这个利用sqlite写的数据库为什么退出应用后再进去数据库就无法找到了啊

Question

package com.sample.Sample_8_4;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.ContentValues;
import android.content.DialogInterface;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class Sample_8_4 extends Activity {
private Button baseButton;
private Button tableButton;
private Button btn_sqlmod, btn_cvmod, btn_qur;
private final String dbName = "mydb";
private final String tableName = "users";
private SQLiteDatabase db = null;
private int i = 1;

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.setContentView(R.layout.main);
    baseButton = (Button) findViewById(R.id.base); // 实例化Button对象
    baseButton.setOnClickListener(new OnClickListener() { // 为Button对象添加监听
                public void onClick(View v) {
                    db = openOrCreateDatabase(dbName, MODE_PRIVATE, null); // 创建名为“mydb”的数据库
                    db.endTransaction();
                    Toast.makeText(getApplicationContext(), "创建数据库成功", 1000)
                            .show();
                }
            });

    tableButton = (Button) findViewById(R.id.table); // 实例化Button对象
    tableButton.setOnClickListener(new OnClickListener() { // 为Button对象添加监听
                public void onClick(View v) {
                    if (db != null) {
                        creatTable(); // 开始创建数据库表
                    } else {
                        Toast.makeText(getApplicationContext(), "没有数据库",
                                1000).show();
                    }
                }
            });

    btn_sqlmod = (Button) findViewById(R.id.sql_mod);
    btn_sqlmod.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            if (db != null) {
                sql_executeData();
            } else {
                Toast.makeText(getApplicationContext(), "没有数据库", 1000)
                        .show();
            }
        }
    });

    btn_cvmod = (Button) findViewById(R.id.cv_mod);
    btn_cvmod.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            if (db != null) {
                cv_executeData();
            } else {
                Toast.makeText(getApplicationContext(), "没有数据库", 1000)
                        .show();
            }
        }
    });

    btn_qur = (Button) findViewById(R.id.que);
    btn_qur.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            if (db != null) {
                queryData();
            } else {
                Toast.makeText(getApplicationContext(), "没有数据库", 1000)
                        .show();
            }

        }
    });
}

public void creatTable() {
    // 创建表的SQL语句，创建一个名为users的表，该表有id,uname和pwd三个字段
    String sql = "CREATE TABLE IF NOT EXISTS "
            + tableName
            + " (id INTEGER PRIMARY KEY AUTOINCREMENT, uname VARCHAR(50), pwd VARCHAR(50));";
    db.execSQL(sql); // 执行sql语句
    db.endTransaction();
    // 查询sqlite_master表中类型为table的记录的name字段
    Cursor cursor = db.query("sqlite_master", new String[] { "name" },
            "type = ?", new String[] { "table" }, null, null, null, null);
    String tables = "";
    if (cursor.getCount() != 0) { // 判断查询结果的条数是否为0
        cursor.moveToFirst(); // 游标指向第一条记录
        for (int i = 0; i < cursor.getCount(); i += 1) {
            tables = tables + cursor.getString(0) + " "; // 累加字符串
            cursor.moveToNext(); // 游标下移
        }
    }
    // 把累加的结果显示在一个信息框中
    new AlertDialog.Builder(Sample_8_4.this).setTitle("Message")
            .setMessage(tables)
            .setNegativeButton("确定", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                }
            }).show();
}

public void sql_executeData() {
    String sql;
    for (; i < 4; i++) {
        sql = "insert into " + tableName + " values ('" + i
                + "','a','123456')"; // 添加一条记录
        db.execSQL(sql);
        db.endTransaction();
    }

    sql = "update " + tableName + " set pwd='654321' where id='1'"; // 修改一条记录
    db.execSQL(sql);
    db.endTransaction();
    sql = "delete from " + tableName + " where id='2'"; // 删除一条记录
    db.execSQL(sql);
    db.endTransaction();
    Toast.makeText(getApplicationContext(), "使用SQL语句修改数据成功", 1000).show();
}

public void cv_executeData() {
    ContentValues cv = new ContentValues(); // 实例化ContentValues对象
    cv.put("uname", "b"); // 插入字段值
    cv.put("pwd", "987654"); // 插入字段值
    db.insert(tableName, null, cv); // 执行insert方法
    db.insert(tableName, null, cv); // 执行insert方法
    db.insert(tableName, null, cv); // 执行insert方法

    cv = new ContentValues(); // 实例化ContentValues对象
    cv.put("pwd", "456789"); // 插入字段值
    db.update(tableName, cv, "id=?", new String[] { "4" }); // 执行update方法

    db.delete(tableName, "id=?", new String[] { "5" }); // 执行delete操作

    Toast.makeText(getApplicationContext(), "使用Android语句修改数据成功", 1000)
            .show();
}

public void queryData() {
    Cursor cursor = db.query(tableName, null, null, null, null, null, null,
            null); // 执行query操作获得Cursor对象
    String str = "";
    if (cursor.getCount() != 0) { // 判断返回的记录条数
        cursor.moveToFirst(); // 游标指向第一条记录
        for (int i = 0; i < cursor.getCount(); i += 1) {
            str = str + cursor.getString(0) + " " + cursor.getString(1)
                    + " " + cursor.getString(2) + "\n"; // 获取一条记录的每个字段的值
            cursor.moveToNext(); // 游标指向下一条记录
        }
    }
    // 在信息框上显示所有记录信息
    new AlertDialog.Builder(Sample_8_4.this).setTitle("Message")
            .setMessage(str)
            .setNegativeButton("确定", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                }
            }).show();
}

}

Answer 1

看一下有没有commit