在SQLite数据库中插入或读取值没有产生任何结果。

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英文:

Inserting or Reading values in an SQLite database doesn't yield any results

问题

DatabaseHelperContract.java

package com.practice.sqlitepractice;

import android.provider.BaseColumns;

// 防止继承的最终类
public final class DatabaseHelperContract {

    // 创建 Entry 字符串
    public static final String CREATE_ENTRY = "CREATE TABLE " + DatabaseSchema.TABLE_NAME + " (" +
            DatabaseSchema._ID + " INTEGER PRIMARY KEY, " + DatabaseSchema.COLUMN_NAME + " TEXT)";

    // 防止意外实例化的私有构造函数
    private DatabaseHelperContract(){
    }

    /* 定义单个表内容的内部类 */
    public static class DatabaseSchema implements BaseColumns{
        public static final String TABLE_NAME = "details";
        public static final String COLUMN_NAME = "name";
    }
}

DatabaseHelper.class

package com.practice.sqlitepractice;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

import androidx.annotation.Nullable;

class DatabaseHelper extends SQLiteOpenHelper {

    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "table.db";

    public DatabaseHelper(@Nullable Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL(DatabaseHelperContract.CREATE_ENTRY);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

    }
}

MainActivity.java

package com.practice.sqlitepractice;

import androidx.appcompat.app.AppCompatActivity;

import android.content.ContentValues;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

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

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText nameEditText;
    private Button insertButton;
    private Button readButton;
    private ListView detailsListView;

    private DatabaseHelper databaseHelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initializeWidgetsMethod();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        databaseHelper.close();
    }

    private void insertionMethod(String name) {
        // 获取可写模式下的数据仓库
        SQLiteDatabase sqLiteDatabase = databaseHelper.getWritableDatabase();

        // 创建一个新的值映射,其中列是键
        ContentValues contentValues = new ContentValues();
        contentValues.put(DatabaseHelperContract.DatabaseSchema.COLUMN_NAME, name);

        try {
            long newRowID = sqLiteDatabase.insert(DatabaseHelperContract.DatabaseSchema.TABLE_NAME, null, contentValues);
            Toast.makeText(this, "新创建行的ID:" + newRowID, Toast.LENGTH_SHORT).show();
        } catch (SQLException exception) {
            exception.printStackTrace();
        }
    }

    private void readMethod() {
        SQLiteDatabase sqLiteDatabase = databaseHelper.getReadableDatabase();
        String[] projection = {DatabaseHelperContract.DatabaseSchema.COLUMN_NAME};

        try {
            Cursor cursor = sqLiteDatabase.query(
                    DatabaseHelperContract.DatabaseSchema.TABLE_NAME,
                    projection,
                    null,
                    null,
                    null,
                    null,
                    null);

            List<String> itemIDs = new ArrayList<>();
            while (cursor.moveToNext()) {
                String name = cursor.getString(cursor.getColumnIndex(DatabaseHelperContract.DatabaseSchema.COLUMN_NAME));
                itemIDs.add(name);
            }

            cursor.close();
            displayMethod(itemIDs);
        } catch (SQLiteException exception) {
            exception.printStackTrace();
        }
    }

    private void displayMethod(List<String> list) {
        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, list);
        detailsListView.setAdapter(arrayAdapter);
    }

    @Override
    public void onClick(View view) {
        if (view == readButton) {
            readMethod();
        }

        if (view == insertButton) {
            String name = nameEditText.getText().toString();
            insertionMethod(name);
        }
    }

    private void initializeWidgetsMethod() {
        nameEditText = findViewById(R.id.name_edit_text);
        insertButton = findViewById(R.id.insert_button);
        readButton = findViewById(R.id.read_button);
        detailsListView = findViewById(R.id.details_list_view);
        databaseHelper = new DatabaseHelper(getApplicationContext());
    }
}

注:我知道数据库事务很昂贵,应始终在异步中执行。此代码仅用于练习,稍后我会在后续阶段执行所有这些操作。

英文:

Started with SQLite in Android this evening. Practiced all the code from the Google documentation but it's still going wrong and I can't figure out where I'm going wrong. Take a look and see if you can find out any details.

DatabaseHelperContract.java

package com.practice.sqlitepractice;
import android.provider.BaseColumns;
//final class to prevent inheritance
public final class DatabaseHelperContract {
//Create Entry String
public static final String CREATE_ENTRY = &quot;CREATE TABLE &quot; + DatabaseSchema.TABLE_NAME + &quot; (&quot; +
DatabaseSchema._ID + &quot; INTEGER PRIMARY KEY, &quot; + DatabaseSchema.COLUMN_NAME + &quot; TEXT)&quot;;
//private constructor to prevent accidental instantiation
private DatabaseHelperContract(){
}
/*Inner class that defines individual table contents*/
public static class DatabaseSchema implements BaseColumns{
public static final String TABLE_NAME = &quot;details&quot;;
public static final String COLUMN_NAME = &quot;name&quot;;
}
}

