NullPointerException：创建SQLite数据库时

Question

这是我从android studio收到的日志。

08-07 07：54：57.379 10573-10573 / com.sifdon.shipping E / AndroidRuntime：FATAL EXCEPTION：main                                                                      处理：com.sifdon.shipping，PID：10573                                                                      java.lang.RuntimeException：无法恢复活动{com.sifdon.shipping / com.sifdon.shipping.DisplayActivity}：java.lang.NullPointerException：尝试调用虚方法'android.database.sqlite.SQLiteDatabase com.sifdon.shipping空对象引用上的.DbHelper.getWritableDatabase（）'                                                                          在android.app.ActivityThread.performResumeActivity（ActivityThread.java:3192）                                                                          在android.app.ActivityThread.handleResumeActivity（ActivityThread.java:3223）                                                                          在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2471）                                                                          在android.app.ActivityThread.access $ 1200（ActivityThread.java:152）                                                                          在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1341）                                                                          在android.os.Handler.dispatchMessage（Handler.java:102）                                                                          在android.os.Looper.loop（Looper.java:135）                                                                          在android.app.ActivityThread.main（ActivityThread.java:5538）                                                                          at java.lang.reflect.Method.invoke（Native Method）                                                                          在java.lang.reflect.Method.invoke（Method.java:372）                                                                          在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:960）                                                                          在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:755）                                                                       引发者：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'android.database.sqlite.SQLiteDatabase com.sifdon.shipping.DbHelper.getWritableDatabase（）'                                                                          at com.sifdon.shipping.DisplayActivity.displayData（DisplayActivity.java:157）                                                                          在com.sifdon.shipping.DisplayActivity.onResume（DisplayActivity.java:149）                                                                          在android.app.Instrumentation.callActivityOnResume（Instrumentation.java:1249）                                                                          在android.app.Activity.performResume（Activity.java:6102）                                                                          在android.app.ActivityThread.performResumeActivity（ActivityThread.java:3167）                                                                          在android.app.ActivityThread.handleResumeActivity（ActivityThread.java:3223）                                                                          在android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2471）                                                                          在android.app.ActivityThread.access $ 1200（ActivityThread.java:152）                                                                          在android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1341）                                                                          在android.os.Handler.dispatchMessage（Handler.java:102）                                                                          在android.os.Looper.loop（Looper.java:135）                                                                          在android.app.ActivityThread.main（ActivityThread.java:5538）                                                                          at java.lang.reflect.Method.invoke（Native Method）                                                                          在java.lang.reflect.Method.invoke（Method.java:372）                                                                          在com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:960）                                                                          在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:755） 08-07 07：54：57.719 10573-10573 / com.sifdon.shipping I / Process：发送信号。 PID：10573 SIG：9

Answer 1

在致电

时，您在NullPointerException中获得了DisplayActivity

DbHelper.getWritableDatabase()'。也许DbHelper是空的，你不能

从中调用任何方法。