我有一个带有登录页面的应用程序。
首先,我有我的MainActivity
一切开始的地方:
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_activity);
// here i launch LoginActivity
startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);
emailText = findViewById(R.id.nav_text_email);
usernameText = findViewById(R.id.nav_text_username);
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data){
if(requestCode == REQUEST_CODE){
if(resultCode != Activity.RESULT_OK) {
finish();
}
// if login succesful, i attach to variable instance of
// logged user class.
// that part works correct, in object vars exist
loggedUser = loggedUser.getInstance();
// i want to attach strings after succesful login
emailText.setText(loggedUser.getEmail());
usernameText.setText(loggedUser.getUsername());
}
}
使用上述代码运行应用时,成功后LoginActivity
应用崩溃并出现以下异常:
2019-01-26 20:46:48.015 13071-13071/com.example.admin.keystroke_dynamics E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.admin.keystroke_dynamics, PID: 13071
java.lang.RuntimeException: Unable to resume activity {com.example.admin.keystroke_dynamics/com.example.admin.keystroke_dynamics.Activities.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3586)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2876)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:156)
at android.app.ActivityThread.main(ActivityThread.java:6523)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.admin.keystroke_dynamics.Activities.MainActivity.onResume(MainActivity.java:89)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1279)
at android.app.Activity.performResume(Activity.java:7022)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3561)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2876)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:156)
at android.app.ActivityThread.main(ActivityThread.java:6523)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
当我从'onActivityResult()中删除setText()
方法时,我没有收到该异常。当我单击"登录"按钮并且应用程序崩溃时出现错误。
在调试时,编译器甚至不与setText()
我的问题是,如何在我上面编码的场景中textviews
设置文本?
更新
即使我将构造函数中loggedUser
字符串填充为空,loggedUser = loggedUser.getInstance()
移动并setText()
移动到onCreat
,我仍然得到那个 NPE。
编辑:
我添加了onResume
方法:
@Override
protected void onResume()
{
if(isLogged) {
emailText.setText(loggedUser.getEmail());
usernameText.setText(loggedUser.getUsername());
}
super.onResume();
}
并编辑onActivityResult()
:
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data){
if(requestCode == REQUEST_CODE){
if(resultCode != Activity.RESULT_OK) {
finish();
}
isLogged = true; //var for if statement in onResume()
loggedUser = loggedUser.getInstance();
}
}
当我设置TextViews
时,我仍然onResume()
方法上收到相同的错误。
2019-01-26 21:21:57.752 18262-18262/com.example.admin.keystroke_dynamics E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.admin.keystroke_dynamics, PID: 18262
java.lang.RuntimeException: Unable to resume activity {com.example.admin.keystroke_dynamics/com.example.admin.keystroke_dynamics.Activities.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3586)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1618)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:156)
at android.app.ActivityThread.main(ActivityThread.java:6523)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.admin.keystroke_dynamics.Activities.MainActivity.onResume(MainActivity.java:90)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1279)
at android.app.Activity.performResume(Activity.java:7022)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3561)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3626)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1618)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:156)
at android.app.ActivityThread.main(ActivityThread.java:6523)
空视图通常是由于布局不包含具有您在 findViewById() 中指定的 id 的视图的结果。 因此,findViewById() 返回 null,并且在调用 onResume() 时变量保持 null。 在main_activity.xml中仔细检查您的视图 ID。
如果您使用的是更复杂的布局,则 onCreate() 中也可能没有完整的内容视图。 如果您发现视图在 onCreate() 中为空,请尝试将 findViewById() 移动到生命周期的稍后点。
尝试像这样对代码重新排序
emailText = findViewById(R.id.nav_text_email);
usernameText = findViewById(R.id.nav_text_username);
startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);
在访问变量之前尝试初始化变量。
public class MainActivity extends AppCompatActivity {
TextView emailText;
TextView usernameText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_activity);
// here i launch LoginActivity
startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);
emailText = findViewById(R.id.nav_text_email);
usernameText = findViewById(R.id.nav_text_username);
}
可以放在评论中
您需要从活动访问导航视图
更新
NavigationView navigationView = findViewById(R.id.nav_id)
View header = navigationView.getHeaderView(0)
TextView usernameText = header.findViewById(R.id.username_text_view_id);