我的Android项目中有2个活动和1个配置文件：
这是我的LoginActivity代码：
public class LoginActivity extends AppCompatActivity implements View.OnClickListener {
//Defining views
private EditText editTextUsername;
private EditText editTextPassword;
private AppCompatButton buttonLogin;
//boolean variable to check user is logged in or not
//initially it is false
private boolean loggedIn = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
//Initializing views
editTextUsername = (EditText) findViewById(R.id.editTextUsername);
editTextPassword = (EditText) findViewById(R.id.editTextPassword);
buttonLogin = (AppCompatButton) findViewById(R.id.buttonLogin);
//Adding click listener
buttonLogin.setOnClickListener(this);
}
@Override
protected void onResume() {
super.onResume();
//In onresume fetching value from sharedpreference
SharedPreferences sharedPreferences = getSharedPreferences(Config.SHARED_PREF_NAME,Context.MODE_PRIVATE);
//Fetching the boolean value form sharedpreferences
loggedIn = sharedPreferences.getBoolean(Config.LOGGEDIN_SHARED_PREF, false);
//If we will get true
if(loggedIn){
//We will start the Profile Activity
Intent intent = new Intent(LoginActivity.this, MainActivity.class);
startActivity(intent);
}
}
private void login(){
//Getting values from edit texts
final String username = editTextUsername.getText().toString().trim();
final String password = editTextPassword.getText().toString().trim();
//Creating a string request
StringRequest stringRequest = new StringRequest(Request.Method.POST, Config.LOGIN_URL,
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
//If we are getting success from server
if(response.equalsIgnoreCase(Config.LOGIN_SUCCESS)){
//Creating a shared preference
SharedPreferences sharedPreferences = LoginActivity.this.getSharedPreferences(Config.SHARED_PREF_NAME, Context.MODE_PRIVATE);
//Creating editor to store values to shared preferences
SharedPreferences.Editor editor = sharedPreferences.edit();
//Adding values to editor
editor.putBoolean(Config.LOGGEDIN_SHARED_PREF, true);
editor.putString(Config.USERNAME_SHARED_PREF, username);
//Saving values to editor
editor.commit();
//Starting profile activity
Intent intent = new Intent(LoginActivity.this, MainActivity.class);
startActivity(intent);
}else{
//If the server response is not success
//Displaying an error message on toast
Toast.makeText(LoginActivity.this, "Invalid username or password", Toast.LENGTH_LONG).show();
}
}
},
new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
//You can handle error here if you want
}
}){
@Override
protected Map<String, String> getParams() throws AuthFailureError {
Map<String,String> params = new HashMap<>();
//Adding parameters to request
params.put(Config.KEY_EMAIL, username);
params.put(Config.KEY_PASSWORD, password);
//returning parameter
return params;
}
};
//Adding the string request to the queue
RequestQueue requestQueue = Volley.newRequestQueue(this);
requestQueue.add(stringRequest);
}
@Override
public void onClick(View v) {
//Calling the login function
login();
}
}
这是我的MainActivity代码：
public class MainActivity extends AppCompatActivity {
private AdView mAdView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
AppCompatTextView name = (AppCompatTextView) toolbar.findViewById(R.id.name);
AppCompatTextView job = (AppCompatTextView) toolbar.findViewById(R.id.job);
AppCompatTextView gender = (AppCompatTextView) toolbar.findViewById(R.id.gender);
AppCompatTextView born = (AppCompatTextView) toolbar.findViewById(R.id.born);
CircleImageView profile_image = (CircleImageView) toolbar.findViewById(R.id.profile_image);
final CollapsingToolbarLayout collapsingToolbarLayout = (CollapsingToolbarLayout) findViewById(R.id.toolbar_layout);
AppBarLayout appBarLayout = (AppBarLayout) findViewById(R.id.app_bar);
collapsingToolbarLayout.setTitle(" ");
appBarLayout.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
boolean isShow = false;
int scrollRange = -1;
@Override
public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
if (scrollRange == -1) {
scrollRange = appBarLayout.getTotalScrollRange();
}
if (scrollRange + verticalOffset == 0) {
collapsingToolbarLayout.setTitle("MIS");
isShow = true;
} else if(isShow) {
