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提示：若报错，检查包名是否一致，还有在AndroidManifest.xml中是否配置好

MainActivity.java文件代码

    package com.example.wzx.myapplication;
    
    import android.content.ContentValues;
    import android.content.Context;
    import android.database.Cursor;
    import android.database.sqlite.SQLiteDatabase;
    import android.database.sqlite.SQLiteOpenHelper;
    import android.support.v7.app.AppCompatActivity;
    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 {
   
        MyHelper myHelper;
        private EditText mEtName;
        private EditText mEtPhone;
        private EditText mEtEmail;
        private TextView mTvShow;
        private Button mBtAdd;
        private Button mBtQuery;
        private Button mBtUpdate;
        private Button mBtDelete;
        public String name;
        public String phone;
        public String email;
        public SQLiteDatabase db;
        public ContentValues values;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
   
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            myHelper = new MyHelper(this);
            init();
        }
        //实例化界面，获取各个控件的值
        private void init() {
   
            mEtName = (EditText) findViewById(R.id.et_name);
            mEtPhone = (EditText) findViewById(R.id.et_phone);
            mEtEmail = (EditText) findViewById(R.id.et_email);
            mTvShow = (TextView) findViewById(R.id.ld_show);
            mBtAdd = (Button) findViewById(R.id.btn_add);
            mBtUpdate = (Button) findViewById(R.id.btn_update);
            mBtQuery = (Button) findViewById(R.id.btn_query);
            mBtDelete = (Button) findViewById(R.id.btn_delete);
            mBtAdd.setOnClickListener(this);
            mBtUpdate.setOnClickListener(this);
            mBtQuery.setOnClickListener(this);
            mBtDelete.setOnClickListener(this);
        }
    
        @Override
        //点击事件
        public void onClick(View v) {
   
            switch(v.getId())