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  • I have a problem with the following Java code

    - by Sanjeev
    public class b { public static void main(String[] args) { byte b = 1; long l = 127; // b = b + l; // 1 if I try this then it does not compile b += l; // 2 if I try this then it does compile System.out.println(b); } } I am using this code but I have problem: I don't understand why b=b+l; is not compiling but if I write b+=l; then it compiles and runs. Please explain why this happens.

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  • Java Simple ActionListener Questions

    - by Allen
    I have a main class in a program that launches another class that handles all the GUI stuff. In the GUI, i have a button that i need to attach an ActionListener to. The only problem is, the code to be executed needs to reside within the main class. How can i get the ActionPerformed() method to execute in the main class when a button is clicked elsewhere?

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  • Java: Make a method abstract for each extending class

    - by Martijn Courteaux
    Hi, Is there any keyword or design pattern for doing this? public abstract class Root { public abstract void foo(); } public abstract class SubClass extends Root { public void foo() { // Do something } } public class SubberClass extends SubClass { // Here is it not necessary to override foo() // So is there a way to make this necessary? // A way to obligate the developer make again the override } Thanks

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  • java.math.BigInteger pow(exponent) question

    - by Jan Kraus
    Hi, I did some tests on pow(exponent) method. Unfortunately, my math skills are not strong enough to handle the following problem. I'm using this code: BigInteger.valueOf(2).pow(var); Results: var | time in ms 2000000 | 11450 2500000 | 12471 3000000 | 22379 3500000 | 32147 4000000 | 46270 4500000 | 31459 5000000 | 49922 See? 2,500,000 exponent is calculated almost as fast as 2,000,000. 4,500,000 is calculated much faster then 4,000,000. Why is that? To give you some help, here's the original implementation of BigInteger.pow(exponent): public BigInteger pow(int exponent) { if (exponent < 0) throw new ArithmeticException("Negative exponent"); if (signum==0) return (exponent==0 ? ONE : this); // Perform exponentiation using repeated squaring trick int newSign = (signum<0 && (exponent&1)==1 ? -1 : 1); int[] baseToPow2 = this.mag; int[] result = {1}; while (exponent != 0) { if ((exponent & 1)==1) { result = multiplyToLen(result, result.length, baseToPow2, baseToPow2.length, null); result = trustedStripLeadingZeroInts(result); } if ((exponent >>>= 1) != 0) { baseToPow2 = squareToLen(baseToPow2, baseToPow2.length, null); baseToPow2 = trustedStripLeadingZeroInts(baseToPow2); } } return new BigInteger(result, newSign); }

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  • Java Collection performance question

    - by Shervin
    I have created a method that takes two Collection<String> as input and copies one to the other. However, I am not sure if I should check if the collections contain the same elements before I start copying, or if I should just copy regardless. This is the method: /** * Copies from one collection to the other. Does not allow empty string. * Removes duplicates. * Clears the too Collection first * @param target * @param dest */ public static void copyStringCollectionAndRemoveDuplicates(Collection<String> target, Collection<String> dest) { if(target == null || dest == null) return; //Is this faster to do? Or should I just comment this block out if(target.containsAll(dest)) return; dest.clear(); Set<String> uniqueSet = new LinkedHashSet<String>(target.size()); for(String f : target) if(!"".equals(f)) uniqueSet.add(f); dest.addAll(uniqueSet); } Maybe it is faster to just remove the if(target.containsAll(dest)) return; Because this method will iterate over the entire collection anyways.

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  • assign keys for combo box in java

    - by adesh kumar
    I want to add a JComboBox in Swing that is simple but I want to assign the values for each items in combo. I have the following code JComboBox jc1= new JComboBox(); jc1.addItem("a"); jc1.addItem("b"); jc1.addItem("c"); Now what I want is that when click on combo box it should return 1, 2 and 3 correspondingly instead of a ,b, c. Is there any way to assign the key values for each items in combo box?

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  • Filtering out specific objects from a search query in Alfresco using Java

    - by Snowright
    I have a HashSet containing all groups I've retrieved from my database. I've been asked to filter this result by removing two specific groups. It seems trivial but I can't seem to come up with a solid solution for storing the specific groups I want to filter out. My idea is to just create an array containing references to the two groups I need to filter out. I can then filter out my search query with whatever is in the array. My concern is that in the future they may ask to filter out more groups and maybe an array may not be a good idea. //Creates the array containing groups to filter out String[] hiddenGroups = {"group1","group2"}; //retrieves all groups Set<String>allGroups = new HashSet<String>(); allGroups.addAll(authorityService.getAllAuthorities(AuthorityType.GROUP); List<String>results = new ArrayList<String>(); //filters out specified groups for (String group : allGroups) { boolean isHidden = false; for (String hiddenGroup : hiddenGroups) { if (hiddenGroup.equalsIgnorecase(group)) { isHidden = true; } } if (!isHidden){ results.add(group); } }

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  • Java Micro Edition (J2ME) - Update Record using recordstore enumeration

    - by Garbit
    Hi there, I have a record store of items which have (name, quantity, owner, status) Now when the user triggers an event i want to set the status of all items in my recordstore with "purchased" re = shoppingListStore.enumerateRecords(null, null, false); while (re.hasNextElement()) { // read current values of item byte [] itemRecord = re.nextRecord(); // deserialise byte array newItemObject.fromByteArray(itemRecord); // set item status to purchased newItemObject.setItemStatus("Purchased"); // create new bytearray and call newitemobject . tobytearray method to return a byte array of the object (using UTF8 encoded strings~) byte[] itemData = newItemObject.toByteArray(); // add new byte array to shoppinglist store shoppingListStore.setRecord(re.nextRecordId(), itemData, 0, itemData.length); } However I am overwriting the next record (using the nextRecordId), i've tried using nextRecordId - 1 but obviously this is out of bounds on the first one Hope you can help, Many thanks, andy

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  • Java Counting # of occurrences of a word in a string

    - by Doug
    I have a large text file I am reading from and I need to find out how many times some words come up. For example, the word "the". I'm doing this line by line each line is a string. I need to make sure that I only count legit "the"'s the the in other would not count. This means I know I need to use regular expressions in some way. What I was trying so far is this: numSpace += line.split("[^a-z]the[^a-z]").length; I realize the regular expression may not be correct at the moment but I tried without that and just tried to find occurrences of the word the and I get wrong numbers to. I was under the impression this would split the string up into an array and how many times that array was split up was how many times the word is in the string. Any ideas I would be grateful.

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  • java number exceeds long.max_value - how to detect?

    - by jurchiks
    I'm having problems detecting if a sum/multiplication of two numbers exceeds the maximum value of a long integer. Example code: long a = 2 * Long.MAX_VALUE; System.out.println("long.max * smth > long.max... or is it? a=" + a); This gives me -2, while I would expect it to throw a NumberFormatException... Is there a simple way of making this work? Because I have some code that does multiplications in nested IF blocks or additions in a loop and I would hate to add more IFs to each IF or inside the loop. Edit: oh well, it seems that this answer from another question is the most appropriate for what I need: http://stackoverflow.com/a/9057367/540394 I don't want to do boxing/unboxing as it adds unnecassary overhead, and this way is very short, which is a huge plus to me. I'll just write two short functions to do these checks and return the min or max long. Edit2: here's the function for limiting a long to its min/max value according to the answer I linked to above: /** * @param a : one of the two numbers added/multiplied * @param b : the other of the two numbers * @param c : the result of the addition/multiplication * @return the minimum or maximum value of a long integer if addition/multiplication of a and b is less than Long.MIN_VALUE or more than Long.MAX_VALUE */ public static long limitLong(long a, long b, long c) { return (((a > 0) && (b > 0) && (c <= 0)) ? Long.MAX_VALUE : (((a < 0) && (b < 0) && (c >= 0)) ? Long.MIN_VALUE : c)); } Tell me if you think this is wrong.

