Question

我目前正在开发一个Android应用程序，我的登出按钮出现问题。每当我在运行时按下它时，应用程序就会停止。此外，在运行我的应用程序时显示ErrorsDashboard屏幕，而我想要显示的是登录屏幕。这将是一个冗长的帖子，所以请耐心等待我。这是我的代码：

对于LoginActivity

package com.example.sabre7;

import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.example.sabre7.library.DatabaseHandler;
import com.example.sabre7.library.UserFunctions;

public class LoginActivity extends Activity {
    Button btnLogin;
    //Button btnLinkToRegister;
    EditText inputEmail;
    EditText inputPassword;
    TextView loginErrorMsg;

    // JSON Response node names
    private static String KEY_SUCCESS = "success";
    private static String KEY_UID = "uid";
    private static String KEY_NAME = "name";
    private static String KEY_EMAIL = "email";
    private static String KEY_CREATED_AT = "created_at";

    private class MyAsyncTask extends AsyncTask<String, Void, JSONObject> {

        protected JSONObject doInBackground(String... params) {
                UserFunctions userFunction = new UserFunctions();
                if (params.length != 2)
                        return null;
                JSONObject json = userFunction.loginUser(params[0], params[1]);
                return json;
        }

        protected void onPostExecute(JSONObject json) {
                try {
            if (json != null && json.getString(KEY_SUCCESS) != null) {
                loginErrorMsg.setText("");
                String res = json.getString(KEY_SUCCESS);
                if(Integer.parseInt(res) == 1){
                    // user successfully logged in
                    // Store user details in SQLite Database
                    DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                    JSONObject json_user = json.getJSONObject("user");

                    // Clear all previous data in database
                    UserFunctions userFunction = new UserFunctions();
                    userFunction.logoutUser(getApplicationContext());
                    db.addUser(json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));                        

                    // Launch Main Screen
                    Intent main = new Intent(getApplicationContext(), MainActivity.class);

                    // Close all views before launching Dashboard
                    main.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                    startActivity(main);

                    // Close Login Screen
                    finish();
                }else{
                    // Error in login
                    loginErrorMsg.setText("Incorrect username/password");
                }
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }
        }
}

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

        // Importing all assets like buttons, text fields
        inputEmail = (EditText) findViewById(R.id.loginEmail);
        inputPassword = (EditText) findViewById(R.id.loginPassword);
        btnLogin = (Button) findViewById(R.id.btnLogin);
        //btnLinkToRegister = (Button) findViewById(R.id.btnLinkToRegisterScreen);
        loginErrorMsg = (TextView) findViewById(R.id.login_error);

        // Login button Click Event
        btnLogin.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {
                String email = inputEmail.getText().toString();
                String password = inputPassword.getText().toString();                
                new MyAsyncTask().execute(email, password);
            }

        });
    }
}

对于MainActivity

package com.example.sabre7;

import com.example.sabre7.library.UserFunctions;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity{
    UserFunctions userFunctions;
    Button Errorsbtn;
    Button Serversbtn;
    Button btnLogout;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Check login status in database
        userFunctions = new UserFunctions();
        if(userFunctions.isUserLoggedIn(getApplicationContext())){
        // user already logged in show databoard
            setContentView(R.layout.main);
            btnLogout = (Button) findViewById(R.id.btnLogout);

            btnLogout.setOnClickListener(new View.OnClickListener() {

                public void onClick(View arg0) {
                    // TODO Auto-generated method stub
                    userFunctions.logoutUser(getApplicationContext());
                    Intent login = new Intent(getApplicationContext(), LoginActivity.class);
                    login.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                    startActivity(login);
                    // Closing dashboard screen
                    finish();
                }
            });

            Errorsbtn.setOnClickListener(new View.OnClickListener() {

                public void onClick(View arg0) {
                    // TODO Auto-generated method stub
                    Intent Errors = new Intent(getApplicationContext(), ErrorsDashboardActivity.class);
                    startActivity(Errors);
                    // Closing dashboard screen
                    finish();
                }
            });

        }else{
            // user is not logged in show login screen
            Intent login = new Intent(getApplicationContext(), LoginActivity.class);
            login.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            startActivity(login);
            // Closing dashboard screen
            finish();
        }

    }

}

对于ErrorsDashboardActivity

package com.example.sabre7;

import org.achartengine.ChartFactory;
import org.achartengine.GraphicalView;
import org.achartengine.chart.BarChart.Type;
import org.achartengine.chart.PointStyle;
import org.achartengine.model.CategorySeries;
import org.achartengine.model.SeriesSelection;
import org.achartengine.model.XYMultipleSeriesDataset;
import org.achartengine.model.XYSeries;
import org.achartengine.renderer.XYMultipleSeriesRenderer;
import org.achartengine.renderer.XYSeriesRenderer;

import android.app.Activity;
import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Toast;

import com.example.sabre7.library.UserFunctions;

public class ErrorsDashboardActivity extends Activity {
    UserFunctions userFunctions;
    Button btnLogout;

