尝试将字符串数组从一个类传递到另一个类时出现NullPointerException

Question

In my Android app, there are some strings that I need to use in two different places in my app. So I wrote a class from which I can get these strings. When I try to call the return method from the class that returns the string array, the app crashes with java.lang.NullPointerException . Here is the class with the return method:

public class MetaDataFetcher {

    String[] metaData;

    public String[] getMetaData() {
        //Gets the metadata strings from HarvasterAsync
        try {
            metaData = new HarvesterAsync().execute("urlhere").get();



        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        return metaData;
    }
}

I am attempting to retrieve the string array like this:

public void updateMetaData(){
//Gets the metadata strings from MetaDataFetcher

String[] receivedMetaData = metaDataFetcher.getMetaData(); 

//More code below...

The NullPointerException occurs at the line String[] receivedMetaData = metaDataFetcher.getMetaData(); .

What am I doing wrong?

Edit:

I initialize the MetaDataFetcher class with the line MetaDataFetcher metaDataFetcher; above the onCreate method in my MainActivity class.

HarvesterAsync is an AsyncTask . You can see it here .

Answer 1

The problem is concurrency. AsyncTask is non-blocking. So basically this

String[] receivedMetaData = metaDataFetcher.getMetaData(); 

will immediately return the receivedMetaData from MetaDataFetcher's field not the result of HarvesterAsync.

Answer 2

In these sort of situations you need to make use of . You cannot call execute() of an AsyncTask and right away start using the value returned because the AsyncTask runs on a separate thread and you will have the problem of concurrency as mentioned by @inmyth as well as others in the comments.

So what you need to do in-order to make it work is use callbacks. At the beginning this might look like a far-fetched solution but trust me it pays off when you are trying to implement large projects.

First off, declare a listener interface inside your HarvesterAsync class and you can implement these listeners wherever you want to get the value from the AsncTask

Here is a sample:

public class HarvesterAsync extends AsyncTask<...> {
    // declare your variables
    private List<HarvesterAsyncListener> harvestListeners;

    public HarvesterAsync() {
        // initialise all variables
        harvestListeners = new ArrayList<HarvestAsyncListener>();
    }

    @Override
    protected void doInBackground() {
        // ...
        // fetch your data from somewhere and load it into say "metaData"
        String[] metaData = fetchYourData();

        // notify all listeners that the data has been successfully fetched
        for(listener: harvestListeners) {
            // pass in your result to the method of the interface
            listener.onFetchComplete(metaData);
        }
    }

    // use this method to add new listeners to harvestListeners
    public void addListener(HarvestListener listener) {
        harvestListeners.add(listener);
    }

    public interface HarvestAsyncListener {
        public void onFetchComplete(String[] metaData);
    }
}

Now to obtain the result you can implement the listener interface like this:

public class MetaDataFetcher implements HarvesterAsync.HarvesterAsyncListener {

    String[] metaData;

    public void getMetaData() {
        // initailize the AsyncTask
        HarvestAsync harvestAsnc = new HarvestAsync();

        // add the listener
        harvestAsync.addListener(this);

        ...

    }

    @Override
    public void onFetchComplete(String[] metaData) {
        // do whatever you want with metaData.
        this.metaData = metaData;
        ....
    }

}

You could of course change the method signature in the interface to suit your requirements.

Here you don't need to return the metaData because once it is loaded by the AsyncTask , the AsyncTask does a callback to the onFetchComplete .