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• How do I bind HTML table data to a java object in a spring controller?

  - by predhme

  I have a spring MVC application using JSP as my view technologies with Jquery for AJAX. I have a table such as the following: <table> <tr> <td>name1</td> <td>value1</td> <td>setting1</td> </tr> <tr> <td>name2</td> <td>value2</td> <td>setting2</td> </tr> </table> I need to serialize this table so that it can later be bound to an object in my controller. However the jquery serialize() method only works on form fields. What would be the best approach to get the table data into the HTTP request so that I can later bind it to a java object?

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• Best (Java) book for understanding 'under the bonnet' for programming?

  - by Ben

  What would you say is the best book to buy to understand exactly how programming works under the hood in order to increase performance? I've coded in assembly at university, I studied computer architecture and I obviously did high level programming, but what I really dont understand is things like: -what is happening when I perform a cast -whats the difference in performance if I declare something global as opposed to local? -How does the memory layout for an ArrayList compare with a Vector or LinkedList? -Whats the overhead with pointers? -Are locks more efficient than using synchronized? -Would creating my own array using int[] be faster than using ArrayList -Advantages/disadvantages of declaring a variable volatile I have got a copy of Java Performance Tuning but it doesnt go down very low and it contains rather obvious things like suggesting a hashmap instead of using an ArrayList as you can map the keys to memory addresses etc. I want something a bit more Computer Sciencey, linking the programming language to what happens with the assembler/hardware. The reason im asking is that I have an interview coming up for a job in High Frequency Trading and everything has to be as efficient as possible, yet I cant remember every single possible efficiency saving so i'd just like to learn the fundamentals. Thanks in advance

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• Do the 'up to date' guarantees provided by final field in Java's memory model extend to indirect ref

  - by mattbh

  The Java language spec defines semantics of final fields in section 17.5: The usage model for final fields is a simple one. Set the final fields for an object in that object's constructor. Do not write a reference to the object being constructed in a place where another thread can see it before the object's constructor is finished. If this is followed, then when the object is seen by another thread, that thread will always see the correctly constructed version of that object's final fields. It will also see versions of any object or array referenced by those final fields that are at least as up-to-date as the final fields are. My question is - does the 'up-to-date' guarantee extend to the contents of nested arrays, and nested objects? An example scenario: Thread A constructs a HashMap of ArrayLists, then assigns the HashMap to final field 'myFinal' in an instance of class 'MyClass' Thread B sees a (non-synchronized) reference to the MyClass instance and reads 'myFinal', and accesses and reads the contents of one of the ArrayLists In this scenario, are the members of the ArrayList as seen by Thread B guaranteed to be at least as up to date as they were when MyClass's constructor completed?

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• Which options do I have for Java process communication?

  - by Dmitriy Matveev

  We have a place in a code of such form: void processParam(Object param) { wrapperForComplexNativeObject result = jniCallWhichMayCrash(param); processResult(result); } processParam - method which is called with many different arguments. jniCallWhichMayCrash - a native method which is intended to do some complex processing of it's parameter and to create some complex object. It can crash in some cases. wrapperForComplexNativeObject - wrapper type generated by SWIG processResult - a method written in pure Java which processes it's parameter by creation of several kinds (by the kinds I'm not meaning classes, maybe some like hierarchies) of objects: 1 - Some non-unique objects which are referencing each other (from the same hierarchy), these objects can have duplicates created from the invocations of processParam() method with different parameter values. Since it's costly to keep all the duplicates it's necessary to cache them. 2 - Some unique objects which are referencing each other (from the same hierarchy) and some of the objects of 1st kind. After processParam is executed for each of the arguments from some set the data created in processResult will be processed together. The problem is in fact that jniCallWhichMayCrash method may crash the entire JVM and this will be very bad. The reason of crash may be such that it can happen for one argument value and not for the other. We've decided that it's better to ignore crashes inside of JVM and just skip some chunks of data when such crashes occur. In order to do this we should run processParam function inside of separate process and pass the result somehow (HOW? HOW?! This is a question) to the main process and in case of any crashes we will only lose some part of data (It's ok) without lose of everything else. So for now the main problem is implementation of transport between different processes. Which options do I have? I can think about serialization and transmitting of binary data by the streams, but serialization may be not very fast due to object complexity. Maybe I have some other options of implementing this?

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• Client Server communication in Java - which approach to use?

  - by markovuksanovic

  I have a typical client server communication - Client sends data to the server, server processes that, and returns data to the client. The problem is that the process operation can take quite some time - order of magnitude - minutes. There are a few approaches that could be used to solve this. Establish a connection, and keep it alive, until the operation is finished and the client receives the response. Establish connection, send data, close the connection. Now the processing takes place and once it is finished the server could establish a connection to the client to send the data. Establish a connection, send data, close the connection. Processing takes place. client asks server, every n minutes/seconds if the operation is finished. If the processing is finished the client fetches the data. I was wondering which approach would be the best way to use. Is there maybe some "de facto" standard for solving this problem? How "expensive" is opening a socket in Java? Solution 1. seems pretty nasty to me, but 2. and 3. could do. The problem with solution 2. is that the server needs to know on which port the client is listening, while solution 3. adds some network overhead.

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• How Do I Parse this XML in Java SAX?

  - by Tiever

  I am using the SAX parser in java. I am not sure: 1) What classes I need for this kind of situation? I am guessing I want to have Classes for (please let me know if my thoughts are completely wrong): -FosterHome (Contains an Arraylist of Family and Child) -Family (Contains ArrayList for Child and a String fro parent) -Child (contains ArrayList for ChildID) 2) How to handle this situation in the startElement and endElement method What complicates is due to the ChildID appearing in both the ChildList and the RemainingChildList. Appreciate anyone who can help me out. <FosterHome> <Orphanage>Happy Days Daycare</Orphanage> <Location>Apple Street</Location> <Families> <Family> <Parent>Adams</ParentID> <ChildList> <ChildID>Child1</ChildID> <ChildID>Child2</ChildID> </ChildList> </Family> <Family> <Parent>Adams</ParentID> <ChildList> <ChildID>Child3</ChildID> <ChildID>Child4</ChildID> </ChildList> </Family> </Families> <RemainingChildList> <ChildID>Child5</ChildID> <ChildID>Child6</ChildID> </RemainingChildList> </FosterHome>

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• Passing the Class<T> in java of a generic list?

  - by Rob Stevenson-Leggett

  I have a method for reading JSON from a service, I'm using Gson to do my serialization and have written the following method using type parameters. public T getDeserializedJSON(Class<T> aClass,String url) { Reader r = getJSONDataAsReader(url); Gson gson = new Gson(); return gson.fromJson(r, aClass); } I'm consuming json which returns just an array of a type e.g. [ { "prop":"value" } { "prop":"value" } ] I have a java class which maps to this object let's call it MyClass. However to use my method I need to do this: RestClient<ArrayList<MyClass>> restClient = new RestClient<ArrayList<MyClass>>(); ArrayList<MyClass> results = restClient.getDeserializedJSON(ArrayList<MyClass>.class, url); However, I can't figure out the syntax to do it. Passing just ArrayList.class doesn't work. So is there a way I can get rid of the Class parameter or how do I get the class of the ArrayList of MyClass?

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• How to call a method in another class in Java?

  - by Puchatek

  Currently I have two classes. a classroom class and a School class. I would like to write a method in the School class to call public void setTeacherName(String newTeacherName) from the classroom class. public class classroom { private String classRoomName; private String teacherName; public void setClassRoomName(String newClassRoomName) { classRoomName = newClassRoomName; } public String returnClassRoomName() { return classRoomName; } public void setTeacherName(String newTeacherName) { teacherName = newTeacherName; } public String returnTeacherName() { return teacherName; } } import java.util.ArrayList; public class School { private ArrayList<classroom> classrooms; private String classRoomName; private String teacherName; public School() { classrooms = new ArrayList<classroom>(); } public void addClassRoom(classroom newClassRoom, String theClassRoomName) { classrooms.add(newClassRoom); classRoomName = theClassRoomName; } // how to write a method to add a teacher to the classroom by using the classroom parameter // and the teachers name }

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• Java: Cleaing up connection reset (but not by peer).

  - by Zombies

  There seems to be some confusion as well contradicting statements on various SO answers: http://stackoverflow.com/questions/585599/whats-causing-my-java-net-socketexception-connection-reset . You can see here that the accepted answer states that the connecteion was closed by other side. But this is not true, closing a connection doesn't cause a connection reset. It is cauesed by "an underlying TCP/IP error." What I want to know is if a SocketException: Connection reset means really besides "unerlying TCP/IP Error." What really causes this? As I doubt it has anything to do with the connection being closed (since closing a connection isn't an exception worthy flag, and reading from a closed connection is, but that isn't an "underlying TCP/IP error." My hypothesis is this Connection reset is caused from a server's failure to acknowledge an ACK packet (either wholly or just improperly as per TCP/IP). And that a SocketTimeoutException is generated only when no data is generated to be read (since this is thrown during a read after a certain duration, and read is waiting for data, but is not concerned with ACK packets). In other words, read() throws SocketTimeoutException if it didn't read any bytes of actual data (DATA LAYER) in its allotted time.

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• How to call a method in another class using the arraylist index in java?

  - by Puchatek

  Currently I have two classes. A Classroom class and a School class. public void addTeacherToClassRoom(Classroom myClassRoom, String TeacherName) I would like my method addTeacherToClassRoom to use the Classroom Arraylist index number to setTeacherName e.g. int 0 = maths int 1 = science I would like to setTeacherName "Daniel" in int 1 science. many, thanks public class Classroom { private String classRoomName; private String teacherName; public void setClassRoomName(String newClassRoomName) { classRoomName = newClassRoomName; } public String returnClassRoomName() { return classRoomName; } public void setTeacherName(String newTeacherName) { teacherName = newTeacherName; } public String returnTeacherName() { return teacherName; } } import java.util.ArrayList; public class School { private ArrayList<Classroom> classrooms; private String classRoomName; private String teacherName; public School() { classrooms = new ArrayList<Classroom>(); } public void addClassRoom(Classroom newClassRoom, String theClassRoomName) { classrooms.add(newClassRoom); classRoomName = theClassRoomName; } public void addTeacherToClassRoom(Classroom myClassRoom, String TeacherName) { myClassRoom.setTeacherName(TeacherName); } }

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• How to specify hash algorithm when updating LDAP via Java?

  - by JuanZe

  Is there a way to specify the hash algorithm (MD5, SHA1, etc.) to use for storing the passwords when you update an Open LDAP directory using Java APIs with code like this: private void resetPassword(String principal, String newPassword) throws NamingException { InitialDirContext ctxAdmin = null; Hashtable<String, String> ctxData = new Hashtable<String, String>(); ctxData.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory"); ctxData.put(Context.PROVIDER_URL, "ldap://myserver:389"); ctxData.put(Context.SECURITY_AUTHENTICATION, "simple"); ctxData.put(Context.SECURITY_PRINCIPAL, "admin_dn"); ctxData.put(Context.SECURITY_CREDENTIALS, "admin_passwd"); InitialDirContext ctxAdmin = new InitialDirContext(ctxData); if (newPassword == null || newPassword.equals("")) { String msg = "Password can't be null"; throw new NamingException(msg); } else { if (principal == null || principal.equals("")) { String msg = "Principal can't be null"; throw new NamingException(msg); } else { if (ctxAdmin == null) { String errCtx = "Can't get LDAP context"; throw new NamingException(errCtx); } } } BasicAttribute attr = new BasicAttribute("userpassword", newPassword); ModificationItem modItem = new ModificationItem(DirContext.REPLACE_ATTRIBUTE, attr); ModificationItem[] items = new ModificationItem[1]; items[0] = modItem; ctxAdmin.modifyAttributes("cn=" + principal + ",ou=Users,dc=com", items); }

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• How to iteratively generate k elements subsets from a set of size n in java?

  - by Bea Metitiri

  Hi, I'm working on a puzzle that involves analyzing all size k subsets and figuring out which one is optimal. I wrote a solution that works when the number of subsets is small, but it runs out of memory for larger problems. Now I'm trying to translate an iterative function written in python to java so that I can analyze each subset as it's created and get only the value that represents how optimized it is and not the entire set so that I won't run out of memory. Here is what I have so far and it doesn't seem to finish even for very small problems: public static LinkedList<LinkedList<Integer>> getSets(int k, LinkedList<Integer> set) { int N = set.size(); int maxsets = nCr(N, k); LinkedList<LinkedList<Integer>> toRet = new LinkedList<LinkedList<Integer>>(); int remains, thresh; LinkedList<Integer> newset; for (int i=0; i<maxsets; i++) { remains = k; newset = new LinkedList<Integer>(); for (int val=1; val<=N; val++) { if (remains==0) break; thresh = nCr(N-val, remains-1); if (i < thresh) { newset.add(set.get(val-1)); remains --; } else { i -= thresh; } } toRet.add(newset); } return toRet; } Can anybody help me debug this function or suggest another algorithm for iteratively generating size k subsets? EDIT: I finally got this function working, I had to create a new variable that was the same as i to do the i and thresh comparison because python handles for loop indexes differently.

