Question

设置我的适配器时，我一直收到Null Pointer Exceptions。我有一个包含多个片段的ViewPager。我已经搜索过并尽力而为。本质上，希望将适配器附加到ViewPager可以使用的片段上的列表视图。

import android.graphics.Paint;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.TextView;

import com.parse.FindCallback;
import com.parse.ParseACL;
import com.parse.ParseException;
import com.parse.ParseQuery;
import com.parse.ParseUser;

import java.util.ArrayList;
import java.util.List;

public class MyFragment extends Fragment implements AdapterView.OnItemClickListener {

    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";
    private EditText mTaskInput;
    private ListView mListView;
    private TaskAdapter mAdapter = null;
    private Button mButton;

    public static final MyFragment newInstance(String message)
    {
        MyFragment f = new MyFragment();
        Bundle bdl = new Bundle(1);
        bdl.putString(EXTRA_MESSAGE, message);
        f.setArguments(bdl);
        return f;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
        Bundle savedInstanceState) {
        String message = getArguments().getString(EXTRA_MESSAGE);
        View v = inflater.inflate(R.layout.myfragment_layout, container, false);
        TextView messageTextView = (TextView)v.findViewById(R.id.textView);
        messageTextView.setText(message);

        mAdapter = new TaskAdapter(getActivity(), new ArrayList<Task>());

        mTaskInput = (EditText) getActivity().findViewById(R.id.task_input);
        mListView = (ListView) getActivity().findViewById(R.id.task_list);
        mButton = (Button) getActivity().findViewById(R.id.submit_button);
        mListView.setAdapter(mAdapter);
        mListView.setOnItemClickListener(this);

        updateData();

        return v;
    }

    public void updateData(){
        ParseQuery<Task> query = ParseQuery.getQuery(Task.class);
        query.whereEqualTo("user", ParseUser.getCurrentUser());
        query.setCachePolicy(ParseQuery.CachePolicy.CACHE_THEN_NETWORK);
        query.findInBackground(new FindCallback<Task>() {
            @Override
            public void done(List<Task> tasks, ParseException error) {
                if(tasks != null){
                    mAdapter.clear();
                    for (int i = 0; i < tasks.size(); i++) {
                        mAdapter.add(tasks.get(i));
                    }
                }
            }
        });
    }

    public void createTask(View v) {
        if (mTaskInput.getText().length() > 0){
            Task t = new Task();
            t.setACL(new ParseACL(ParseUser.getCurrentUser()));
            t.setUser(ParseUser.getCurrentUser());
            t.setDescription(mTaskInput.getText().toString());
            t.setCompleted(false);
            t.saveEventually();
            mAdapter.insert(t, 0);
            mTaskInput.setText("");
        }
    }

    @Override
    public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
        Task task = mAdapter.getItem(position);
        TextView taskDescription = (TextView) view.findViewById(R.id.task_description);

        task.setCompleted(!task.isCompleted());

        if(task.isCompleted()){
            taskDescription.setPaintFlags(taskDescription.getPaintFlags() | Paint.STRIKE_THRU_TEXT_FLAG);
        }else{
            taskDescription.setPaintFlags(taskDescription.getPaintFlags() & (~Paint.STRIKE_THRU_TEXT_FLAG));
        }

        task.saveEventually();
    }
}

Answer 1

你的ListView在片段中，而不在活动中（根据你的陈述）。因此，使用这一行：

mListView = (ListView) getActivity().findViewById(R.id.task_list);

将尝试从您的活动中获取参考。你应该这样做：

mListView = (ListView) v.findViewById(R.id.task_list);

否则，它将编译（R.id存在，但不存在于活动中）。请注意更改：

 v.findViewById

这会创建对膨胀片段的引用，而不是活动。

setAdapter导致空指针异常

1 个答案: