Question

我有一个ListView的活动，我一直在尝试使用字符串ArrayList中的数据填充列表视图这是活动代码

package com.example.arun.db;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import java.util.ArrayList;
public class List_view extends AppCompatActivity {
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            ArrayList<String> throwlist = new ArrayList<String>();
            ListView Trainlist;
            Trainlist=(ListView)findViewById(R.id.station_list);
            SQLiteDatabase db = openOrCreateDatabase("Station11.db", SQLiteDatabase.CREATE_IF_NECESSARY, null);
            setContentView(R.layout.activity_list_view);
            try{
                Cursor data_fetch = db.rawQuery("Select Station_name From Station", null);

                String[] station_array = new String[data_fetch.getCount()];
                int i= 0;
                while (data_fetch.moveToNext()) {
                    String name = data_fetch.getString(data_fetch.getColumnIndex("Station_name"));
                    station_array[i] = name;
                    Log.d("STATION_GET","Retrieved station " + station_array[i]);
                    //Toast.makeText(List_view.this, "Retrieved station " + station_array[i] , Toast.LENGTH_SHORT).show();
                    throwlist.add(name);
                    i++;

                }
                data_fetch.close();
            }catch(Exception e)
            {
                Toast.makeText(List_view.this, "An Error occured Retrieving your data" , Toast.LENGTH_LONG).show();
            }
            try {

                ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
                        this, android.R.layout.simple_list_item_1, throwlist);
                Trainlist.setAdapter(arrayAdapter);
            }catch (Exception e){
                Log.d("Error in listview ",e.toString());
                Toast.makeText(List_view.this, "Listview Error" , Toast.LENGTH_LONG).show();
            }
        }
    }

我已经确定了该部分 `

try {

                ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(
                        this, android.R.layout.simple_list_item_1, throwlist);
                Trainlist.setAdapter(arrayAdapter);
            }catch (Exception e){
                Log.d("Error in listview ",e.toString());
                Toast.makeText(List_view.this, "Listview Error" , Toast.LENGTH_LONG).show();
            }`

抛出错误

10-28 18:03:42.004 11549-11549/com.example.arun.db D/Error in listview: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

在进一步调查此错误后，我知道这个错误可能是由于here

中没有在xml文件中为特定活动声明的列表视图引起的

但是ListView在xml中有明确的定义下面是xml代码：

<ListView
    android:id="@+id/station_list"
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    tools:layout_constraintBottom_creator="1"
    tools:layout_constraintLeft_creator="1"
    tools:layout_constraintRight_creator="1"
    tools:layout_constraintTop_creator="1"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintVertical_bias="1.0" />

关于为什么会这样做的任何想法

Answer 1

你必须先设置setContentView(R.layout.activity_list_view);而不是像下面的代码

那样设置controll绑定

@Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_list_view);
            ArrayList<String> throwlist = new ArrayList<String>();
            ListView Trainlist;
            Trainlist=(ListView)findViewById(R.id.station_list);
            SQLiteDatabase db = openOrCreateDatabase("Station11.db", SQLiteDatabase.CREATE_IF_NECESSARY, null);

            // other code
           }

将arraylist中的数据插入ListView会引发尝试调用虚拟错误的尝试

1 个答案: