android studio 运行活动闪退Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'vo

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method ‘void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)’ on a null object reference at com.example.mylogin0310.LoginActivity.onCreate(LoginActivity.java:75)

问题描述：FATAL EXCEPTION: main

Process: com.example.mylogin0310, PID: 19367
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mylogin0310/com.example.mylogin0310.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method ‘void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)’ on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2462)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2524)
at android.app.ActivityThread.access$800(ActivityThread.java:167) at android.app.ActivityThread$800(ActivityThread.java:167)atandroid.app.ActivityThreadH.handleMessage(ActivityThread.java:1419)
at android.os.Handler.dispatchMessage(Handler.java:111)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5546)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$MethodAndArgsCaller.run(ZygoteInit.java:964)atcom.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)Causedby:java.lang.NullPointerException:Attempttoinvokevirtualmethod′voidandroid.widget.Button.setOnClickListener(android.view.ViewOnClickListener)’ on a null object reference

笔者再进行修改主页面活动时出现的问题，我新建个mainActivity，将原来的主活动复制到了loginActivity中，结果导致跳转loginActivity时APP闪退。

原因：闪退一定是有问题，有可能是背景图格式不对，或者背景图过大，但是明显我这个是代码问题。

如下图，我忘记修改了 .mxl了

总结，学习过程一定要严谨，有错误仔细读取下方的debug、error。