Why do I get nullPointerException with this code?

Question

I have an activity and am getting a nullPointerException on the last line of the clicked method

public class MainActivity extends AppCompatActivity {

List<Item> it;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);   
    }

    public void clicked(View view) {          
        getCompany(s);
        Integer.toString(it.size());  
    }

    public void getCompany(String name) {

        apiCall.setCallback(new ApiCall.NetworkCallback() {
            @Override
            public void onResponse(List<Item> items) {
                 it = items;
                for (int i = 0; i < items.size(); i++) {    
                    Log.d("Name", it.get(i).getTitle());
                }
            }
        });
    }

I have removed some of the code to keep it short but the issue is the 'it' list is accessible in the onResponse method but gives a nullPointerException in the clicked method.

What would be causing this and how can I store the list to be used outside of the onResponse method?

Answer 1

because you try to get size of list before initialize it . in Integer.toString(it.size());
because it not initialized yet getCompany(s); is asynchronous method try to remove Integer.toString(it.size());
because in not used and not useful. public class MainActivity extends AppCompatActivity {

List<Item> it;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);   
}

public void clicked(View view) {          
    getCompany(s);
}

public void getCompany(String name) {

    apiCall.setCallback(new ApiCall.NetworkCallback() {
        @Override
        public void onResponse(List<Item> items) {
             it = items;
            for (int i = 0; i < items.size(); i++) {    
                Log.d("Name", it.get(i).getTitle());
            }
        }
    });
}

Answer 2

In clicked you call getCompany where you set a callback where it is set. Problem is that onResponse will most likely be called some time after that and apiCall.setCallback will return immediately. So getCompany returns immediately and it will still be null when you try to access it.

Whatever you want to do after doResponse is called, it should be initiated in doResponse. How to do that depends on the actual use case which is not clear with your example but maybe my explanation helps you to have some ideas for yourself. If not, start a new question here with more details (new question because it has nothing to do with NullPointerExceptions).

Answer 3

I'll go straight with a possible solution for you which give you a quick example of an interface

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);   
    }

    public void clicked(View view) {          
        getCompany(s, new OnReceiveCompany() {
            @Override
            public void onReceiveCompany(List<Item> it) {
                Integer.toString(it.size());                  
            }
        });
    }

    public void getCompany(String name, final OnReceiveCompany callback) {
        apiCall.setCallback(new ApiCall.NetworkCallback() {
            @Override
            public void onResponse(List<Item> items) {
                for (int i = 0; i < items.size(); i++) {    
                    Log.d("Name", it.get(i).getTitle());
                }
                // make sure this run on UI thread if you need to perform ui operation. You can wrap it with new Hanler(Looper.getMainLooper).post(...
                callback.onReceiveCompany(items);
            }
        });
    }

    interface OnReceiveCompany {
        void onReceiveCompany(List<Item> it);
    } 
}

why your code is not working:

    public void clicked(View view) {
        // this method is gonna set "it" in the block of code in onResponse, which will happens after you call Integer.toString probably in a separate thread
        getCompany(s);
        Integer.toString(it.size());  
    }

Answer 4

A NullPointerException occurs when you try to use a null variable (or a variable that has not been initialized).

Even though onResponse() is called from within clicked(), you have to remember that onResponse is within an asynchronous call. This simply means that it is not guaranteed that onResponse would finish executing before clicked() is called.

I'd suggest adding Integer.toString(it.size()) to your onResponse() method.

I hope this helps. Struggle no more ;-) Merry coding!