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• How to have variables with dynamic data types in Java?

  - by Nazgulled

  Hi, I need to have a UserProfile class that it's just that, a user profile. This user profile has some vital user data of course, but it also needs to have lists of messages sent from the user friends. I need to save these messages in LinkedList, ArrayList, HashMap and TreeMap. But only one at a time and not duplicate the message for each data structure. Basically, something like a dynamic variable type where I could pick the data type for the messages. Is this, somehow, possible in Java? Or my best approach is something like this? I mean, have 2 different classes (for the user profile), one where I host the messages as Map<K,V> (and then I use HashMap and TreeMap where appropriately) and another class where I host them as List<E> (and then I use LinkedList and ArrayList where appropriately). And probably use a super class for the UserProfile so I don't have to duplicate variables and methods for fields like data, age, address, etc... Any thoughts?

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• Why aren't my threads start at the same time? Java

  - by Ada

  Hi, I have variable number of threads which are used for parallel downloading. I used this, for(int i = 0; i< sth; i++){ thrList.add(new myThread (parameters)); thrList.get(i).start(); thrList.get(i).join(); } I don't know why but they wait for each other to complete. When using threads, I am supposed get mixed print outs, since right then there are several threads running that code. However, when I print them out, they are always in order and one thread waits for the previous one to finish first. I only want them to join the main thread, not wait for each other. I noticed that when I measured time while downloading in parallel. How can I fix this? Why are they doing it in order? In my .java, there is MyThread class with run and there is Downloader class with static methods and variables. Would they be the cause of this? The static methods and variables? How can I fix this problem?

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• PDFBox Pagebreak strange Nullpointer exception

  - by schneiti

  I currently try printing out text on multiple pages. For this, I count the number of rows and when they reach a fixed amount a method called pagebreakis executed. After the first pagebreak, when I try setting a font using contentstream.setFont(PDType1Font.HELVETICA, 12); it yields the following errormessage occuring at the described setFont-row. java.lang.NullPointerException at org.apache.pdfbox.pdmodel.edit.PDPageContentStream.setFont(PDPageContentStream.java:321) at com.xy.deu.xy.abc.db.schemavergleich.PDFDocumenter.drawBGCS(PDFDocumenter.java:781) at com.xy.deu.xy.abc.db.schemavergleich.PDFDocumenter.createPDFDocumentation(PDFDocumenter.java:205) at com.xy.deu.xy.abc.db.schemavergleich.MainClass.createPDFandOutput(MainClass.java:361) at com.xy.deu.xy.abc.db.schemavergleich.MainClass.start(MainClass.java:231) at com.xy.deu.xy.abc.db.schemavergleich.MainClass.main(MainClass.java:180) Below is the code that gets executed as the error occurs. ... // If Table gets to long for a page -> pagebreak: if(currentLines > 37) { pageBreak(currentLinePos); // TODO Currently causing app to crash } ... private void pageBreak(int currentLine) throws Exception { contentStream.endText(); contentStream.close(); // Create new page page = new PDPage(PDPage.PAGE_SIZE_A4); doc.addPage( page ); // Create a new font object selecting one of the PDF base fonts font = PDType1Font.HELVETICA; // Start a new content stream which will "hold" the to be created content contentStream = new PDPageContentStream(doc, page); currentLines = 0; mediabox = page.findMediaBox(); contentStream.beginText(); contentStream.moveTextPositionByAmount(startX, startY); contentStream.setFont(PDType1Font.HELVETICA, 12); } Now comes the strange thing: Debugging yields into nothing that is not set. I'll attach a screenshot for you right at the position where the error occurs:

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• How to implement the API/SPI Pattern in Java?

  - by Adam Tannon

  I am creating a framework that exposes an API for developers to use: public interface MyAPI { public void doSomeStuff(); public int getWidgets(boolean hasRun); } All the developers should have to do is code their projects against these API methods. I also want them to be able to place different "drivers"/"API bindings" on the runtime classpath (the same way JDBC or SLF4J work) and have the API method calls (doSomeStuff(), etc.) operate on different 3rd party resources (files, servers, whatever). Thus the same code and API calls will map to operations on different resources depending on what driver/binding the runtime classpath sees (i.e. myapi-ftp, myapi-ssh, myapi-teleportation). How do I write (and package) an SPI that allows for such runtime binding, and then maps MyAPI calls to the correct (concrete) implementation? In other words, if myapi-ftp allows you to getWidgets(boolean) from an FTP server, how would I could this up (to make use of both the API and SPI)? Bonus points for concrete, working Java code example! Thanks in advance!

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• Is there a Java data structure that is effectively an ArrayList with double indicies and built-in in

  - by Bob Cross

  I am looking for a pre-built Java data structure with the following characteristics: It should look something like an ArrayList but should allow indexing via double-precision rather than integers. Note that this means that it's likely that you'll see indicies that don't line up with the original data points (i.e., asking for the value that corresponds to key "1.5"). As a consequence, the value returned will likely be interpolated. For example, if the key is 1.5, the value returned could be the average of the value at key 1.0 and the value at key 2.0. The keys will be sorted but the values are not ensured to be monotonically increasing. In fact, there's no assurance that the first derivative of the values will be continuous (making it a poor fit for certain types of splines). Freely available code only, please. For clarity, I know how to write such a thing. In fact, we already have an implementation of this and some related data structures in legacy code that I want to replace due to some performance and coding issues. What I'm trying to avoid is spending a lot of time rolling my own solution when there might already be such a thing in the JDK, Apache Commons or another standard library. Frankly, that's exactly the approach that got this legacy code into the situation that it's in right now.... Is there such a thing out there in a freely available library?

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• Java: How do I implement a method that takes 2 arrays and returns 2 arrays?

  - by HansDampf

  Okay, here is what I want to do: I want to implement a crossover method for arrays. It is supposed to take 2 arrays of same size and return two new arrays that are a kind of mix of the two input arrays. as in [a,a,a,a] [b,b,b,b] ------ [a,a,b,b] [b,b,a,a]. Now I wonder what would be the suggested way to do that in Java, since I cannot return more than one value. My ideas are: - returning a Collection(or array) containing both new arrays. I dont really like that one because it think would result in a harder to understand code. - avoiding the need to return two results by calling the method for each case but only getting one of the results each time. I dont like that one either, because there would be no natural order about which solution should be returned. This would need to be specified, though resulting in harder to understand code. Plus, this will work only for this basic case, but I will want to shuffle the array before the crossover and reverse that afterwards. I cannot do the shuffling isolated from the crossover since I wont want to actually do the operation, instead I want to use the information about the permutation while doing the crossover, which will be a more efficient way I think.

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• Learning Java and logic using debugger. Did I cheat?

  - by centr0

  After a break from coding in general, my way of thinking logically faded (as if it was there to begin with...). I'm no master programmer. Intermediate at best. I decided to see if i can write an algorithm to print out the fibonacci sequence in Java. I got really frustrated because it was something so simple, and used the debugger to see what was going on with my variables. solved it in less than a minute with the help of the debugger. Is this cheating? When I read code either from a book or someone else's, I now find that it takes me a little more time to understand. If the alghorithm is complex (to me) i end up writing notes as to whats going on in the loop. A primitive debugger if you will. When you other programmers read code, do you also need to write things down as to whats the code doing? Or are you a genius and and just retain it?

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• Java: multi-threaded maps: how do the implementations compare?

  - by user346629

  I'm looking for a good hash map implementation. Specifically, one that's good for creating a large number of maps, most of them small. So memory is an issue. It should be thread-safe (though losing the odd put might be an OK compromise in return for better performance), and fast for both get and put. And I'd also like the moon on a stick, please, with a side-order of justice. The options I know are: HashMap. Disastrously un-thread safe. ConcurrentHashMap. My first choice, but this has a hefty memory footprint - about 2k per instance. Collections.sychronizedMap(HashMap). That's working OK for me, but I'm sure there must be faster alternatives. Trove or Colt - I think neither of these are thread-safe, but perhaps the code could be adapted to be thread safe. Any others? Any advice on what beats what when? Any really good new hash map algorithms that Java could use an implementation of? Thanks in advance for your input!

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• Can I make a derived class inherit a derived member from its base class in Java?

  - by Eric

  I have code that looks like this: public class A { public void doStuff() { System.out.print("Stuff successfully done"); } } public class B extends A { public void doStuff() { System.out.print("Stuff successfully done, but in a different way"); } public void doMoreStuff() { System.out.print("More advanced stuff successully done"); } } public class AWrapper { public A member; public AWrapper(A member) { this.member = member; } public void doStuffWithMember() { a.doStuff(); } } public class BWrapper extends AWrapper { public B member; public BWrapper(B member) { super(member); //Pointer to member stored in two places: this.member = member; //Not great if one changes, but the other does not } public void doStuffWithMember() { member.doMoreStuff(); } } However, there is a problem with this code. I'm storing a reference to the member in two places, but if one changes and the other does not, there could be trouble. I know that in Java, an inherited method can narrow down its return type (and perhaps arguments, but I'm not certain) to a derived class. Is the same true of fields?

