多列 ListView 显示零而不是数据库值
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【中文标题】多列 ListView 显示零而不是数据库值【英文标题】：Multi-Column ListView shows zeros instead of database values 【发布时间】：2020-03-25 04:09:49 【问题描述】：我正在 android studio 中制作一个速度计应用程序，每次用户超过保存的速度限制时，他当前的速度、经度、纬度和时间都应该存储在数据库中。然后用户将能够查看列表中的条目。我已经创建了everythinf，但是列表视图（使用适配器有 4 列）只显示零而不是数据库值。
数据库助手
package com.mortoglou.speedometer;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteOpenHelper;
import androidx.annotation.Nullable;
public class DatabaseHelper extends SQLiteOpenHelper
public static final String DATABASE_NAME = "limits_list_final.db";
public static final String TABLE_NAME = "limits_list";
public static final String COL1 = "ID";
public static final String COL2 = "SPEED";
public static final String COL3 = "LONGITUDE";
public static final String COL4 = "LATITUDE";
public static final String COL5 = "TIMESTAMP";
public DatabaseHelper(@Nullable Context context)
super(context, DATABASE_NAME, null, 3);
@Override
public void onCreate(SQLiteDatabase db)
String createTable = "CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, " +
" SPEED TEXT, LONGITUDE TEXT, LATITUDE TEXT, TIMESTAMP TEXT)";
db.execSQL(createTable);
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
db.execSQL("DROP TABLE IF EXISTS TABLE_NAME");
onCreate(db);
public boolean addData(float curSpeed, double longitude, double latitude, long timestamp)
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(COL2, curSpeed);
contentValues.put(COL3, longitude);
contentValues.put(COL4, latitude);
contentValues.put(COL5, timestamp);
long result = db.insert(TABLE_NAME, null, contentValues);
if (result == -1)
return false;
else
return true;
public Cursor getListContents()
SQLiteDatabase db = this.getWritableDatabase();
Cursor data = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
return data;
主要活动
'package com.mortoglou.speedometer;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import android.Manifest;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity implements LocationListener, PopUP.PopUpListener
private TextView textView;
private String savedSpeedText;
private Button updateSpeedButton;
private Button checkLimits;
DatabaseHelper myDB;
public double longi;
public int ch = 1; //choice
public float savedSpeed;
public static final String SHARED_PREFS = "sharedprefs";
public static final String SAVED_SPEED = "savedspeed";
@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
textView = (TextView) findViewById(R.id.textView5);
updateSpeedButton = (Button) findViewById(R.id.button);
checkLimits = (Button) findViewById(R.id.button2);
myDB = new DatabaseHelper(this);
updateSpeedButton.setOnClickListener(new View.OnClickListener()
@Override
public void onClick(View v)
popUp();
);
checkLimits.setOnClickListener(new View.OnClickListener()
@Override
public void onClick(View v)
//setContentView(R.layout.speed_limits);
Intent intent = new Intent(MainActivity.this, LimitsList.class);
startActivity(intent);
);
if (ContextCompat.checkSelfPermission(this,
Manifest.permission.ACCESS_FINE_LOCATION)
!= PackageManager.PERMISSION_GRANTED)
int MY_PERMISSIONS_REQUEST_ACCESS_FINE_LOCATION = 0;
ActivityCompat.requestPermissions(this,
new String[]Manifest.permission.ACCESS_FINE_LOCATION,
MY_PERMISSIONS_REQUEST_ACCESS_FINE_LOCATION);
LocationManager lm = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, this);
// this.onLocationChanged(null);
//savedSpeed();
loadSpeed();
updateSpeed();
@Override
public boolean onCreateOptionsMenu(Menu menu)
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.speed_menu, menu);
return true;
@Override
public boolean onOptionsItemSelected(@NonNull MenuItem item)
switch (item.getItemId())
case R.id.speed1:
ch = 1; // choice 1 = m/s
return true;
case R.id.speed2:
ch = 2; // choice 2 = km/h
return true;
return super.onOptionsItemSelected(item);
@Override
public void onLocationChanged(Location location)
TextView txt = (TextView) this.findViewById(R.id.textView2);
float curSpeed = location.getSpeed();
double longitude = location.getLongitude();
double latitude = location.getLatitude();
long timestamp = location.getTime();
if (location == null)
txt.setText("NO LOCATION FOUND");
else
if (curSpeed > 0)
if (ch == 1)
txt.setText(String.format(curSpeed + "m/s"));
else if (ch == 2)
txt.setText(String.format(curSpeed * 3.6 + "km/h"));
else
txt.setText("-");
speedLimit(curSpeed, longi, latitude, timestamp);
public void savedSpeed()
SharedPreferences sharedPreferences = getSharedPreferences(SHARED_PREFS, MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putFloat(SAVED_SPEED, savedSpeed);
editor.apply();
public void loadSpeed()
SharedPreferences sharedPreferences = getSharedPreferences(SHARED_PREFS, MODE_PRIVATE);
savedSpeedText = String.valueOf(sharedPreferences.getFloat(SAVED_SPEED, 0));
public void updateSpeed()
textView.setText(savedSpeedText);
public void popUp()
PopUP popUP = new PopUP();
