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！
答案 0 :(得分：0)
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
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。
答案 1 :(得分：0)
btnLogout = (Button) findViewById(R.id.btnLogout);
userFunctions = new UserFunctions(); // need to add this line