    // First Create a Graphical View object called mChart.
    private GraphicalView mChart;
    private String[] mErrors = new String[] {"E0001", "E0002" , "E0003", "E0004", "E0005"};

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.dashboard);
        btnLogout = (Button) findViewById(R.id.btnLogout);

        btnLogout.setOnClickListener(new View.OnClickListener() {

            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                userFunctions.logoutUser(getApplicationContext());  //this is line 43
                Intent login = new Intent(getApplicationContext(), LoginActivity.class);
                login.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(login);
                // Closing dashboard screen
                finish();
            }
        });
        LineChart();
        BarChart();
    }

    //LINE CHART
     private void LineChart()
     {
     // Define the number of elements you want in the chart.
     int z[]={0,1,2,3,4};
     int x[]={10,18,32,21,48};


     // Create XY Series for X Series.
     XYSeries xSeries=new XYSeries("X Series");

     //  Adding data to the X Series.
     for(int i = 0; i < z.length; i++)
     {
      xSeries.add(z[i], x[i]);

     }

     // Create a Dataset to hold the XSeries.
     XYMultipleSeriesDataset dataset = new XYMultipleSeriesDataset();

     // Add X series to the Dataset.   
     dataset.addSeries(xSeries);

     // Create XYSeriesRenderer to customize XSeries
     XYSeriesRenderer Xrenderer=new XYSeriesRenderer();
     Xrenderer.setColor(Color.GREEN);
     Xrenderer.setPointStyle(PointStyle.SQUARE);
     Xrenderer.setDisplayChartValues(true);
     Xrenderer.setLineWidth(2);
     Xrenderer.setFillPoints(true);

     // Create XYMultipleSeriesRenderer to customize the whole chart
     XYMultipleSeriesRenderer mRenderer=new XYMultipleSeriesRenderer();
     mRenderer.setChartTitle("Top 5 Errors");
     mRenderer.setXTitle("Errors");
     mRenderer.setYTitle("Instances");
     mRenderer.setLabelsTextSize(16);
     mRenderer.setLegendTextSize(20);
     mRenderer.setShowGrid(true);
     mRenderer.setExternalZoomEnabled(true);
     mRenderer.setYLabelsPadding(2);
     mRenderer.setAxesColor(Color.GREEN);
     //mRenderer.setChartValuesTextSize(15);
     mRenderer.setZoomButtonsVisible(true);
     mRenderer.setBackgroundColor(Color.WHITE);
     mRenderer.setXLabels(0);
     mRenderer.setShowGrid(true);
     mRenderer.setClickEnabled(true);
     for(int i=0;i<z.length;i++)
     {
      mRenderer.addXTextLabel(i, mErrors[i]);
     }

     // Adding the XSeriesRenderer to the MultipleRenderer. 
     mRenderer.addSeriesRenderer(Xrenderer);     
     LinearLayout chart_container=(LinearLayout)findViewById(R.id.Chart_layout);

     // Creating an intent to plot line chart using dataset and multipleRenderer
     mChart=(GraphicalView)ChartFactory.getLineChartView(getBaseContext(), dataset, mRenderer);

     //  Adding click event to the Line Chart.
     mChart.setOnClickListener(new View.OnClickListener() {
     @Override
     public void onClick(View arg0) {
     // TODO Auto-generated method stub

     SeriesSelection series_selection=mChart.getCurrentSeriesAndPoint();

     if(series_selection!=null)
     {
     int series_index=series_selection.getSeriesIndex();

     String select_series="X Series";
     if(series_index==0)
     {
      select_series="X Series";
     }else
     {
      select_series="Y Series";
     }

     String servers=mErrors[(int)series_selection.getXValue()];
     int amount=(int)series_selection.getValue();
     Toast.makeText(getBaseContext(), select_series + "in" + servers + ":" + amount, Toast.LENGTH_LONG).show();
     }
     }
     });

    // Add the graphical view mChart object into the Linear layout.
     chart_container.addView(mChart);
    }


     //BAR CHART
     private void BarChart(){

         //Defining the number of elements in chart
         int z[]={0,5,10,15,20};
         int x[]={14,5,10,20,16};

         CategorySeries series = new CategorySeries("Error Bar Graph");
         for (int i = 0; i < x.length; i++ ){
             series.add("Bar " + (i+1), x[i]);

         }

         XYMultipleSeriesDataset dataset = new XYMultipleSeriesDataset();
         dataset.addSeries(series.toXYSeries());