collapsingToolbarLayout.setTitle(" "); //carefull there should a space between double quote otherwise it wont work
isShow = false;
}
}
});
/*mAdView = (AdView) findViewById(R.id.adView);
AdRequest adRequest = new AdRequest.Builder().build();
mAdView.loadAd(adRequest);
*/
//Fetching email from shared preferences
SharedPreferences myPrefs = this.getSharedPreferences(Config.SHARED_PREF_NAME, Context.MODE_PRIVATE);
String uName = myPrefs.getString(Config.USERNAME_SHARED_PREF,"");
name.setText(uName);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
/*Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
*/
logout();
}
});
}
//Logout function
private void logout(){
//Creating an alert dialog to confirm logout
AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
alertDialogBuilder.setMessage("Are you sure you want to logout?");
alertDialogBuilder.setPositiveButton("Yes",
new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface arg0, int arg1) {
//Getting out sharedpreferences
SharedPreferences preferences = getSharedPreferences(Config.SHARED_PREF_NAME,Context.MODE_PRIVATE);
//Getting editor
SharedPreferences.Editor editor = preferences.edit();
//Puting the value false for loggedin
editor.putBoolean(Config.LOGGEDIN_SHARED_PREF, false);
//Putting blank value to email
editor.putString(Config.USERNAME_SHARED_PREF, "");
//Saving the sharedpreferences
editor.commit();
//Starting login activity
Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
startActivity(intent);
finish();
}
});
alertDialogBuilder.setNegativeButton("No",
new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface arg0, int arg1) {
}
});
//Showing the alert dialog
AlertDialog alertDialog = alertDialogBuilder.create();
alertDialog.show();
}
}
这是我的配置：
public class Config {
//URL to our login.php file
public static final String LOGIN_URL = "";
//Keys for email and password as defined in our $_POST['key'] in login.php
public static final String KEY_EMAIL = "username";
public static final String KEY_PASSWORD = "password";
//If server response is equal to this that means login is successful
public static final String LOGIN_SUCCESS = "success";
//Keys for Sharedpreferences
//This would be the name of our shared preferences
public static final String SHARED_PREF_NAME = "mis_app";
//This would be used to store the email of current logged in user
public static final String USERNAME_SHARED_PREF = "username";
//We will use this to store the boolean in sharedpreference to track user is loggedin or not
public static final String LOGGEDIN_SHARED_PREF = "loggedin";
}
运行项目并单击“登录”按钮后，我在logcat中收到此错误：
引起：java.lang.NullPointerException：尝试调用虚方法＆＃39; void android.widget.TextView.setText（java.lang.CharSequence）＆＃39;在空对象引用上
错误在 uName 字符串中，我不知道这里发生了什么。
有人可以帮帮我吗？
答案 0 :(得分：0)
获取空指针异常的原因是您将数据保存到MainActivity的SharedPreferences并尝试从另一个活动的SharedPreferences，plesase try语句而不是您的语句中检索。
@RequestMapping(value = "/events", method = RequestMethod.GET)
public List<Event> getEvents() {
return eventRepo.findAllByUserId(1L);
}
答案 1 :(得分：0)
最后我解决了这个问题。
我更改了此代码：
AppCompatTextView name =（AppCompatTextView）toolbar.findViewById（R.id.name）; AppCompatTextView job =（AppCompatTextView）toolbar.findViewById（R.id.job）; AppCompatTextView gender =（AppCompatTextView）toolbar.findViewById（R.id.gender）; AppCompatTextView born =（AppCompatTextView）toolbar.findViewById（R.id.born）; CircleImageView profile_image =（CircleImageView）toolbar.findViewById（R.id.profile_image）;
成：
AppCompatTextView toolbar_name = (AppCompatTextView) toolbar.findViewById(R.id.name);
AppCompatTextView toolbar_job = (AppCompatTextView) toolbar.findViewById(R.id.job);
AppCompatTextView toolbar_gender = (AppCompatTextView) toolbar.findViewById(R.id.gender);
AppCompatTextView toolbar_born = (AppCompatTextView) toolbar.findViewById(R.id.born);
CircleImageView toolbar_profile_image = (CircleImageView) toolbar.findViewById(R.id.profile_image);
然后在它下面添加了这段代码：
AppCompatTextView name = (AppCompatTextView)findViewById(R.id.name);
AppCompatTextView job = (AppCompatTextView)findViewById(R.id.job);
AppCompatTextView gender = (AppCompatTextView)findViewById(R.id.gender);
AppCompatTextView born = (AppCompatTextView)findViewById(R.id.born);
CircleImageView profile_image = (CircleImageView)findViewById(R.id.profile_image);
瞧！错误消失了。