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• Java ORM related question - SQL Vs Google DB (Big Table?) GAE

  - by StackerFlow

  I was wondering about the following two options when one is not using SQL tables but ORM based DBs (Example - when you are using GAE) Would the second option be less efficient? Requirement: There is an object. The object has a collection of similar items. I need to store this object. Example, say the object is a tree and it has a collection of leaves. Option 1: Traditional SQL type structure: Table for the Tree (with TreeId as the identifier for a row in the Table.) Table for the Leaves (where each leaf has a TreeId and to show the leaves of a tree, I query all leaves where the TreeId is the Id of the tree.) Here, the Tree structure DOES NOT have a field with leaves. Option 2: ORM / GAE Tables: Using the same example above, I have an object for Tree where the object has a collection (Set/List in Java/C++) of leaves. I store and retrieve the Tree together with the leaves (as the leaves are implemented as a Set in the Tree object) My question is, will the second one be less efficient that the first option? If so, why? Are there other alternatives? Thank you!

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• How can I make my code work in a GUI using Swing in Java?

  - by Razor Shadow

  I'm making a tool that does multiple things, and I'm onto the last feature for now: You type in a date and it tells you what day of the week it would be on that date. So I have two problems here: I can't use I can't use arguments, it doesn't like them On the last line, I can't use jTextArea2.setText(d0); because it doesn't like that either... My code is here: public static void main(String[] args) { int d = Integer.parseInt(args[0]); int m = Integer.parseInt(args[1]); int y = Integer.parseInt(args[2]); int y0 = y - (14 - m) / 12; int x = y0 + y0/4 - y0/100 + y0/400; int m0 = m + 12 * ((14 - m) / 12) - 2; int d0 = (d + x + (31*m0)/12) % 7; System.out.println("Sunday = 0\nMonday = 1\nTuesday = 2\nWednesday = 3\nThursday = 4\nFriday = 5\nSaturday = 6\nThis date is on a:"); System.out.println(d0); Basically, this code was at first for use with the console, and now I want to implement it into a Swing GUI app. I'm only a week into learning Java, so excuse me if the problem is obvious or easy to fix... But can anyone figure out how to work it? Thanks!

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• Filling in uninitialized array in java? (or workaround!)

  - by AlexRamallo

  Hello all, I'm currently in the process of creating an OBJ importer for an opengles android game. I'm relatively new to the language java, so I'm not exactly clear on a few things. I have an array which will hold the number of vertices in the model(along with a few other arrays as well): float vertices[]; The problem is that I don't know how many vertices there are in the model before I read the file using the inputstream given to me. Would I be able to fill it in as I need to like this?: vertices[95] = 5.004f; //vertices was defined like the example above or do I have to initialize it beforehand? if the latter is the case then what would be a good way to find out the number of vertices in the file? Once I read it using inputstreamreader.read() it goes to the next line until it reads the whole file. The only thing I can think of would be to read the whole file, count the number of vertices, then read it AGAIN the fill in the newly initialized array. Is there a way to dynamically allocate the data as is needed?

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• How do I use requestFocus in a Java JFrame GUI?

  - by JohnWong

  I am given an assignment but I am totally new to Java (I have been programming in C++ and Python for two years). So we are doing GUI and basically we extended JFrame and added a couple fields. Say we have a field named "Text 1" and "Text 2". When user presses enter with the cursor in Text 1, move the focus to Text 2. I tried to add private JTextField textfield1() { textfield1 = new JTextField(); textfield1.setPreferredSize(new Dimension(200, 20)); textfield1.addActionListener( new ActionListener() { public void actionPerformed(ActionEvent e) { textfield1text = textfield1.getText().trim(); textfield1.setText(textfield1text); System.out.println(textfield1text); textfield1.requestFocus(); } }); return textfield1; } But that doesn't work at all. I noticed that requestFocus is not recommended, and instead one should use requestFocusWindows. But I tried that too. Upon some readings it seems like I have to do keyboard action and listener? But my teacher said it only requires 1 line...

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