popUP.show(getSupportFragmentManager(), "pop up");
public void speedLimit(float curSpeed, double longitude, double latitude, long timestamp)
if (curSpeed > savedSpeed)
Toast.makeText(this, "Ayo you goin too fast", Toast.LENGTH_SHORT).show();
AddData(curSpeed, longi, latitude, timestamp);
@Override
public void onStatusChanged(String provider, int status, Bundle extras)
@Override
public void onProviderEnabled(String provider)
@Override
public void onProviderDisabled(String provider)
@Override
public void passText(float speedText)
savedSpeed = speedText;
savedSpeed();
loadSpeed();
updateSpeed();
public void AddData(float curSpeed, double longitude, double latitude, long time)
boolean insertData = myDB.addData(curSpeed, longitude, latitude, time);
if (insertData == true)
Toast.makeText(MainActivity.this, "Successfully Entered Data!", Toast.LENGTH_LONG).show();
else
Toast.makeText(MainActivity.this, "Something went wrong :(.", Toast.LENGTH_LONG).show();
（查看内容）
package com.mortoglou.speedometer;
import android.database.Cursor;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.Toast;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import java.util.ArrayList;
public class LimitsList extends AppCompatActivity
DatabaseHelper myDB;
ArrayList<Entry> entryList;
ListView listView;
Entry entry;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState)
super.onCreate(savedInstanceState);
setContentView(R.layout.speed_limits);
//listView = (ListView) findViewById(R.id.limits_list);
myDB = new DatabaseHelper(this);
//populate an ArrayList<String> from the database and then view it
entryList = new ArrayList<>();
Cursor data = myDB.getListContents();
if (data.getCount() == 0)
Toast.makeText(this, "There are no contents in this list!", Toast.LENGTH_LONG).show();
else
int i=0;
while(data.moveToNext())
entry = new Entry(data.getFloat(1),data.getDouble(2),data.getDouble(3), data.getLong(4));
entryList.add(entry);
//System.out.println(data.getFloat(1)+" "+data.getDouble(2)+" "+data.getDouble(3), data.getLong(4));
System.out.println(entryList.get(i).getSpeed());
i++;
LimitsAdapter adapter = new LimitsAdapter(this, R.layout.final_list_adapter, entryList);
listView = findViewById(R.id.limits_list);
listView.setAdapter(adapter);
进入
package com.mortoglou.speedometer;
public class Entry
private float Speed;
private double Longitude;
private double Latitude;
private long Time;
public Entry(float speed, double longitude, double latitude, long time)
speed = Speed;
longitude = Longitude;
latitude = Latitude;
time = Time;
public float getSpeed()
return Speed;
public void setSpeed(float speed)
Speed = speed;
public double getLongitude()
return Longitude;
public void setLongitude(double longitude)
Longitude = longitude;
public double getLatitude()
return Latitude;
public void setLatitude(double latitude)
Latitude = latitude;
public long getTime()
return Time;
public void setTime(long time)
Time = time;
适配器
package com.mortoglou.speedometer;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;
import java.util.ArrayList;
public class LimitsAdapter extends ArrayAdapter<Entry>
private LayoutInflater mInflater;
private ArrayList<Entry> entries;
private int mViewResourceId;
public LimitsAdapter(Context context, int textViewResourceId, ArrayList<Entry> entries)
super(context, textViewResourceId, entries);
this.entries = entries;
mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
mViewResourceId = textViewResourceId;
public View getView(int position, View convertView, ViewGroup parent)
convertView = mInflater.inflate(mViewResourceId, null);
Entry entry = entries.get(position);
TextView speed = (TextView) convertView.findViewById(R.id.speed);
TextView longitude = (TextView) convertView.findViewById(R.id.longitude);
TextView latitude = (TextView) convertView.findViewById(R.id.latitude);
TextView timestamp = (TextView) convertView.findViewById(R.id.timestamp);
speed.setText(Float.toString(entry.getSpeed()));
longitude.setText(Double.toString((entry.getLongitude())));
latitude.setText(Double.toString((entry.getLatitude())));
timestamp.setText(Long.toString(entry.getTime()));
return convertView;
【问题讨论】：
【参考方案1】：我相信您的问题在于 Entry 对象的构造函数。
你有：-
public Entry(float speed, double longitude, double latitude, long time)
speed = Speed;
longitude = Longitude;
latitude = Latitude;
time = Time;
它使用对象变量中保存的值设置/覆盖传递给构造函数的值，这将是变量类型的默认值（0），因为没有分配任何值。
默认值
并不总是需要赋值 字段被声明。已声明但未初始化的字段将 由编译器设置为合理的默认值。通常来说，一般来说， 此默认值将为零或空，具体取决于数据类型。依靠 然而，在这样的默认值上，通常被认为是不好的 编程风格。
下图总结了以上数据的默认值 类型。
Data Type Default Value (for fields) byte 0 short 0 int 0 long 0L float 0.0f double 0.0d char '\u0000' String (or any object) null boolean falsePrimitive Data Types
您应该使用传递的值设置对象的变量。所以使用：-
public Entry(float speed, double longitude, double latitude, long time)
Speed = speed;
Longitude = longitude;
Latitude = latitude;
Time = time;
【讨论】：
是的！这就是问题所在。非常感谢您的回答，我现在可以继续我的项目了！再次感谢！以上是关于多列 ListView 显示零而不是数据库值的主要内容，如果未能解决你的问题，请参考以下文章