android 按返回键程序没反回到前一个activity 直接关掉了

Question

MainActivity 跳转代码
        fgSwitchBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent = new Intent(MainActivity.this, OtherFragment.class);
                startActivityForResult(intent,REQUEST_ONE);
            }
        });另一个Activity 的返回键设置
public void onBackPressed() {
        this.finish();
    }没有从第二个Activity返回到第一个Activity 程序直接关掉了example.yyh.pagechange E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            Process: com.example.yyh.pagechange, PID: 29728
                                                                            java.lang.RuntimeException: Unable to resume activity {com.example.yyh.pagechange/com.example.yyh.pagechange.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
                                                                                at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3124)
                                                                                at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3165)
                                                                                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1452)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                at android.os.Looper.loop(Looper.java:179)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5491)
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:961)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
                                                                             Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
                                                                                at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
                                                                                at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:441)
                                                                                at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:432)
                                                                                at com.example.yyh.pagechange.MainActivity.switchFragment(MainActivity.java:140)
                                                                                at com.example.yyh.pagechange.MainActivity.onResume(MainActivity.java:123)
                                                                                at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1260)
                                                                                at android.app.Activity.performResume(Activity.java:6269)
                                                                                at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3113)
                                                                                at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3165) 
                                                                                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1452) 
                                                                                at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                at android.os.Looper.loop(Looper.java:179) 
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5491) 
                                                                                at java.lang.reflect.Method.invoke(Native Method) 
                                                                                at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:961) 
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

Answer 1

  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference看到这一行了吗，空指针报错了at com.example.yyh.pagechange.MainActivity.switchFragment(MainActivity.java:140)MainActivity 的第140行