Question

使用Android Studio进行Android开发的新手。我正在构建一个可以创建和查看记录的应用程序。但是在点击创建时，它会将我重定向到主页，当我点击查看时，应用程序将关闭。

这是主页的MainActivity，只有两个按钮，创建和查看数据

public class MainActivity extends Activity {

public void createButtonClicked(View v){
    Intent create = new Intent(MainActivity.this, AddActivity2Activity.class);
    startActivity(create);
}
public void viewButtonClicked(View v){
    Intent create = new Intent(MainActivity.this, ViewActivity.class);
    startActivity(create);
}

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

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

这是用于创建新记录的输入活动

class MyDbHelper extends SQLiteOpenHelper {

private static final String DB_NAME = "mydb";
private static final int DB_VERSION = 1;

public static final String TABLE_NAME = "student";
public static final String COL_NAME = "pName";
public static final String COL_EMAIL = "pEmail";
public static final String COL_TEXT = "pText";
public static final String COL_DATE = "pDate";
private static final String STRING_CREATE = "CREATE TABLE "+TABLE_NAME+" (_id INTEGER PRIMARY KEY AUTOINCREMENT, "
        +COL_NAME+" TEXT, "+COL_EMAIL+"TEXT," +COL_TEXT +" TEXT," +COL_DATE +" DATE);";

public MyDbHelper(Context context) {
    super(context, DB_NAME, null, DB_VERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(STRING_CREATE);
    ContentValues cv = new ContentValues();
    cv.put(COL_NAME, "New Name");
    cv.put(COL_EMAIL, "New Email");
    cv.put(COL_TEXT, "New Text");
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    cv.put(COL_DATE, dateFormat.format(new Date()));
    db.insert(TABLE_NAME, null, cv);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
    onCreate(db);
}
}

public class AddActivity2Activity extends Activity implements View.OnClickListener{

EditText mText;
EditText mText1;
EditText mText2;
Button mAdd;

MyDbHelper mHelper;
SQLiteDatabase mDb;
Cursor mCursor;
SimpleCursorAdapter mAdapter;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add_activity2);

    mText = (EditText)findViewById(R.id.name);
    mText1 = (EditText)findViewById(R.id.email);
    mText2 = (EditText)findViewById(R.id.comment);
    mAdd = (Button)findViewById(R.id.add);
    mAdd.setOnClickListener(this);


    mHelper = new MyDbHelper(this);
}

 @Override
public void onPause() {
    super.onPause();
    mDb.close();
    mCursor.close();
}

public void onClick(View v) {
    ContentValues cv = new ContentValues();
    cv.put(MyDbHelper.COL_NAME, mText.getText().toString());
    cv.put(MyDbHelper.COL_EMAIL, mText1.getText().toString());
    cv.put(MyDbHelper.COL_TEXT, mText2.getText().toString());
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    cv.put(MyDbHelper.COL_DATE, dateFormat.format(new Date())); //Insert 'now' as the date
    mDb.insert(MyDbHelper.TABLE_NAME, null, cv);
    mCursor.requery();
    mAdapter.notifyDataSetChanged();
    mText.setText(null);
    mText1.setText(null);
    mText2.setText(null);
   Toast.makeText(getApplicationContext(), "Record Created Successfully" , Toast.LENGTH_LONG).show();
}

public void viewButtonClicked(View v){
    Intent intent = new Intent(AddActivity2Activity.this, ViewActivity.class);
    startActivity(intent);
}
}

这是视图活动

public class ViewActivity extends Activity {

MyDbHelper mHelper;
SQLiteDatabase mDb;
Cursor mCursor;
private ListView mList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view);
    mList = (ListView)findViewById(R.id.list);
    mDb = mHelper.getWritableDatabase();
    String[] columns = new String[] {"_id", MyDbHelper.COL_NAME, MyDbHelper.COL_EMAIL, MyDbHelper.COL_TEXT, MyDbHelper.COL_DATE};
    mCursor = mDb.query(MyDbHelper.TABLE_NAME, columns, null, null, null, null, null, null);
    String[] headers = new String[] {MyDbHelper.COL_NAME,  MyDbHelper.COL_EMAIL, MyDbHelper.COL_TEXT, MyDbHelper.COL_DATE};
    List<String> itemsList = new ArrayList<String>();
    itemsList.add( MyDbHelper.COL_NAME );
    itemsList.add(MyDbHelper.COL_EMAIL) ;
    itemsList.add ( MyDbHelper.COL_TEXT ) ;
    itemsList.add ( MyDbHelper.COL_DATE ) ;
    ArrayAdapter<String> adapter = new ArrayAdapter<String>( this, android.R.layout.simple_list_item_1, itemsList ) ;
    mList.setAdapter(adapter);

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_view, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

如果你能帮助我修复这个错误，我将非常感激

Answer 1

首先，确保您已在Manifest.xml中添加了所有活动

<activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".ViewActivity"/>
        <activity android:name=".AddActivity2Activity"/>

其次，您尚未在AddActivity2Activity中初始化数据库对象。所以在onCreate函数中添加以下内容：

mDb = mHelper.getWritableDatabase();

代码中还有其他问题，我不知道你究竟想用它做什么。 - 当您为其调用requery（）函数时，您的游标将为null。 - 同样删除以下行，您在活动中不需要它们：

mCursor.requery();
mAdapter.notifyDataSetChanged();

