android volley错误whle使用imageloader

Question

我的下面代码有什么问题？我正在跟进android tutorial 我在这个代码行中有nullpointerexception，可能是什么问题？

mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);

这是完整的代码

 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  RequestQueue mRequestQueue;

  final String IMAGE_URL =
            "http://developer.android.com/images/training/system-ui.png";

//Instantiate the cache
Cache cache = new DiskBasedCache(getCacheDir(), 1024 * 1024); // 1MB cap

//Set up the network to use HttpURLConnection as the HTTP client.
Network network = new BasicNetwork(new HurlStack());

//Instantiate the RequestQueue with the cache and network.
mRequestQueue = new RequestQueue(cache, network);

//Start the queue
mRequestQueue.start();


ImageLoader mImageLoader = new ImageLoader(mRequestQueue, new LruBitmapCache(
            LruBitmapCache.getCacheSize(mContext)));
mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);
}

这是单身人士

package com.example.p;

import android.content.Context;
import android.graphics.Bitmap;
import android.support.v4.util.LruCache;
import android.util.DisplayMetrics;
import com.android.volley.toolbox.ImageLoader.ImageCache;

public class LruBitmapCache extends LruCache<String, Bitmap>
        implements ImageCache {

    public LruBitmapCache(int maxSize) {
        super(maxSize);
    }

    public LruBitmapCache(Context ctx) {
        this(getCacheSize(ctx));
    }

    @Override
    protected int sizeOf(String key, Bitmap value) {
        return value.getRowBytes() * value.getHeight();
    }

    @Override
    public Bitmap getBitmap(String url) {
        return get(url);
    }

    @Override
    public void putBitmap(String url, Bitmap bitmap) {
        put(url, bitmap);
    }

    // Returns a cache size equal to approximately three screens worth of images.
    public static int getCacheSize(Context ctx) {
        final DisplayMetrics displayMetrics = ctx.getResources().
                getDisplayMetrics();
        final int screenWidth = displayMetrics.widthPixels;
        final int screenHeight = displayMetrics.heightPixels;
        // 4 bytes per pixel
        final int screenBytes = screenWidth * screenHeight * 4;

        return screenBytes * 3;
    }
}