Question

我创建了一个用法适配器来绑定我的项目。但是，我不知道为什么这个错误（尝试在空对象引用上调用虚拟方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）'）显示我运行应用程序时。

我的适配器

package com.example.redir.wealthtrack;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;

import com.example.redir.wealthtrack.model.Receipt;
import com.example.redir.wealthtrack.model.Usage;

import java.io.FileNotFoundException;
import java.io.InputStream;
import java.util.ArrayList;

/**
 * Created by User on 2/3/2016.
 */
public class UsageAdapter extends ArrayAdapter<Usage> {
    private static final String TAG = "Usage" ;
    ArrayList<Usage> list;
    Context context;
    public static final String LOGTAG = "EXPLORECE";


    public UsageAdapter(Context context, int resource, ArrayList<Usage> lists){
        super(context, resource, lists);
        this.context = context;
        this.list = lists;

    }

    public View getView(int position, View convertView, ViewGroup parent) {
        Usage myusage = list.get(position);

        LayoutInflater inflater =
                (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);

        View view = inflater.inflate(R.layout.usage_item, null);




        return view;
    }


}

我的片段

package com.example.redir.wealthtrack;


import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.text.Html;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.ListView;
import android.widget.Toast;

import com.example.redir.wealthtrack.db.ReceiptsDataSource;
import com.example.redir.wealthtrack.db.UsageDataSource;
import com.example.redir.wealthtrack.model.Receipt;
import com.example.redir.wealthtrack.model.Usage;

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


/**
 * A simple {@link Fragment} subclass.
 */
public class UsageFragment extends Fragment {


    public UsageDataSource Usagedatasource;
    ArrayAdapter<Usage> UsageAdapter;
    ListView listView;
    List<Usage> usages;
    Usage selectedUsage;


    public UsageFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        showToast(getActivity().getApplicationContext(), "Usage's history");


        ((MainActivity) getActivity()).setActionBarTitle("View Usage");

        Usagedatasource = new UsageDataSource(getActivity().getApplicationContext());
        Usagedatasource.open();

        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_receipts, container, false);

        usages = (List<Usage>) Usagedatasource.getAll();
        UsageAdapter = new UsageAdapter(getActivity().getApplicationContext(),0, (ArrayList<Usage>) usages);
        listView = (ListView) view.findViewById(R.id.listView3);
        listView.setAdapter(UsageAdapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                selectedUsage = usages.get(position);
                AlertDialog.Builder a_builder = new AlertDialog.Builder(getActivity());
                a_builder.setMessage("Receipt/Bill captured on: " + selectedUsage.getUsagedate())
                        .setCancelable(false)
                        .setPositiveButton("Ok", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                            }
                        });
                AlertDialog alert = a_builder.create();
                alert.setTitle(Html.fromHtml("<font color='#08ae9e'>Information</font>"));
                alert.show();

            }
        });


        listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
            @Override
            public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {

                selectedUsage = usages.get(position);
                AlertDialog.Builder a_builder = new AlertDialog.Builder(getActivity());
                a_builder.setMessage("Are you sure you want to delete?")
                        .setCancelable(false)
                        .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                deleteItem(selectedUsage);
                            }
                        })
                        .setNegativeButton("No", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                dialog.cancel();
                            }
                        });
                AlertDialog alert = a_builder.create();
                alert.setTitle(Html.fromHtml("<font color='#08ae9e'>Alert!!</font>"));
                alert.show();
                return true;
            }
        });
        return view;
    }
    public static void showToast(Context context, String text) {
        Toast.makeText(context, text, Toast.LENGTH_LONG).show();
    }

    private void deleteItem(Usage selectedUsage) {
        Usagedatasource.removeUsage(selectedUsage);
        showToast(getActivity().getApplicationContext(), "Usage deleted");
        UsageAdapter.notifyDataSetChanged();
        UsageAdapter.notifyDataSetInvalidated();
        refreshDisplay();
    }


    private void refreshDisplay() {
        usages = Usagedatasource.getAll();
        UsageAdapter.clear();
        UsageAdapter.addAll(usages);
    }


}

我的项目

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Medium Text"
        android:id="@+id/textView2"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="70dp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Medium Text"
        android:id="@+id/textView3"
        android:layout_below="@+id/textView2"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="57dp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Medium Text"
        android:id="@+id/textView4"
        android:layout_centerVertical="true"
        android:layout_alignStart="@+id/textView3" />
</RelativeLayout>

//收据片段

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ReceiptFragment"
    android:weightSum="1">

    <!-- TODO: Update blank fragment layout -->

    <GridView
        android:layout_width="wrap_content"
        android:layout_height="503dp"
        android:id="@+id/gridView"
        android:numColumns="auto_fit"
        android:horizontalSpacing="5dp"
        android:columnWidth="120dp"
        android:layout_marginLeft="10dp"
        android:layout_marginRight="10dp"
        android:layout_marginTop="70dp"
        android:padding="8dp"
       />

</LinearLayout>

//我的使用数据库（getALL方法）

 public List<Usage> getAll() {
        List<Usage> usages = new ArrayList<Usage>();

        Cursor cursor = database.query(WealthDBOpenHelper.TABLE_USAGE, allColumns,
                null, null, null, null, null);

        if (cursor.getCount() > 0) {
            while (cursor.moveToNext()) {
                Usage usage = new Usage();
                usage.setUsagedate(cursor.getString(cursor.getColumnIndex(WealthDBOpenHelper.COLUMN_USAGE_DATE)));
                usage.setAmount(cursor.getDouble(cursor.getColumnIndex(WealthDBOpenHelper.COLUMN_AMOUNT)));
                usage.setCategory(cursor.getString(cursor.getColumnIndex(WealthDBOpenHelper.COLUMN_CATEGORY)));
                usages.add(usage);
            }
        }
        return usages;


    }

Answer 1

您使用的是GridView而不是ListView，您使用的ID也是错误的。

像这样改变

GridView gridView;
gridView = (GridView) view.findViewById(R.id.gridView);
gridView.setAdapter(UsageAdapter);

在xml中更改

tools:context=".UsageFragment"

错误：尝试在空对象引用上调用虚方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）'

1 个答案: