Question

我试图检查用户是否在上次离开应用程序之前/仍然登录之前已登录。

现在，如果我第一次启动应用程序，它会给出强制停止错误。

但是当我点击确定时，它只会继续登录屏幕。

检查用户是否已登录我正在使用＆＃39; if else＆＃39;

MainActivity.java oncreate我在哪里检查用户是否已登录

@Override
protected 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.activity_main);
    }else{
        // user is not logged in show login screen
        setContentView(R.layout.login);
        finish();

        return;
    }

logcat的

10-14 15:31:33.751: E/AndroidRuntime(30585): FATAL EXCEPTION: main
10-14 15:31:33.751: E/AndroidRuntime(30585): Process: info.androidhive.listviewfeed, PID: 30585
10-14 15:31:33.751: E/AndroidRuntime(30585): java.lang.RuntimeException: Unable to start activity              ComponentInfo{info.androidhive.listviewfeed/com.BijCasperApp.slidingmenu.MainActivity}: java.lang.NullPointerException
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2413)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2471)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread.access$900(ActivityThread.java:175)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.os.Handler.dispatchMessage(Handler.java:102)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.os.Looper.loop(Looper.java:146)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread.main(ActivityThread.java:5602)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at java.lang.reflect.Method.invokeNative(Native Method)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at java.lang.reflect.Method.invoke(Method.java:515)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at dalvik.system.NativeStart.main(Native Method)
10-14 15:31:33.751: E/AndroidRuntime(30585): Caused by: java.lang.NullPointerException
10-14 15:31:33.751: E/AndroidRuntime(30585):    at com.BijCasperApp.slidingmenu.MainActivity.onCreate(MainActivity.java:103)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.Activity.performCreate(Activity.java:5451)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
10-14 15:31:33.751: E/AndroidRuntime(30585):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377)
10-14 15:31:33.751: E/AndroidRuntime(30585):    ... 11 more

为什么在启动应用程序时会出现nullpointerexception？

Answer 1

您已从“未登录”分支中注释掉finish()。放回去并添加return。

代码执行在if-else之后继续，并且您正在使用findViewById()访问视图，但您只在“登录”分支中调用setContentView()。因此findViewById()返回null并且在null上调用方法会导致NPE。

检查用户是否已登录时为nullpointerexception

1 个答案: