试图结束会话时java应用程序崩溃。。空对象引用
我试图实现一个注销按钮的应用程序,我正在开发。当我从导航菜单会话中单击注销时。logoutUser();调用,并在我的IDE中导致以下错误
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ravi.myapplication, PID: 32694
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.ravi.myapplication.SessionHandler.logoutUser()' on a null object reference
at com.example.ravi.myapplication.BaseActivity$1.onNavigationItemSelected(BaseActivity.java:57)
at 安卓.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:154)
at 安卓.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
at 安卓.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
at 安卓.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
at 安卓.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342)
at 安卓.view.View.performClick(View.java:6597)
at 安卓.view.View.performClickInternal(View.java:6574)
at 安卓.view.View.access$3100(View.java:778)
at 安卓.view.View$PerformClick.run(View.java:25885)
at 安卓.os.Handler.handleCallback(Handler.java:873)
at 安卓.os.Handler.dispatchMessage(Handler.java:99)
at 安卓.os.Looper.loop(Looper.java:193)
at 安卓.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.安卓.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:858)
当我在基本活动类的开头调用session类时,我收到一个从未分配的私有字段警告。如果有人能解释为什么会发生这种情况,我们将非常感激!课程代码如下:
基本活动。java
package com.example.ravi.myapplication;
import 安卓.content.Intent;
import 安卓.net.Uri;
import 安卓.os.Bundle;
import 安卓.support.design.widget.NavigationView;
import 安卓.support.v4.widget.DrawerLayout;
import 安卓.support.v7.app.ActionBarDrawerToggle;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.support.v7.widget.Toolbar;
import 安卓.view.MenuItem;
import 安卓.widget.Spinner;
public class BaseActivity extends AppCompatActivity {
private SessionHandler session;
DrawerLayout drawerLayout;
ActionBarDrawerToggle actionBarDrawerToggle;
Toolbar toolbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_base);
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawerLayout.setDrawerListener(actionBarDrawerToggle);
navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
@Override
public boolean onNavigationItemSelected(MenuItem item) {
final String appPackageName = getPackageName();
switch (item.getItemId()) {
case R.id.nav_dashboard:
Intent dash = new Intent(getApplicationContext(), MainActivity.class);
startActivity(dash);
// finish();
drawerLayout.closeDrawers();
break;
case R.id.nav_about_us:
Intent anIntent = new Intent(getApplicationContext(), AboutUS.class);
startActivity(anIntent);
// finish();
drawerLayout.closeDrawers();
break;
case R.id.nav_logout:
session.logoutUser();
Intent i = new Intent(getApplicationContext(), LoginActivity.class);
startActivity(i);
break;
}
return false;
}
});
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
actionBarDrawerToggle.syncState();
}
@Override
public void finish() {
super.finish();
overridePendingTransitionExit();
}
@Override
public void startActivity(Intent intent) {
super.startActivity(intent);
overridePendingTransitionEnter();
}
/**
* Overrides the pending Activity transition by performing the "Enter" animation.
*/
protected void overridePendingTransitionEnter() {
overridePendingTransition(R.anim.slide_from_right, R.anim.slide_to_left);
}
/**
* Overrides the pending Activity transition by performing the "Exit" animation.
*/
protected void overridePendingTransitionExit() {
overridePendingTransition(R.anim.slide_from_left, R.anim.slide_to_right);
}
}
会话处理程序。java
package com.example.ravi.myapplication;
import 安卓.content.Context;
import 安卓.content.SharedPreferences;
import java.util.Date;
public class SessionHandler {
private static final String PREF_NAME = "UserSession";
private static final String KEY_USERNAME = "username";
private static final String KEY_EXPIRES = "expires";
private static final String KEY_FULL_NAME = "full_name";
private static final String KEY_EMPTY = "";
private Context mContext;
private SharedPreferences.Editor mEditor;
private SharedPreferences mPreferences;
public SessionHandler(Context mContext) {
this.mContext = mContext;
mPreferences = mContext.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
this.mEditor = mPreferences.edit();
}
/**
* Logs in the user by saving user details and setting session
*/
public void loginUser(String username, String fullName) {
mEditor.putString(KEY_USERNAME, username);
mEditor.putString(KEY_FULL_NAME, fullName);
Date date = new Date();
//Set user session for next 7 days
long millis = date.getTime() + (7 * 24 * 60 * 60 * 1000);
mEditor.putLong(KEY_EXPIRES, millis);
mEditor.commit();
}
/**
* Checks whether user is logged in
*/
public boolean isLoggedIn() {
Date currentDate = new Date();
long millis = mPreferences.getLong(KEY_EXPIRES, 0);
/* If shared preferences does not have a value
then user is not logged in
*/
if (millis == 0) {
return false;
}
Date expiryDate = new Date(millis);
/* Check if session is expired by comparing
current date and Session expiry date
*/
return currentDate.before(expiryDate);
}
/**
* Fetches and returns user details
*/
public User getUserDetails() {
//Check if user is logged in first
if (!isLoggedIn()) {
return null;
}
User user = new User();
user.setUsername(mPreferences.getString(KEY_USERNAME, KEY_EMPTY));
user.setFullName(mPreferences.getString(KEY_FULL_NAME, KEY_EMPTY));
user.setSessionExpiryDate(new Date(mPreferences.getLong(KEY_EXPIRES, 0)));
return user;
}
/**
* Logs out user by clearing the session
*/
public void logoutUser(){
mEditor.clear();
mEditor.commit();
}
}
# 1 楼答案
类
BaseActivity
中的session
字段没有赋值,因此为空