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• Should a Stack have a maximum size?

  - by Sotirios Delimanolis

  I'm practicing my knowledge of ADTs by implementing some data structures, even if most already exist. With Stacks, a lot of books and other documentation I've read talk about the stack throwing an error when you try to add an element but the stack is full. In a java implementation (or any other), should I specifically keep track of a maximum stack size (from constructor), check to see if that size is reached, and throw an overflow exception if it is? Or is not such a big deal?

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• Java Runtime.freeMemory() returning bizarre results when adding more objects

  - by Sotirios Delimanolis

  For whatever reason, I wanted to see how many objects I could create and populate a LinkedList with. I used Runtime.getRuntime().freeMemory() to get the approximation of free memory in my JVM. I wrote this: public static void main(String[] arg) { Scanner kb = new Scanner(System.in); List<Long> mem = new LinkedList<Long>(); while (true) { System.out.println("Max memory: " + Runtime.getRuntime().maxMemory() + ". Available memory: " + Runtime.getRuntime().freeMemory() + " bytes. Press enter to use more."); String s = kb.nextLine(); if (s.equals("m")) for (int i = 0; i < 1000000; i++) { mem.add(new Long((new Random()).nextLong())); } } } If I write in m, the app adds a million Long objects to the list. You would think the more objects (to which we have references, so can't be gc'ed), the less free memory. Running the code: Max memory: 1897725952. Available memory: 127257696 bytes. m Max memory: 1897725952. Available memory: 108426520 bytes. m Max memory: 1897725952. Available memory: 139873296 bytes. m Max memory: 1897725952. Available memory: 210632232 bytes. m Max memory: 1897725952. Available memory: 137268792 bytes. m Max memory: 1897725952. Available memory: 239504784 bytes. m Max memory: 1897725952. Available memory: 169507792 bytes. m Max memory: 1897725952. Available memory: 259686128 bytes. m Max memory: 1897725952. Available memory: 189293488 bytes. m Max memory: 1897725952. Available memory: 387686544 bytes. The available memory fluctuates. How does this happen? Is the GC cleaning up other things (what other things are there on the heap to really clean up?), is the freeMemory() method returning an approximation that's way off? Am I missing something or am I crazy?

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• Session scoped bean as class attribute of Spring MVC Controller

  - by Sotirios Delimanolis

  I have a User class: @Component @Scope("session") public class User { private String username; } And a Controller class: @Controller public class UserManager { @Autowired private User user; @ModelAttribute("user") private User createUser() { return user; } @RequestMapping(value = "/user") public String getUser(HttpServletRequest request) { Random r = new Random(); user.setUsername(new Double(r.nextDouble()).toString()); request.getSession().invalidate(); request.getSession(true); return "user"; } } I invalidate the session so that the next time i got to /users, I get another user. I'm expecting a different user because of user's session scope, but I get the same user. I checked in debug mode and it is the same object id in memory. My bean is declared as so: <bean id="user" class="org.synchronica.domain.User"> <aop:scoped-proxy/> </bean> I'm new to spring, so I'm obviously doing something wrong. I want one instance of User for each session. How?

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• Sending an HTTP POST request through the android emulator doesn't work

  - by Sotirios Delimanolis

  I'm running a tomcat servlet on my local machine and an Android emulator with an app that makes a post request to the servlet. The code for the POST is below (without exceptions and the like): String strUrl = "http://10.0.2.2:8080/DeviceDiscoveryServer/server/devices/"; Device device = Device.getUniqueInstance(); urlParameters += URLEncoder.encode("user", "UTF-8") + "=" + URLEncoder.encode(device.getUser(), "UTF-8"); urlParameters += "&" + URLEncoder.encode("port", "UTF-8") + "=" + URLEncoder.encode(new Integer(Device.PORT).toString(), "UTF-8"); urlParameters += "&" + URLEncoder.encode("address", "UTF-8") + "=" + URLEncoder.encode(device.getAddress().getHostAddress(), "UTF-8"); URL url = new URL(strUrl); HttpURLConnection connection = (HttpURLConnection) url.openConnection(); connection.setDoOutput(true); connection.setRequestMethod("POST"); OutputStreamWriter wr = new OutputStreamWriter(connection.getOutputStream()); wr.write(urlParameters); wr.flush(); wr.close(); Whenever this code is executed, the servlet isn't called. However if I change the type of the request to 'GET' and don't write anything to the outputstream, the servlet gets called and everything works fine. Am I just not making the POST correctly or is there some other error?

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