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，他的当前速度、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 ListView (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 Activity
'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;
}
}
最佳答案
我认为您的问题出在 Entry 对象的构造函数上。
你有:-
public Entry(float speed, double longitude, double latitude, long time){
speed = Speed;
longitude = Longitude;
latitude = Latitude;
time = Time;
}
这是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)，因为尚未分配任何值。
-
Default Values
It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Generally speaking, this default will be zero or null, depending on the data type. Relying on such default values, however, is generally considered bad programming style.
The following chart summarizes the default values for the above data types.
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 false
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;
}
关于android - 多列 ListView 显示零而不是数据库值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/59106859/