Question

我有一个带有登录页面的应用程序。

首先，我有我的 MainActivity ：

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程序后，成功的 LoginActivity 应用程序因以下异常而崩溃：

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，没有得到该异常。当我单击登录按钮时，该错误就会出现，并且应用程序崩溃。

在调试时，编译器甚至不符合setText()

我的问题是，在我上面编码的情况下，如何在textviews中设置文本？

更新

即使我将构造函数中loggedUser中的字符串弹出为空，将loggedUser = loggedUser.getInstance()移至setText()并将onCreat移至onResume，我仍然会得到该NPE。

编辑：

我添加了@Override protected void onResume() { if(isLogged) { emailText.setText(loggedUser.getEmail()); usernameText.setText(loggedUser.getUsername()); } super.onResume(); }方法：

onActivityResult()

并编辑了@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(); } }：

onResume()

设置TextViews时，我在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)方法上仍然遇到相同的错误。

def highlight_max(s):
    '''
    highlight the maximum in a Series yellow.
    '''
    is_max = s == s.max()
    return ['background-color: yellow' if v else '' for v in is_max]
s=df.style.apply(highlight_max)

Answer 1

空视图通常是由于布局不包含具有在findViewById（）中指定的ID的视图而导致的。因此，当调用onResume（）时，findViewById（）返回null，并且变量保持为null。在main_activity.xml中仔细检查您的视图ID。

如果使用更复杂的布局，则onCreate（）中的完整内容视图也可能不可用。如果在onCreate（）中发现视图为空，请尝试将findViewById（）移至生命周期中的更高位置。

Answer 2

尝试像这样重新排序代码

emailText = findViewById(R.id.nav_text_email);
usernameText = findViewById(R.id.nav_text_username);
startActivityForResult(new Intent(this, LoginActivity.class), REQUEST_CODE);

Answer 3

尝试在访问变量之前对其进行初始化。

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);
}

可以加入评论

您需要从活动中访问NavigationView

更新

NavigationView navigationView = findViewById(R.id.nav_id)
View header = navigationView.getHeaderView(0)
TextView usernameText = header.findViewById(R.id.username_text_view_id);

在onActivityResult之后设置TextView文本

3 个答案: