Question

//packages
//imports
public class MainActivity extends Activity 
{
 // some string declarations;
 // object creations etc etc
     DatabaseManipulation dbManage = new DatabaseManipulation();//whereDatabaseManipulation is a class bdManage -->object
protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    //creating contentViews;
}

 public void methods()
 {
 //works
 }
  public void methods2(String name)
  {
   try{
  dbManage.functioncall(name);
      }
   catch(Exception e)
      {
        System.out.println(e);// value of e is NullPointerException
      }
   }


}

第二类是DatabaseManipulation.class，如下所示

public class DatabaseManipulation extends Activity
{

  public void functioncall(String name)
 {

    System.out.println(bookName+ "  bookname in addBookmark");

      SQLiteDatabase db;
      db = openOrCreateDatabase("epub.db", SQLiteDatabase.CREATE_IF_NECESSARY,null);// error occurs here
      db.setLocale(Locale.getDefault());
      db.setLockingEnabled(true);
      db.setVersion(1);

      String temp_address="nothing";


      try
      {
          String selectQuery = "SELECT lastchapter FROM Bookdetails WHERE bookpath=?";
          Cursor c = db.rawQuery(selectQuery, new String[] { name });
          if (c.moveToFirst()) {
              temp_address = c.getString(c.getColumnIndex("lastchapter")); // assigning value in lastchapter to temp_address
          }
          System.out.println(temp_address+" result of select Query");  // 
      }

  catch(Exception e)
      {
       System.out.println(e);
      }

      try
      {



      System.out.println("BLOW FILE111");



      System.out.println("BLOW FILE222");
      final String createtabBook="CREATE TABLE IF NOT EXISTS BookMark(lid INTEGER UNIQUE AUTOINCREMENT,  bookpath TEXT , lastchapter TEXT, PRIMARY KEY(bookpath,lastchapter));";
      db.execSQL(createtabBook);


      ContentValues val=new ContentValues();
      val.put("bookpath",name );
      val.put("lastchapter", temp_address);
     db.insert("Bookdetails", val, "bookpath="+name, null);
      System.out.println("BLOW FILE333");
      }
      catch(Exception e)
      {
          System.out.println(e+"  errors happens");
      }
      finally
      {
          db.close();
      }

}

}

我只是想从main活动到DatabaseManipulation.java调用这个函数，我做错了什么？在log cat中，Null指针异常显示为错误，因为我在try catch中调用它我们真的可以做这种类型的函数调用吗？请帮忙

Answer 1

您无法实例化此类活动：

DatabaseManipulation dbManage = new DatabaseManipulation();

您需要使用Intent。像这样：

Intent intent = new Intent(this, DatabaseManipulation.class);
startActivity(intent);

Answer 2

您无法在

之前初始化数据库类

super.onCreate(savedInstanceState);

方法。为您的数据库类创建全局变量

DatabaseManipulation dbManage;

并在

之后的onCreate（）方法之后初始化它

setContentView(R.layout.urlayout)

方法

dbManage = new DatabaseManipulation();

将字符串值从一个类传递到另一个类并在那里初始化db时出现空指针异常

2 个答案: