Further adapt a Volley Request by specifying GET parameter, Json response decoding, and cache and retry policies. Part of http://develop-for-android.blogspot.com.es/2014/01/using-volley-in-your-application.html

CachedStringGETRequest.java

package net.colaborativa.exampleapp.api.volley;

import java.io.UnsupportedEncodingException;

import com.android.volley.Cache;
import com.android.volley.DefaultRetryPolicy;
import com.android.volley.NetworkResponse;
import com.android.volley.ParseError;
import com.android.volley.Request;
import com.android.volley.Response;
import com.android.volley.Response.ErrorListener;
import com.android.volley.Response.Listener;
import com.android.volley.toolbox.HttpHeaderParser;

public class CachedStringGETRequest extends Request<String> {

	protected static final int defaultClientCacheExpiry = 
	  1000 * 60 * 15; // milliseconds; = 15 min

	protected final Listener<String> listener;

	public CachedStringGETRequest(
	  String url, 
	  Listener<String> listener, 
	  ErrorListener errorListener
	  ) {
		super(Method.GET, url, errorListener);
		this.listener = listener;
		setRetryPolicy(new DefaultRetryPolicy(
				20000, // 20 seconds, slow VPN and slow service
				3, // 3 retries 
				DefaultRetryPolicy.DEFAULT_BACKOFF_MULT));
	}
    
	@Override
	protected Response<String> parseNetworkResponse(
	  NetworkResponse response
	  ) {
			String data;
			String charset = 
			  HttpHeaderParser.parseCharset(response.headers);
			
			try {
				data = new String(response.data, charset);
				return Response.success(
						data, 
						enforceClientCaching(
						  HttpHeaderParser.parseCacheHeaders(response), 
						  response)
					);
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
				return Response.error(new ParseError(e));
			}
			
	}
	
	protected Cache.Entry enforceClientCaching(
	  Cache.Entry entry, 
	  NetworkResponse response
	  ) {
	    if (getClientCacheExpiry() == null) return entry;

	    long now = System.currentTimeMillis();

	    if (entry == null) {
	        entry = new Cache.Entry();
	        entry.data = response.data;
	        entry.etag = response.headers.get("ETag");
	        entry.softTtl = now + getClientCacheExpiry();
	        entry.ttl = entry.softTtl;
	        entry.serverDate = now;
	        entry.responseHeaders = response.headers;
	    } else if (entry.isExpired()) {
	        entry.softTtl = now + getClientCacheExpiry();
	        entry.ttl = entry.softTtl;
	    }

	    return entry;
	}

	protected Integer getClientCacheExpiry() {
	    return defaultClientCacheExpiry;
	}

	@Override
	protected void deliverResponse(String response) {
		listener.onResponse(response);
	}

}