4.2

主要用了一些sql语句和学习了基本的增删改查的语句，完成了数据库的链接。

代码如下：

mainactivity代码：

package com.example.sqlite;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private Button generateBtn, addBtn, deleteBtn, updateBtn, selectBtn;
    private EditText et_username, et_password, et_selection;
    private TextView showInfo;
    private MyDbHelper myDbHelper;//全局变量
    private SQLiteDatabase db;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myDbHelper = new MyDbHelper(MainActivity.this, "MyDatabase.db", null, 666);
        initView();

        addBtn.setOnClickListener(this);
        selectBtn.setOnClickListener(this);
        generateBtn.setOnClickListener(this);
        deleteBtn.setOnClickListener(this);
        updateBtn.setOnClickListener(this);
    }

    private void initView() {
        generateBtn = findViewById(R.id.generate);
        addBtn = findViewById(R.id.add);
        deleteBtn = findViewById(R.id.delete);
        updateBtn = findViewById(R.id.update);
        selectBtn = findViewById(R.id.select);
        et_username = findViewById(R.id.username);
        et_password = findViewById(R.id.password);
        et_selection = findViewById(R.id.selection);
        showInfo = findViewById(R.id.showInfo);

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {

            case R.id.add:
                db = myDbHelper.getWritableDatabase();
                String username = et_username.getText().toString();
                String password = et_password.getText().toString();
                //创建一个ContentValues对象，用于存储记录的字段值，以键值对的方式存储，“键“对应就是字段名，”值"对应的就是某个字段的具体的值
                // ContentValues contentValues=new ContentValues();
                //contentValues.put("username",username);
                //contentValues.put("password",password);
                //db.insert("user",null,contentValues);
                db.execSQL("insert into user(username,password) values(?,?)", new Object[]{username, password});
                //?占位符
                db.close();
                break;
            case R.id.select:
                showInfo.setText(" ");
                db = myDbHelper.getWritableDatabase();
                //Cursor:结果集，结果集中会有游标，游标会指向结果集中的某一条记录，我们获取的记录是哪一条，初始时指向第一条记录
                Cursor cursor = db.query("user", new String[]{"username", "password"}, null, null, null, null, null, null);

                while (cursor.moveToNext()) {
                    showInfo.append("\n" + "用户名:" + cursor.getString(0) + ",密码：" + cursor.getString(1));
                }

                cursor.close();
                db.close();
                break;
            case R.id.generate:
                db = myDbHelper.getWritableDatabase();
                String selection = et_selection.getText().toString();
                //Cursor cursor1 = db.query("user", new String[]{"username", "password"}, "username=?", new String[]{selection}, null, null, null, null);
                Cursor cursor1 = db.rawQuery("select username,password from user where username=?", new String[]{selection});

                showInfo.setText("查询结果如下：");
                while (cursor1.moveToNext()) {
                    showInfo.append("\n" + "用户名:" + cursor1.getString(0) + ",密码：" + cursor1.getString(1));
                }

                cursor1.close();
                db.close();
                break;
            case R.id.delete:
                db = myDbHelper.getWritableDatabase();
                String selection1 = et_selection.getText().toString();
                int i = db.delete("user", "username=?", new String[]{selection1});
                if (i > 0) {
                    Toast.makeText(MainActivity.this, "删除成功，删除了" + i + "条！", Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(MainActivity.this, "删除失败！", Toast.LENGTH_SHORT).show();

                }
                db.close();
                break;
            case R.id.update:
                db = myDbHelper.getWritableDatabase();
                String username1 = et_username.getText().toString();
                String password1= et_password.getText().toString();
                String selection2 = et_selection.getText().toString();
                //创建一个ContentValues对象，用于存储记录的字段值，以键值对的方式存储，“键“对应就是字段名，”值"对应的就是某个字段的具体的值
                ContentValues contentValues=new ContentValues();
                contentValues.put("username",username1);
                contentValues.put("password",password1);
                db.update("user",contentValues,"username=?",new String[]{selection2});
                db.close();
                break;


        }
    }


    //快速格式化的快捷键ctrl+alt+l
    //数据库帮助类
    class MyDbHelper extends SQLiteOpenHelper {

        //构造器作用，参数含义：上下文，数据库文件名称，结果集工厂，版本号
        public MyDbHelper(@Nullable Context context, @Nullable String name, @Nullable SQLiteDatabase.CursorFactory factory, int version) {
            super(context, name, factory, version);
        }

        //数据库初始化的时候，用于创建表或者视图文件
        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL("create table user(user_id integer primary key autoincrement,userName varchar(10),password varchar(10))");
        }

        //升级方法
        @Override
        public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

        }
    }


}

布局代码如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical">

    <ImageView
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_gravity="center"
        android:src="@mipmap/ic_launcher"

        ></ImageView>

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="用户名："

            android:textColor="@color/black"></TextView>

        <EditText
            android:id="@+id/username"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:hint="请输入用户名"
            android:textColorHint="@color/black"></EditText>

    </LinearLayout>

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        >

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="密码："

            android:textColor="@color/black"></TextView>

        <EditText
            android:id="@+id/password"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:hint="请输入密码"
            android:password="true"
            android:textColorHint="@color/black"></EditText>

    </LinearLayout>
 <LinearLayout
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:orientation="vertical">
<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="条件查询"
    android:id="@+id/generate"
    ></Button>
<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hint="请输入查询用户名:"
    android:layout_gravity="center"
    android:id="@+id/selection"></EditText>

 </LinearLayout>
    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/add"
            android:onClick="dengluclick"
            android:text="添加"

            ></Button>

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/delete"
            android:onClick="zhuceclick"
            android:text="删除"

            ></Button>
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="zhuceclick"
            android:id="@+id/update"
            android:text="修改"

            ></Button>
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="zhuceclick"
            android:id="@+id/select"
            android:text="查询"

            ></Button>

    </LinearLayout>
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="显示查询结果"
        android:gravity="center"

android:layout_gravity="center"
        android:id="@+id/showInfo"
        android:textColor="@color/black"></TextView>
</LinearLayout>