DatabaseHelper.class

package com.practice.sqlitepractice;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import androidx.annotation.Nullable;
class DatabaseHelper extends SQLiteOpenHelper {
public static final int DATABASE_VERSION = 1;
public static final String DATABASE_NAME = &quot;table.db&quot;;
public DatabaseHelper(@Nullable Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase sqLiteDatabase) {
sqLiteDatabase.execSQL(DatabaseHelperContract.CREATE_ENTRY);
}
@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
}
}

MainActivity.java

package com.practice.sqlitepractice;
import androidx.appcompat.app.AppCompatActivity;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private EditText nameEditText;
private Button insertButton;
private Button readButton;
private ListView detailsListView;
private DatabaseHelper databaseHelper;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initializeWidgetsMethod();
}
@Override
protected void onDestroy() {
super.onDestroy();
databaseHelper.close();
}
private void insertionMethod(String name) {
//gets the data repository in write mode
SQLiteDatabase sqLiteDatabase = databaseHelper.getWritableDatabase();
//create a new map of values, where the columns are the keys
ContentValues contentValues = new ContentValues();
contentValues.put(DatabaseHelperContract.DatabaseSchema.COLUMN_NAME, name);
try {
long newRowID = sqLiteDatabase.insert(DatabaseHelperContract.DatabaseSchema.TABLE_NAME, null, contentValues);
Toast.makeText(this, &quot;ID for newly created row: &quot; + newRowID, Toast.LENGTH_SHORT).show();
} catch (SQLException exception) {
exception.printStackTrace();
}
}
private void readMethod() {
SQLiteDatabase sqLiteDatabase = databaseHelper.getReadableDatabase();
String[] projection = {DatabaseHelperContract.DatabaseSchema.COLUMN_NAME};
try {
Cursor cursor = sqLiteDatabase.query(
DatabaseHelperContract.DatabaseSchema.TABLE_NAME,
projection,
null,
null,
null,
null,
null);
List&lt;String&gt; itemIDs = new ArrayList&lt;&gt;();
while (cursor.moveToNext()) {
String name = cursor.getString(cursor.getColumnIndex(DatabaseHelperContract.DatabaseSchema.COLUMN_NAME));
itemIDs.add(name);
}
cursor.close();
displayMethod(itemIDs);
} catch (SQLiteException exception) {
exception.printStackTrace();
}
}
private void displayMethod(List&lt;String&gt; list) {
ArrayAdapter&lt;String&gt; arrayAdapter = new ArrayAdapter&lt;&gt;(this, android.R.layout.simple_list_item_1, R.id.details_list_view, list);
detailsListView.setAdapter(arrayAdapter);
}
@Override
public void onClick(View view) {
if (view == readButton) {
readMethod();
}
if (view == insertButton) {
String name = nameEditText.getText().toString();
insertionMethod(name);
}
}
private void initializeWidgetsMethod() {
nameEditText = findViewById(R.id.name_edit_text);
insertButton = findViewById(R.id.insert_button);
readButton = findViewById(R.id.read_button);
detailsListView = findViewById(R.id.details_list_view);
databaseHelper = new DatabaseHelper(getApplicationContext());
}
}

Note: I know that database transactions are expensive and should always be performed asynchronously. This code is just for practice and I will be performing all those at a later stage.

答案1

得分: 1

你代码中的问题在于在 displayMethod() 函数中初始化 ArrayAdapter 的方式。这个定义的第三个参数:

ArrayAdapter<String> arrayAdapter = new ArrayAdapter<>(
   this, 
   android.R.layout.simple_list_item_1, 
   R.id.details_list_view, 
   list
);

R.id.details_list_view,它是 ListView 的资源 ID,而你应该使用一个 TextView 的资源 ID,但前提是你是否已经在 ListView 内创建了一个要使用的 TextView。你创建了这样的 TextView 吗?
如果没有的话,你可以使用默认的 TextView,即不传递这个参数。
所以将代码修改为这样:

ArrayAdapter<String> arrayAdapter = new ArrayAdapter<>(
    this, 
    android.R.layout.simple_list_item_1, 
    list
);
英文:

The problem with your code is the way that you initialize the ArrayAdapter in displayMethod(). <br/>
The 3d argument of this definition:

ArrayAdapter&lt;String&gt; arrayAdapter = new ArrayAdapter&lt;&gt;(
this, 
android.R.layout.simple_list_item_1, 
R.id.details_list_view, 
list
);

is R.id.details_list_view which is the ListView resource id, while you should have used a TextView resource id but only if you have created one to use inside the ListView.<br/>
Did you create such a TextView?<br/>
If not then you can use the default TextView, by not passing this argument.<br/>
So change the code to this:

ArrayAdapter&lt;String&gt; arrayAdapter = new ArrayAdapter&lt;&gt;(
this, 
android.R.layout.simple_list_item_1, 
list
);

huangapple
  • 本文由 发表于 2020年9月7日 22:39:32
  • 转载请务必保留本文链接:https://go.coder-hub.com/63779753.html
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