是否为contextWrapper.getResources（）提供了空指针异常？ 使用自定义侦听器时

Question

getting a strange null pointer exception for a common type of custom listener that i have implemented before without any problems.

a crash will happen if i put any code in the callback method that communicates with the main user interface thread of the activity. if i put any code inside this method that does not touch the user interface thread then there is no crash. it does not make sense because all the code in this class runs on the UI thread and does not need to use a handler or runonui method.

how do i fix this problem?

from the logcat:

09-30 10:55:15.360: E/AndroidRuntime(2207): FATAL EXCEPTION: main
09-30 10:55:15.360: E/AndroidRuntime(2207): java.lang.NullPointerException
09-30 10:55:15.360: E/AndroidRuntime(2207):     at 
android.content.ContextWrapper.getResources(ContextWrapper.java:81)
09-30 10:55:15.360: E/AndroidRuntime(2207):     at android.widget.Toast.<init>(Toast.java:92)
09-30 10:55:15.360: E/AndroidRuntime(2207):     at android.widget.Toast.makeText(Toast.java:233)
09-30 10:55:15.360: E/AndroidRuntime(2207):     at StartActivity.onResultReturned(StartActivity.java:124)
09-30 10:55:15.360: E/AndroidRuntime(2207):     at jp.co.forever.tankinspectionsystem.Synchronizer.sendInt(Synchronizer.java:215)
09-30 10:55:15.360: E/AndroidRuntime(2207):     at 
jp.co.forever.tankinspectionsystem.Synchronizer$SendOutMsgAndPack$2.run(Synchronizer.java:162)

code for the listener in the StartActivity class, this is utility class that does not extend activity, and all the code here runs in a new thread run method separate from the UI thread

in class variables declarations

 public OnResultReturnedListener listener;

in oncreate

 listener = new StartActivity();

listener is an interface as a nested subclass in this activity class

 public interface OnResultReturnedListener {
public void  onResultReturned(int result);
 }

code in the other class that implements the listener, all code runs on UI main thread, no additional thread are created

 public class StartActivity extends Activity
 implements Synchronizer.OnResultReturnedListener {

 // method onResultReturned is override for custom listerner class
 // OnResultReturnedListener interface inside of the Synchronizer class

    @Override
public void onResultReturned(int result) {

    // toast like this or any other method that touched the
    // UI thread will result in a crash
Toast.makeText(StartActivity.this, "value returned "
    + result , Toast.LENGTH_SHORT).show();


    // putting the Toast in a handler will not help, still results in crash
    handler.post(new Runnable() {
        @Override
        public void run() {
        Toast.makeText(StartActivity.this, "value returned "
                    + result, Toast.LENGTH_SHORT).show();
        }
        });

    //adding a handler.post to the other class, Synchronizer,
    // that the method call comes from
    // will not help and still causes crashes, for example
    //handler.post(new Runnable() {
    //@Override
    //public void run() {
    //fileTransferStatus = 1;
    //        listener.onResultReturned(fileTransferStatus);
    //   }
    //});

    // this statement will not cause crash
    int x = 13;
    }
 }

Answer 1

I believe the problem is in listener = new StartActivity(). You have created activity manually that's why it doesn't have attached context and unable to interact with ui thread. You can pass StartActivity reference to your Synchronizer.