在ViewActivity中，您应首先初始化您的助手：

mHelper = new MyDbHelper(this);

最后一个：在MyDbHelper类中创建表时，请将STRING_CREATE替换为以下内容：

private static final String STRING_CREATE = "CREATE TABLE "+TABLE_NAME+" (_id INTEGER PRIMARY KEY AUTOINCREMENT, "
            + COL_NAME + " TEXT, " + COL_EMAIL + " TEXT, " + COL_TEXT + " TEXT," + COL_DATE + " DATE);";

事实上，你错过了几个空格字符。

干杯

编辑：

这是AddActivity2Activity

import android.app.Activity;
import android.content.ContentValues;
import android.content.Intent;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.SimpleCursorAdapter;
import android.widget.Toast;

import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * Created by amintavassolian on 15-06-05.
 */
public class AddActivity2Activity extends Activity implements View.OnClickListener{

    EditText mText;
    EditText mText1;
    EditText mText2;
    Button mAdd;

    MyDbHelper mHelper;
    SQLiteDatabase mDb;
    Cursor mCursor;
    SimpleCursorAdapter mAdapter;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_activity2);

        mText = (EditText)findViewById(R.id.name);
        mText1 = (EditText)findViewById(R.id.email);
        mText2 = (EditText)findViewById(R.id.comment);
        mAdd = (Button)findViewById(R.id.add);
        mAdd.setOnClickListener(this);


        mHelper = new MyDbHelper(this);
        mDb = mHelper.getWritableDatabase();
    }

    @Override
    public void onPause() {
        super.onPause();
        mDb.close();
    }

    public void onClick(View v) {
        ContentValues cv = new ContentValues();
        cv.put(MyDbHelper.COL_NAME, mText.getText().toString());
        cv.put(MyDbHelper.COL_EMAIL, mText1.getText().toString());
        cv.put(MyDbHelper.COL_TEXT, mText2.getText().toString());
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        cv.put(MyDbHelper.COL_DATE, dateFormat.format(new Date())); //Insert 'now' as the date
        mDb.insert(MyDbHelper.TABLE_NAME, null, cv);

        mText.setText(null);
        mText1.setText(null);
        mText2.setText(null);
        Toast.makeText(getApplicationContext(), "Record Created Successfully", Toast.LENGTH_LONG).show();
    }

    public void viewButtonClicked(View v){
        Intent intent = new Intent(AddActivity2Activity.this, ViewActivity.class);
        startActivity(intent);
    }
}

这是MyDbHelper：

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

import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * Created by amintavassolian on 15-06-05.
 */
class MyDbHelper extends SQLiteOpenHelper {

    private static final String DB_NAME = "mydb";
    private static final int DB_VERSION = 1;

    public static final String TABLE_NAME = "student";
    public static final String COL_NAME = "pName";
    public static final String COL_EMAIL = "pEmail";
    public static final String COL_TEXT = "pText";
    public static final String COL_DATE = "pDate";
    private static final String STRING_CREATE = "CREATE TABLE "+TABLE_NAME+" (_id INTEGER PRIMARY KEY AUTOINCREMENT, "
            + COL_NAME + " TEXT, " + COL_EMAIL + " TEXT, " + COL_TEXT + " TEXT," + COL_DATE + " DATE);";

    public MyDbHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(STRING_CREATE);
        ContentValues cv = new ContentValues();
        cv.put(COL_NAME, "New Name");
        cv.put(COL_EMAIL, "New Email");
        cv.put(COL_TEXT, "New Text");
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        cv.put(COL_DATE, dateFormat.format(new Date()));
        db.insert(TABLE_NAME, null, cv);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
        onCreate(db);
    }
}

这是ViewActivity

import android.app.Activity;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView;

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

/**
 * Created by amintavassolian on 15-06-05.
 */
public class ViewActivity extends Activity {

    MyDbHelper mHelper;
    SQLiteDatabase mDb;
    Cursor mCursor;
    private ListView mList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view);
        mHelper = new MyDbHelper(this);
        mList = (ListView)findViewById(R.id.list);
        mDb = mHelper.getWritableDatabase();
        String[] columns = new String[] {"_id", MyDbHelper.COL_NAME, MyDbHelper.COL_EMAIL, MyDbHelper.COL_TEXT, MyDbHelper.COL_DATE};
        mCursor = mDb.query(MyDbHelper.TABLE_NAME, columns, null, null, null, null, null, null);
        String[] headers = new String[] {MyDbHelper.COL_NAME,  MyDbHelper.COL_EMAIL, MyDbHelper.COL_TEXT, MyDbHelper.COL_DATE};
        List<String> itemsList = new ArrayList<String>();
        itemsList.add( MyDbHelper.COL_NAME );
        itemsList.add(MyDbHelper.COL_EMAIL) ;
        itemsList.add ( MyDbHelper.COL_TEXT ) ;
        itemsList.add ( MyDbHelper.COL_DATE ) ;
        ArrayAdapter<String> adapter = new ArrayAdapter<String>( this, android.R.layout.simple_list_item_1, itemsList ) ;
        mList.setAdapter(adapter);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_view, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

干杯甲

android：如何在数组列表视图中获取和显示数据

1 个答案: