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• Java, HashMaps and using Strings as the keys - does the string value get stored twice?

  - by MalcomTucker

  If I have a HashMap that looks like this: HashMap<String, MyObject> where the String key is a field in MyObject, does this string value get stored twice? So when I add entries: _myMap.put(myObj.getName(), myObj); Am I using double the String size in terms of memory? Or does Java do something clever behind the scenes? Thanks

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• Analysing a JVisualVM profile - finding the source of large numbers of primitive types?

  - by MalcomTucker

  I am trying to reduce the memory footprint of my application. JVisualVM heap dumps report that the objects taking up the most space are: char[] byte[] int[] Which isn't particularly helpful. How can I track these objects back to the parent classes that are holding them? Thanks

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• Java multi-threading - what is the best way to monitor the activity of a number of threads?

  - by MalcomTucker

  I have a number of threads that are performing a long runing task. These threads themselves have child threads that do further subdivisions of work. What is the best way for me to track the following: How many total threads my process has created What the state of each thread currently is What part of my process each thread has currently got to I want to do it in as efficient a way as possible and once threads finish, I don't want any references to them hanging around becasuse I need to be freeing up memory as early as possible. Any advice?

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• Android - modes of connectivity, device identification and device inter-communication?

  - by MalcomTucker

  Can someone explain a couple of very simple concepts to me - I'm interested in mobile devices running android and how they are identified over networks. Some scenarios: Device is connected over WiFi - presumably the device has a standard IP address as with any host and can communicate with any other android host over TCP/IP (assuming it knows the participating device's IP? Device is connected over bluetooth - how are devices identified in this case? Device is connected over mobile operator's network - this is the one I'm interested in and confused by - is there anyway for two or more devices to discover each other and communicate via the mobile operator's network? How does a device communicate with a backend server in this scenario? In other words, how do apps and devices communicate when not connected to a WiFi network? Thanks for any advice..

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• How to insert into two different tables in one statement with Java and MySQL?

  - by MalcomTucker

  I am using Java, Spring (NamedParameterJdbcTemplate) and MySQL. My statement looks like this: INSERT INTO Table1 (Name) VALUES (?); INSERT INTO Table2 (Path, Table1Id) VALUES (?, LAST_INSERT_ID()) But it is throwing the following error: PreparedStatementCallback; bad SQL grammar [INSERT INTO Table1 (Name) VALUES (?); INSERT INTO Table2 (Path, Table1Id) VALUES (?, LAST_INSERT_ID())] ` Nested exception is: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INSERT INTO Table2 (Path, Table1Id' at line 1 The syntax works fine in MySQL but something is up when combining via the Spring template. Thanks!

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• When does the call() method get called in a Java Executor using Callable objects?

  - by MalcomTucker

  This is some sample code from an example. What I need to know is when call() gets called on the callable? What triggers it? public class CallableExample { public static class WordLengthCallable implements Callable { private String word; public WordLengthCallable(String word) { this.word = word; } public Integer call() { return Integer.valueOf(word.length()); } } public static void main(String args[]) throws Exception { ExecutorService pool = Executors.newFixedThreadPool(3); Set<Future<Integer>> set = new HashSet<Future<Integer>>(); for (String word: args) { Callable<Integer> callable = new WordLengthCallable(word); Future<Integer> future = pool.submit(callable); //**DOES THIS CALL call()?** set.add(future); } int sum = 0; for (Future<Integer> future : set) { sum += future.get();//**OR DOES THIS CALL call()?** } System.out.printf("The sum of lengths is %s%n", sum); System.exit(sum); } }

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• What is the relationship between MIDP and JME (and Android)?

  - by MalcomTucker

  What is the distinction between MIDP and JME? How do both relate to Android? As far as I understand JME isn't supported on Android - is that correct? Thanks

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• How to make a div expand vertically to wrap the content within it?

  - by MalcomTucker

  I have a div which wraps a number of images that are generated dynamically. I don't know how high the list of images is. My problem is the div that contains the dynamically generated images doesn't behave like it is housing any content - I want it to extend to the height of the list of images. Each image is itself wrapped in a div. This is the wrapper div: .block { padding:10px; margin-top:10px; height:auto; background-color:#f9f9f9; } This is the markup dynamically generated for (one of) the images: <div class="block"> <div style="float: left; padding: 2px 2px 2px 2px;"><IMG SRC="45.jpg" BORDER="0"/></div> ..... How do I get the block div to extend down with the images? Thanks

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• How do I get a peak memory usage snapshot from JVisualVM?

  - by MalcomTucker

  I need a memory snapshot at the peak of my application's memory usage - is there an easy way to achieve this? thanks

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• Algorithm design: can you provide a solution to the multiple knapsack problem?

