add 和 notifyDataSetChanged() 但是 DoneListAdapter returns null 并且我的应用程序崩溃了，为什么 returns 为空？

public class ListAdapterA extends ArrayAdapter<Abc> {

private Context context;
private String testString = "a";
DoneListAdapter doneListAdapter;
List<Abc> abc;
List<DoneAbc> doneAbc;


public ListAdapter(Context context, List<Abc> cbA) {
    super(context, R.layout.list_item, cbA);
    this.context = context;
    this.abc = cbA;
    doneListAdapter = new DoneListAdapter(context,doneAbc);
    doneActionHandler = new DoneActionHandler(context);
    actionHandler = new ActionHandler(context);
}

private class ViewHolder {
    ImageButton abcDoneBtn;
}

@Override
public Abc getItem(int position) {
    return abc.get(position);
}

@Override
public long getItemId(int position) {
    return 0;
}

@Override
public int getViewTypeCount() {
    return 2;
}

@Override
public int getItemViewType(int position) {

    return (testString == "a") ? 0:1;
}

@Override
public View getView(final int position, View convertView, ViewGroup parent) {
    ViewHolder holder = null;
    int type = getItemViewType(position);

    if (convertView == null) {

        if (type == 0){
            LayoutInflater inflater = (LayoutInflater) context
                    .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.list_item_finished, null);
        }else {
            LayoutInflater inflater = (LayoutInflater) context
                    .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.list_item, null);
        }

        holder = new ViewHolder();

        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
    }

    if (type == 0){

    holder.abcDoneBtn= (ImageButton)convertView.findViewById(R.id.finished_abc);
        holder.abcDoneBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Abc abc = (Abc) getItem(position);


        DoneABC doneabc = new DoneAC();
                DoneABC.setId(abc.getId());
                doneabc.setName(abc.getName());

                doneActionHandler.save(doneabc);

                doneListAdapter.add(doneabc);
                remove(abc);
                actionHandler.delete(abc);
                notifyDataSetChanged();
                doneListAdapter.notifyDataSetChanged();

            }
        }); 
    }
    return convertView;
}

@Override
public void add(Abc abc) {
    dreams.add(abc);
    notifyDataSetChanged();
    super.add(abc);
}

@Override
public void remove(Abc abc) {
    dreams.remove(abc);
    notifyDataSetChanged();
    super.remove(abc);
}

}

完成适配器：

public class DoneListAdapter extends ArrayAdapter<DoneAbc> {

private Context context;
List<DoneAbc> adoneAbc;


public DoneListAdapter(Context context, List<DoneAbc> doneAbc) {
    super(context, R.layout.list_item, doneAbc);
    this.context = context;
    this.adoneAbc = doneAbc;
}

private class ViewHolder {

}

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

@Override
public DoneAbc getItem(int position) {
    return adoneAbc.get(position);
}

@Override
public long getItemId(int position) {
    return 0;
}


@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder = null;
    int type = getItemViewType(position);

    if (convertView == null) {

            LayoutInflater inflater = (LayoutInflater) context
                    .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            convertView = inflater.inflate(R.layout.list_item, null);


        holder = new ViewHolder();

        convertView.setTag(holder);
    } else {
        holder = (ViewHolder) convertView.getTag();
    }

    return convertView;
}

@Override
public void add(DoneAbc doneAbc) {
    adoneAbc.add(doneAbc);
    notifyDataSetChanged();
    super.add(doneAbc);
}

@Override
public void remove(DoneAbc doneAbc) {
    adoneAbc.remove(doneAbc);
    notifyDataSetChanged();
    super.remove(doneAbc);
}
}

FATAL EXCEPTION: main Process: com.teamkosas.cloudv05, PID: 30693 java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference at com.teamkosas.cloudv05.adapters.DoneListAdapter.add(DoneListAdapter.java:123) at com.teamkosas.cloudv05.adapters.ListAdapterA.onClick(ListAdapterA.java:152) at android.view.View.performClick(View.java:5697) at android.view.View$PerformClick.run(View.java:22526) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:158) at android.app.ActivityThread.main(ActivityThread.java:7224) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

Answer 1

您正在 DoneListAdapter 构造函数中传递空列表。

在您的 class ListAdapterA 中替换此行：

doneListAdapter = new DoneListAdapter(context,doneAbc);

有了这个：

doneListAdapter = new DoneListAdapter(context,new ArrayList<DoneAbc>())

在其他数组适配器中实例化数组适配器时，我得到 null

when instantiating array adapter in other array adapter I'm getting null

arrays

android

arraylist

android-arrayadapter

notifydatasetchanged