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  • Java convert JSONObject to URL parameter

    - by Alex Ivasyuv
    What is the elegant way to convert JSONObject to URL parameters. For example, JSONObject: {stat: {123456: {x: 1, y: 2}, 123457: {z: 5, y: 2}}}} this should be like: stat[123456][x]=1&stat[123456][y]=2&stat[123457][z]=5&stat[123457][y]=2 of course with escaped symbols, and of course JSON object could be more complicated.. Maybe there already exist some mechanisms for that? Thanks,

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  • Simplest way of creating java next/previous buttons

    - by Holly
    I know that when creating buttons, like next and previous, that the code can be somewhat long to get those buttons to function. My professor gave us this example to create the next button: private void jbtnNext_Click() { JOptionPane.showMessageDialog(null, "Next" ,"Button Pressed", JOptionPane.INFORMATION_MESSAGE); try { if (rset.next()) { fillTextFields(false); }else{ //Display result in a dialog box JOptionPane.showMessageDialog(null, "Not found"); } } catch (SQLException ex) { ex.printStackTrace(); } } Though, I do not really understand how that short and simple if statement is what makes the next button function. I see that the fillTextFields(false) uses a boolean value and that you need to initialize that boolean value in the beginning of the code I believe. I had put private fillTextFields boolean = false; but this does not seem to be right... I'm just hoping someone could explain it better. Thanks :)

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  • Java multiple Images Uploader

    - by Padur
    Hello Folks I have this new requirement to develop a software which is a large scale image up loader in a web application. I was able to do the same using swing contains several feature like drag and drop, progress bar, remove file / files , modify, limit file size, verify file information, timer, verify at run time ..and its a very powerful tool which uploads images. I would like to do the same in web based app, like user selects 200 images process it and click upload and it should start uploading, like to know any feasible frameworks or any API's which help me do this faster and achieve the same kind of functionality. Please point me in correct direction. -PD

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  • How do Java mocking frameworks work?

    - by Amir Rachum
    This is NOT a question about which is the best framework, etc. I have never used a mocking framework and I'm a bit puzzled by the idea. How does it know how to create the mock object? Is it done in runtime or generates a file? How do you know its behavior? And most importantly - what is the work flow of using such a framework (what is the step-by-step for creating a test). Can anyone explain? You can choose whichever framework you like for example, just say what it is.

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  • Java do while, while

    - by Pindatjuh
    Hello, what behaviour can I expect when I run this code: do while(testA) { // do stuff } while(testB); Will it behave like: do { while(testA) { // do stuff } } while(testB); Or: if(testA) { do { // do stuff } while(testA && testB); } Or something totally unexpected? I ask this question because I think this is quite ambiguous, and for other people searching on this topic, not because I am lazy to test it out.

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  • how can i acess a variable outside of an if statement in java

    - by themanepalli
    i have defined a variable inside an if statement and I am trying to access it outside of that if statement now. now the error is saying that it cannot find the symbol which is because its being defined as an intance variable, is there a way i can change it so i can access it outside the variable? heres the code if((e.getSource()==userOrder2)&& (orderType==1)) { String buyO= userOrder2.getText(); int buyOrder= Integer.parseInt(buyO); //variable im trying to access } // trying to use buyOrder in a different if statement if(orderType==1 && (stockPrice <= buyOrder)) { orderResult.setText("The Stock" + (stockName2.getText()) + "was bought at" + stockPrice); }

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  • Google App-Engine Java Batch Update

    - by Manjoor
    I need to upload a .csv file and save the records in bigtable. My application successfully parse 200 the records in the csv files and save to table. Here is my code to save the data. for (int i=0;i<lines.length -1;i++) //lines hold total records in csv file { String line = lines[i]; //The record have 3 columns integer,integer,Text if(line.length() > 15) { int n = line.indexOf(","); if (n>0) { int ID = lInteger.parseInt(ine.substring(0,n)); int n1 = line.indexOf(",", n + 2); if(n1 > n) { int Col1 = Integer.parseInt(line.substring(n + 1, n1)); String Col2 = line.substring(n1 + 1); myTable uu = new myTable(); uu.setId(ID); uu.setCol1(MobNo); Text t = new Text(Col2); uu.setCol2(t); PersistenceManager pm = PMF.get().getPersistenceManager(); pm.makePersistent(uu); pm.close(); } } } } But when no of records grow it gives timeout error. The csv file may have upto 800 records. Is it possible to do that in App-Engine? (something like batch update)

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  • Embeddable database better than SQLite for java

    - by dexter
    I am creating a web application that is accessing a SQLite database in the server. I also have "clients" that updates this same database. As we know SQLite locks the entire database during INSERTs which are done by the clients and the web application is also trying to make some UPDATEs at the same time. So my problem now is about concurrency in database access. I would like to use an embeddable database like SQLite. Any suggestions.

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