        // Create XYMultipleSeriesRenderer to customize the whole chart
         XYMultipleSeriesRenderer mRenderer = new XYMultipleSeriesRenderer();
         mRenderer.setBackgroundColor(Color.WHITE);
         mRenderer.setBarSpacing(1);
         mRenderer.setChartTitle("Top 5 Errors");
         mRenderer.setXTitle("Errors");
         mRenderer.setYTitle("Instances");
         mRenderer.setLabelsTextSize(16);
         mRenderer.setLegendTextSize(20);
         mRenderer.setExternalZoomEnabled(true);
         mRenderer.setYLabelsPadding(2);
         mRenderer.setShowGrid(true);
         mRenderer.setClickEnabled(true);
         mRenderer.setAxesColor(Color.GREEN);

         for(int i=0;i<z.length;i++)
         {
          mRenderer.addXTextLabel(i, mErrors[i]);
         }

         // Create XYSeriesRenderer to customize XSeries
         XYSeriesRenderer renderer = new XYSeriesRenderer();
         renderer.setShowLegendItem(true);
         renderer.setColor(Color.GREEN);         

         // Adding the XSeriesRenderer to the MultipleRenderer. 
         mRenderer.addSeriesRenderer(renderer);
         LinearLayout Barchart_container = (LinearLayout)findViewById(R.id.BarChart_layout);

         // Creating an intent to plot bar chart using dataset and multipleRenderer
         mChart=(GraphicalView)ChartFactory.getBarChartView(getBaseContext(), dataset, mRenderer, Type.DEFAULT);

         // Add the graphical view mChart object into the Linear layout.
         Barchart_container.addView(mChart);

         //  Adding click event to the Bar Chart.
         mChart.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View arg0) {
         // TODO Auto-generated method stub

         SeriesSelection series_selection=mChart.getCurrentSeriesAndPoint();

         if(series_selection!=null)
         {
         int series_index=series_selection.getSeriesIndex();

         String select_series="X Series";
         if(series_index==0)
         {
          select_series="X Series";
         }else
         {
          select_series="Y Series";
         }

         String servers=mErrors[(int)series_selection.getXValue()];
         int amount=(int)series_selection.getValue();
         Toast.makeText(getBaseContext(), select_series + "in" + servers + ":" + amount, Toast.LENGTH_LONG).show();
         }
         }
         });
     }
}

对于UserFunctions活动

package com.example.sabre7.library;

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

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONObject;

import android.content.Context;

public class UserFunctions {

    private JSONParser jsonParser;

    // Testing in localhost using wamp or xampp 
    // use http://10.0.2.2/ to connect to your localhost ie http://localhost/
    private static String loginURL = "http://10.0.2.2/Sabre1/";

    private static String login_tag = "login";

    // constructor
    public UserFunctions(){
        jsonParser = new JSONParser();
    }

    /**
     * function make Login Request
     * @param email
     * @param password
     * */
    public JSONObject loginUser(String email, String password){
        // Building Parameters
        List<NameValuePair> params = new ArrayList<NameValuePair>();
        params.add(new BasicNameValuePair("tag", login_tag));
        params.add(new BasicNameValuePair("email", email));
        params.add(new BasicNameValuePair("password", password));
        JSONObject json = jsonParser.makeHttpRequest(loginURL, "POST", params);
        return json;
    }

    /**
     * Function get Login status
     **/
    public boolean isUserLoggedIn(Context context){
        DatabaseHandler db = new DatabaseHandler(context);
        int count = db.getRowCount();
        if(count > 0){
            // user logged in
            return true;
        }
        return false;
    }

    /**
     * Function to logout user
     * Reset Database
     * */
    public boolean logoutUser(Context context){
        DatabaseHandler db = new DatabaseHandler(context);
        db.resetTables();
        return true;
    }
}

对于DatabaseHandler活动

package com.example.sabre7.library;

import java.util.HashMap;

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

public class DatabaseHandler extends SQLiteOpenHelper {


    // All Static variables
    // Database Version
    private static final int DATABASE_VERSION = 1;

    // Database Name
    private static final String DATABASE_NAME = "android_api";

    // Login table name
    private static final String TABLE_LOGIN = "login";

    // Login Table Columns names
    private static final String KEY_ID = "id";
    private static final String KEY_NAME = "name";
    private static final String KEY_EMAIL = "email";
    private static final String KEY_UID = "uid";
    private static final String KEY_CREATED_AT = "created_at";

    public DatabaseHandler(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    // Creating Tables
    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_LOGIN_TABLE = "CREATE TABLE " + TABLE_LOGIN + "("
                + KEY_ID + " INTEGER PRIMARY KEY,"
                + KEY_NAME + " TEXT,"
                + KEY_EMAIL + " TEXT UNIQUE,"
                + KEY_UID + " TEXT,"
                + KEY_CREATED_AT + " TEXT" + ")";
        db.execSQL(CREATE_LOGIN_TABLE);
    }

    // Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_LOGIN);

        // Create tables again
        onCreate(db);
    }

    /**
     * Storing user details in database
     */
    public void addUser(String name, String email, String uid, String created_at) {
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(KEY_NAME, name); // Name
        values.put(KEY_EMAIL, email); // Email
        values.put(KEY_UID, uid); // Email
        values.put(KEY_CREATED_AT, created_at); // Created At

        // Inserting Row
        db.insert(TABLE_LOGIN, null, values);
        db.close(); // Closing database connection
    }

    /**
     * Getting user data from database
     * */
    public HashMap<String, String> getUserDetails(){
        HashMap<String,String> user = new HashMap<String,String>();
        String selectQuery = "SELECT  * FROM " + TABLE_LOGIN;

        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);
        // Move to first row
        cursor.moveToFirst();
        if(cursor.getCount() > 0){
            user.put("name", cursor.getString(1));
            user.put("email", cursor.getString(2));
            user.put("uid", cursor.getString(3));
            user.put("created_at", cursor.getString(4));
        }
        cursor.close();
        db.close();
        // return user
        return user;
    }

    /**
     * Getting user login status
     * return true if rows are there in table
     * */
    public int getRowCount() {
        String countQuery = "SELECT  * FROM " + TABLE_LOGIN;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(countQuery, null);
        int rowCount = cursor.getCount();
        db.close();
        cursor.close();

        // return row count
        return rowCount;
    }

    /**
     * Re create database
     * Delete all tables and create them again
     * */
    public void resetTables(){
        SQLiteDatabase db = this.getWritableDatabase();
        // Delete All Rows
        db.delete(TABLE_LOGIN, null, null);
        db.close();
    }
}

我的dashboard.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@drawable/adservereport">

    <TextView android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="@string/DashboardWelcometxt"
              android:textSize="20sp"
              android:gravity="center"
              android:layout_marginTop="50dip"/>

    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="170dp"
        android:id="@+id/Chart_layout"
        android:orientation="vertical"
        android:textSize="15sp"
        android:layout_marginLeft="45dp" 
        android:layout_marginRight="45dp"
        android:layout_marginTop="20dp" >
    </LinearLayout>

    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="170dp"
        android:id="@+id/BarChart_layout"
        android:orientation="vertical"
        android:textSize="15sp"
        android:layout_marginLeft="45dp" 
        android:layout_marginRight="45dp"
        android:layout_marginTop="0dp" >
    </LinearLayout>

    <Button
        android:id="@+id/btnLogout"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/LogOutbtntxt"
        android:textSize="20sp"
        android:layout_marginLeft="40dp"
        android:layout_marginRight="40dp"/>

</LinearLayout>

我的logcat错误：

01-28 20:52:58.729: E/AndroidRuntime(1290): FATAL EXCEPTION: main
01-28 20:52:58.729: E/AndroidRuntime(1290): Process: com.example.sabre7, PID: 1290
01-28 20:52:58.729: E/AndroidRuntime(1290): java.lang.NullPointerException
01-28 20:52:58.729: E/AndroidRuntime(1290):     at com.example.sabre7.ErrorsDashboardActivity$1.onClick(ErrorsDashboardActivity.java:43)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.view.View.performClick(View.java:4424)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.view.View$PerformClick.run(View.java:18383)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.os.Handler.handleCallback(Handler.java:733)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.os.Handler.dispatchMessage(Handler.java:95)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.os.Looper.loop(Looper.java:137)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at android.app.ActivityThread.main(ActivityThread.java:4998)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at java.lang.reflect.Method.invokeNative(Native Method)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at java.lang.reflect.Method.invoke(Method.java:515)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
01-28 20:52:58.729: E/AndroidRuntime(1290):     at dalvik.system.NativeStart.main(Native Method)

真的希望你能提供帮助。谢谢！

Answer 1

你在这里获得NPE

userFunctions.logoutUser(getApplicationContext());

因为您尚未初始化userFunctions。你在这里声明

UserFunctions userFunctions;

但是你需要在尝试调用函数之前初始化它（给它一个值）。

如果您在Activity开始时显示错误的Activity，我会假设您Activity中声明的manifest.xml错误为启动器。请确保在<actviity>的{{1}}标记中，您Login Activity上只有IntentFilter 。像

这样的东西

Activity

请在发布时使用最相关的代码开始工作。这是太多的代码。如果我们认为需要，我们总是可以请求更多代码。

Answer 2

btnLogout = (Button) findViewById(R.id.btnLogout);
userFunctions = new  UserFunctions(); // need to add this line

NullPointerException和不需要的屏幕加载

2 个答案: