如何解決android studio中的NullPointerException錯誤？

我正在嘗試將我的android應用程序連接到MS SQL Server 2008.我正在使用Android Studio。這是我的MainActivity.java文件：如何解決android studio中的NullPointerException錯誤？

import android.annotation.SuppressLint; 
import android.os.Bundle; 
import android.os.StrictMode; 
import android.support.v7.app.ActionBarActivity; 
import android.util.Log; 
import android.view.View; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.PreparedStatement; 
import java.sql.SQLException; 
import java.sql.Statement; 

public class MainActivity extends ActionBarActivity { 

Button add; 
TextView errorlbl; 
EditText name, address, pincode; 
Connection connect; 
PreparedStatement preparedStatement; 
Statement st; 
String ipaddress, db, username, password; 

@SuppressLint("NewApi") 
private Connection ConnectionHelper(String user, String password, 
            String database, String server) { 
    StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder() 
      .permitAll().build(); 
    StrictMode.setThreadPolicy(policy); 
    Connection connection = null; 
    String ConnectionURL = null; 
    try { 
     Class.forName("net.sourceforge.jtds.jdbc.Driver"); 
     ConnectionURL = "jdbc:jtds:sqlserver://" + server + ";" 
       + "databaseName=" + database + ";user=" + user 
       + ";password=" + password + ";"; 
     connection = DriverManager.getConnection(ConnectionURL); 
    } catch (SQLException se) { 
     Log.e("ERRO", se.getMessage()); 
    } catch (ClassNotFoundException e) { 
     Log.e("ERRO", e.getMessage()); 
    } catch (Exception e) { 
     Log.e("ERRO", e.getMessage()); 
    } 
    return connection; 
} 

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

    add = (Button) findViewById(R.id.btnadd); 

    errorlbl = (TextView) findViewById(R.id.lblerror); 

    name = (EditText) findViewById(R.id.txtname); 
    address = (EditText) findViewById(R.id.txtaddress); 
    pincode = (EditText) findViewById(R.id.txtpincode); 

    ipaddress = "192.168.0.9"; 
    db = "mydatabase"; 
    username = "myusername"; 
    password = "mypassword"; 
    connect = ConnectionHelper(username, password, db, ipaddress); 
    add.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      try { 


     connect = ConnectionHelper(username, password, db, ipaddress); 
     st = connect.createStatement(); 

       preparedStatement = connect 
         .prepareStatement("insert into studentRecord(Name,Address,Pincode) values ('" 
           + name.getText().toString() 
           + "','" 
           + address.getText().toString() 
           + "','" 
           + pincode.getText().toString() + "')"); 
       preparedStatement.executeUpdate(); 
       errorlbl.setText("Data Added successfully"); 
      } catch (SQLException e) { 
       errorlbl.setText(e.getMessage().toString()); 
      } 

     } 
    }); 

} 
}

但我發現了以下錯誤：

java.lang.NullPointerException: Attempt to invoke interface method 'java.sql.Statement java.sql.Connection.createStatement()' on a null object reference at app.mysqlapp.MainActivity$1.onClick(MainActivity.java:77)

錯誤指向下面的代碼

st = connect.createStatement();

如何我解決這個問題？

我已經包含了jtds-1.2.7 jar文件，並且我已經正確設置了清單文件。

我是Android開發的初學者。請幫我解決這個問題。謝謝！

來源

2015-06-30 Alexander Jagannathan

URL format for jTDS是：

jdbc:jtds:<server_type>://<server>[:<port>][/<database>][;<property>=<value>[;...]]

在字符串中有2個額外的;，而遺漏/：

"jdbc:jtds:sqlserver://" + server + ";"  + "databaseName=" + database + 
            ↑ here! ↑ here!                 

";user=" + user + ";password=" + password + ";"; 
              ↑ here!

我想這一點：

"jdbc:jtds:sqlserver://" + server + "/databaseName=" + database + ";user=" + user + ";password=" + password;

您將修復你的問題...如果沒有，請分享你的ConnectionURL字符串（System.out或調試），我會重新檢查結果。

來源

2015-06-30 09:38:47

如何解決android studio中的NullPointerException錯誤？

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