java - android变量空指针

Question

我认为我正在检查的变量为空。我的 on resume 方法是这样做的:

此方法检查网络上的电子邮件，如果出现问题将返回 -1。我认为这会起作用。现在，当我运行它 10 次时，它开始工作了。所以我把它给了一个 friend 。他说这有时会让他异常(exception)。

///   Ok found it it was in a try catch in the OnCreate() 

tvEmail = (TextView) findViewById(R.id.itemz_email);
        tvEmail.setTypeface(TitleFont);

///  and occasionally it didn't even get initialized.
///  I guess ill just init it again in the onResume() just in case.
///  Thanks!


@Override
protected void onResume() {
    super.onResume();
    tvEmail = (TextView) findViewById(R.id.itemz_email);
        tvEmail.setTypeface(TitleFont);

    if (EMAIL != null) {
        tvEmail.setText("E-mail: " + EMAIL);
    } else {
        tvEmail.setText("E-mail: No Email Available");  //<----==LINE 77
    }
}

这是我的用户遇到的错误:

 java.lang.RuntimeException: Unable to resume activity {com.codalata.craigslistchecker/com.codalata.craigslistchecker.ItemView}: java.lang.NullPointerException
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2578)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2606)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2092)
    at android.app.ActivityThread.access$600(ActivityThread.java:133)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1198)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4777)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at com.codalata.craigslistchecker.ItemView.onResume(ItemView.java:77)<------/**/error/**/
    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1184)
    at android.app.Activity.performResume(Activity.java:5082)
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2568)

我应该这样检查并设置它吗？

if(EMAIL == null){
    EMAIL = "E-mail: No Email Available"
    tvEmail.setText(EMAIL);
}else {
    tvEmail.setText("E-mail: " + EMAIL);
}

Answer 1

您在包含 tvEmail.setText(...) 的行上收到了一个 NullReferenceException；交换条件 block 以检查 EMAIL 是否为 null 与否不会改变事情，因为 EMAIL 没有参与导致错误的行。

您需要查看 tvEmail，因为 that 在您的错误发生时是 null。