  - by MalcomTucker

  I am looking for a pseudo-code solution to what is effectively the Multiple Knapsack Problem (optimisation statement is halfway down the page). I think this problem is NP Complete so the solution doesn't need to be optimal, rather if it is fairly efficient and easily implemented that would be good. The problem is this: I have many work items, with each taking a different (but fixed and known) amount of time to complete. I need to divide these work items into groups so as to have the smallest number of groups (ideally), with each group of work items taking no longer than a given total threshold - say 1 hour. I am flexible about the threshold - it doesnt need to be rigidly applied, though should be close. My idea was to allocate work items into bins where each bin represents 90% of the threshold, 80%, 70% and so on. I could then match items that take 90% to those that take 10%, and so on. Any better ideas?

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• How do I ensure data consistency in this concurrent situation?

  - by MalcomTucker

  The problem is this: I have multiple competing threads (100+) that need to access one database table Each thread will pass a String name - where that name exists in the table, the database should return the id for the row, where the name doesn't already exist, the name should be inserted and the id returned. There can only ever be one instance of name in the database - ie. name must be unique How do I ensure that thread one doesn't insert name1 at the same time as thread two also tries to insert name1? In other words, how do I guarantee the uniqueness of name in a concurrent environment? This also needs to be as efficient as possible - this has the potential to be a serious bottleneck. I am using MySQL and Java. Thanks

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• Can I lock tables in an IF statement in MySQL?

  - by MalcomTucker

  This is throwing a syntax error - --from body of a stored proc IF (name = in_name) SET out_id = temp; ELSE LOCK TABLE People WRITE; INSERT INTO People (Name) VALUES (in_name); UNLOCK TABLE; SELECT LAST_INSERT_ID() INTO out_id END IF do I have to lock any tables I need at the start of the SP?

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• What came before web services and SOA?

  - by MalcomTucker

  I'm interested in the history of distributed, collaborative, cross-organisational programming paradigms - web services and SOA are de-facto now, but what came before? What models have been superceded by SOA? Thanks

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• Is there anyway to get the generated keys when using Spring JDBC batchUpdate?

  - by MalcomTucker

  I am using JDBC and want to batch insert, but I need the generated keys for the next round of inserts - is there anyway to accomplish this? MapSqlParameterSource[] batchArgs .... DAL.getNamedParameterTemplate().batchUpdate("INSERT INTO...", batchArgs); Thanks

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• How to make a div expand vertically to wrpa the content within it?

  - by MalcomTucker

  I have a div which wraps a number of images that are generated dynamically. I don't know how high the list of images is. My problem is the div that contains the dynamically generated images doesn't behave like it is housing any content - I want it to extend to the height of the list of images. Each image is itself wrapped in a div. This is the wrapper div: .block { padding:10px; margin-top:10px; height:auto; background-color:#f9f9f9; } This is the markup dynamically generated for (one of) the images: <div class="block"> <div style="float: left; padding: 2px 2px 2px 2px;"><IMG SRC="45.jpg" BORDER="0"/></div> ..... How do I get the block div to extend down with the images? Thanks

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• How do I guarantee row uniqueness in MySQL without the use of a UNIQUE constraint?

  - by MalcomTucker

  Hi I have some fairly simple requirements but I'm not sure how I implement them: I have multiple concurrent threads running the same query The query supplies a 'string' value - if it exists in the table, the query should return the id of the matching row, if not the query should insert the 'string' value and return the last inserted id The 'string' column is (and must be) a text column (it's bigger than varchar 255) so I cannot set it as unique - uniqueness must be enforced through the access mechanism The query needs to be in stored procedure format (which doesnt support table locks in MySQL) How can I guarantee that 'string' is unique? How can I prevent other threads writing to the table after another thread has read it and found no matching 'string' item? Thanks for any advice..

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• What areas of computer science are particularly relevant to mobile development?

  - by MalcomTucker

  This isn't a platform specific question - rather I'm interested in the general platform independent areas of computer science that are particularly relevant to mobile applications development. For example, things like compression techniques, distributed synchronisation algorithims etc.. what theoretical concepts have you found relevant, useful or enabling when building mobile apps?

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• Is there a timeout for threads waiting on a synchronised method in Java?

  - by MalcomTucker

  Is there a default timeout for threads waiting on a synchronised method in Java? Some threads in my app are not completing as expected. Is there anyway to check whether threads have died as a result of timeouts?

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• Connecting to an RMI server that sits behind a firewall?

  - by MalcomTucker

  I know my RMI app works correctly - it works fine when the server is on localhost and inside the LAN but when connecting to an external RMI server it fails when trying to make stub calls So the server is bound to localhost (an internal IP - 192.168.1.73) but the client is specifying an external IP (45.4.234.56) - which then gets forwarded to the internal server. How do you resolve this problem? thanks

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• C++ Template Usage

  - by MalcomTucker

  If I have a template definition like the one below, can someone provide a code sample for how I would actually instantiate an instance of this with two of my own classes? template <class T1, class T2> class LookUpTable { public: LookUpTable(); void set(Tl x, T2* y); T2* get(Tl x); }; Thanks.

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