Android SQLite NullPointer 异常和堆栈溢出异常 [重复]

Question

有sql查询的线路出现NUllpointer异常和堆栈溢出异常。

display.java

 import android.app.Activity;
 import android.content.Context;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.widget.TextView;

  public class display extends Activity {
  private Context context;
  MyDBHandler helper

 @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display);
    helper = new MyDBHandler(context);

    Bundle nameValue=getIntent().getExtras();
    String vname=nameValue.getString("Name");
    TextView tvname=(TextView)findViewById(R.id.TVname);
    tvname.setText(vname);

    String dispname=helper.onSelect(vname);
    TextView tvaddress=(TextView)findViewById(R.id.TVaddress);
    tvaddress.setText(dispname);


  }
   /*public void onDisplay(){
    Bundle nameValue=getIntent().getExtras();
    if(nameValue==null){

    }
    String vname=nameValue.getString("Name");
    String dispname=helper.onSelect(vname);
    TextView tvaddress=(TextView)findViewById(R.id.TVaddress);
    tvaddress.setText(dispname);
}*/




@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);
}
}

MyDBHandler

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

public class MyDBHandler extends SQLiteOpenHelper {

private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "productDB.db";
public static final String TABLE_PRODUCTS = "products";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_NAME = "name";
public static final String COLUMN_ADDRESS = "address";


private final Context myContext;
public MyDBHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.myContext = context;
}


    @Override
     public void onCreate(SQLiteDatabase db) {
    String query = "CREATE TABLE " + TABLE_PRODUCTS + "(" +
            COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
            COLUMN_NAME + " TEXT, " +
            COLUMN_ADDRESS + " TEXT " +
            ");";
    db.execSQL(query);
    String query1="INSERT INTO"+ TABLE_PRODUCTS + "(name,address) VALUES   
    ('name1,address1');";
    String query2="INSERT INTO"+ TABLE_PRODUCTS + "(name,address) VALUES   
    ('name2,address2');";
    db.execSQL(query1);
    db.execSQL(query2);

}

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

public String onSelect(String sname){
    String dbString = "";
    SQLiteDatabase db = getWritableDatabase();
    String displayname= "SELECT address FROM"+ TABLE_PRODUCTS + " WHERE "+       


    COLUMN_NAME + "=\""+ sname + "\";";
    Cursor c = db.rawQuery(displayname, null);

   if (c.getString(c.getColumnIndex("address")) != null) {
           dbString = c.getString(c.getColumnIndex("address"));
   }
    db.close();
    return dbString;
   }

   }

空指针异常

E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.arjun.tablesqlite/com.example.arjun.tablesqlite.display}: java.lang.NullPointerException
at com.example.arjun.tablesqlite.MyDBHandler.onSelect(MyDBHandler.java:57)
at com.example.arjun.tablesqlite.display.onCreate(display.java:27)

我在 MyDbHandler.java 的这一行收到 NullPointerException

SQLiteDatabase db = getWritableDatabase();  

在 display.java 的这一行......

String dispname=helper.onSelect(vname); 

Answer 1

试试这个，

你还没有初始化上下文，你直接尝试用那个上下文初始化 DBHandler，所以它抛出了 NLP。

public class display extends Activity {

MyDBHandler helper;


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

    helper = new MyDBHandler(this);
   .... //your code
}

Answer 2

请尝试下面的 onSelect 方法并让我知道其中的任何错误：-

  public String onSelect(String sname) {
        String dbString = "";
        SQLiteDatabase db = this.getWritableDatabase();
        String displayname = "SELECT address FROM" + TABLE_PRODUCTS + " WHERE " +


                COLUMN_NAME + "='" + sname + "'";
        Cursor c = db.rawQuery(displayname, null);

/*
        if (c.getString(c.getColumnIndex("address")) != null) {
            dbString = c.getString(c.getColumnIndex("address"));
        }
*/
        if (c.moveToFirst()) {
            if (c.getString(c.getColumnIndex("address")) != null) {
                dbString = c.getString(c.getColumnIndex("address"));
            }else{
                dbString="No Address";
            }
        }else{
            dbString="No Address Found";
        }
        db.close();
        return dbString;
    }

还有这个：-

MyDBHandler helper = new MyDBHandler(this);

像下面这样改变这一行

String query1="INSERT INTO "+ TABLE_PRODUCTS + "(name,address) VALUES   
    ('name1','address1');";
String query2="INSERT INTO "+ TABLE_PRODUCTS + "(name,address) VALUES   
    ('name2','address2')";

您的数据库查询如下（供参考）：-

INSERT INTO products (name,address) VALUES ('name1','address1');

您的查询构建如下：-

插入INTOproducts（姓名、地址）VALUES（'name1'、'address1'）；

所以你得到了以下错误。