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  • Java Data Structure

    - by Joe
    Hi there, I'm looking for a data structure that will act like a Queue so that I can hava First In First Out behaviour, but ideally I would also be able to see if an element exists in that Queue in constant time as you can do with a HashMap, rather than the linear time that you get with a LinkedList. I thought a LinkedHashMap might do the job, but although I could make an iterator and just take and then remove the first element of the iteration to produce a sort of poll() method, I'm wondering if there is a better way. Many thanks in advance

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  • Java downcasting dilemma

    - by Shades88
    please have a look at this code here. class Vehicle { public void printSound() { System.out.print("vehicle"); } } class Car extends Vehicle { public void printSound() { System.out.print("car"); } } class Bike extends Vehicle{ public void printSound() { System.out.print("bike"); } } public class Test { public static void main(String[] args) { Vehicle v = new Car(); Bike b = (Bike)v; v.printSound(); b.printSound(); Object myObj = new String[]{"one", "two", "three"}; for (String s : (String[])myObj) System.out.print(s + "."); } } Executing this code will give ClassCastException saying inheritance.Car cannot be cast to inheritance.Bike. Now look at the line Object myObj = new String[]{"one", "two", "three"};. This line is same as Vehicle v = new Car(); right? In both lines we are assigning sub class object to super class reference variable. But downcasting String[]myObj is allowed but (Bike)v is not. Please help me understand what is going on around here.

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  • Java io ugly try-finally block

    - by Tom Brito
    Is there a not so ugly way of treat the close() exception to close both streams then: InputStream in = new FileInputStream(inputFileName); OutputStream out = new FileOutputStream(outputFileName); try { copy(in, out); } finally { try { in.close(); } catch (Exception e) { try { // event if in.close fails, need to close the out out.close(); } catch (Exception e2) {} throw e; // and throw the 'in' exception } out.close(); }

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  • Strange problem with simple multithreading program in Java

    - by Elizabeth
    Hello, I am just starting play with multithreading programming. I would like to my program show alternately character '-' and '+' but it doesn't. My task is to use synchronized keyword. As far I have: class FunnyStringGenerator{ private char c; public FunnyStringGenerator(){ c = '-'; } public synchronized char next(){ if(c == '-'){ c = '+'; } else{ c = '-'; } return c; } } class ThreadToGenerateStr implements Runnable{ FunnyStringGenerator gen; public ThreadToGenerateStr(FunnyStringGenerator fsg){ gen = fsg; } @Override public void run() { for(int i = 0; i < 10; i++){ System.out.print(gen.next()); } } } public class Main{ public static void main(String[] args) throws IOException { FunnyStringGenerator FSG = new FunnyStringGenerator(); ExecutorService exec = Executors.newCachedThreadPool(); for(int i = 0; i < 20; i++){ exec.execute(new ThreadToGenerateStr(FSG)); } } } EDIT: I also testing Thread.sleep in run method instead for loop.

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  • Issue with Java join() method.

    - by gmunk
    First of all here are some code snippets: http://pastebin.com/9ZCwekXs http://pastebin.com/TtLLXPYP I'm trying to animate some images. The thing is that I want the main thread to wait for the animation thread to finish and then to continue. I searched around, read a little bit and decided to use the join() method. It perfectly waits for the thread to finish but I doesn't animate correctly. The repaint() method gets called 2 times instead of nine. I think maybe the problem is because I used singletons. Here is the singleton implementation. http://pastebin.com/bA3qLZJE

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  • How to mix two arrays in Java?

    - by roddik
    Hello. I have some String[] arrays, for example: ['a1', 'a2'] ['b1', 'b2', 'b3', 'b4'] ['c1'] How can I mix them, so that I get ['a1', 'b1', 'c1', 'a2', 'b2', 'b3', 'b4'] (0 element of a, then b, c, 1 element of a, b, c and so on)? Thanks

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  • JADE (Java) - Changing Agent Container

    - by Chad S
    Is there a way to reassign agents to a different container or will I have to create a new container and then create all new instances of the agents within the new container? I have done a lot of searching and can't seem to find anything on container reassignment. Thanks in advance for any info!

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  • Java interface design

    - by Nayn
    Hi, I had an interface initially as below. public interface testMe { public Set<String> doSomething(); } public class A implements testMe { public Set<String> doSomething() { return // Set<String> } } I had similar classes implementing testMe. Now I have to add one more class which returns Set<Some Object> public class X implements testMe() { public Set<Some OBject> doSomething() { } } How could i add this mehtod in the interface without breaking existing classes? Thanks Nayn

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  • Java: Generics, Class.isaAssignableFrom, and type casting

    - by bguiz
    This method that uses method-level generics, that parses the values from a custom POJO, JXlistOfKeyValuePairs (which is exactly that). The only thing is that both the keys and values in JXlistOfKeyValuePairs are Strings. This method wants to taken in, in addition to the JXlistOfKeyValuePairs instance, a Class<T> that defines which data type to convert the values to (assume that only Boolean, Integer and Float are possible). It then outputs a HashMap with the specified type for the values in its entries. This is the code that I have got, and it is obviously broken. private <T extends Object> Map<String, T> fromListOfKeyValuePairs(JXlistOfKeyValuePairs jxval, Class<T> clasz) { Map<String, T> val = new HashMap<String, T>(); List<Entry> jxents = jxval.getEntry(); T value; String str; for (Entry jxent : jxents) { str = jxent.getValue(); value = null; if (clasz.isAssignableFrom(Boolean.class)) { value = (T)(Boolean.parseBoolean(str)); } else if (clasz.isAssignableFrom(Integer.class)) { value = (T)(Integer.parseInt(str)); } else if (clasz.isAssignableFrom(Float.class)) { value = (T)(Float.parseFloat(str)); } else { logger.warn("Unsupporteded value type encountered in key-value pairs, continuing anyway: " + clasz.getName()); } val.put(jxent.getKey(), value); } return val; } This is the bit that I want to solve: if (clasz.isAssignableFrom(Boolean.class)) { value = (T)(Boolean.parseBoolean(str)); } else if (clasz.isAssignableFrom(Integer.class)) { value = (T)(Integer.parseInt(str)); } I get: Inconvertible types required: T found: Boolean Also, if possible, I would like to be able to do this with more elegant code, avoiding Class#isAssignableFrom. Any suggestions? Sample method invocation: Map<String, Boolean> foo = fromListOfKeyValuePairs(bar, Boolean.class);

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  • Java Interfaces/Implementation naming convention

    - by Amir Rachum
    How do you name different classes / interfaces you create? Sometimes I don't have implementation information to add to the implementation name - like interface FileHandler and class SqlFileHandler. hen this happens I usually name the interface in the "normal" name, like Truck and name the actual class TruckClass. How do you name interfaces and classes in this regard?

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  • Most Elegant Way to write isPrime in java

    - by Anantha Kumaran
    public class Prime { public static boolean isPrime1(int n) { if (n <= 1) { return false; } if (n == 2) { return true; } for (int i = 2; i <= Math.sqrt(n) + 1; i++) { if (n % i == 0) { return false; } } return true; } public static boolean isPrime2(int n) { if (n <= 1) { return false; } if (n == 2) { return true; } if (n % 2 == 0) { return false; } for (int i = 3; i <= Math.sqrt(n) + 1; i = i + 2) { if (n % i == 0) { return false; } } return true; } } public class PrimeTest { public PrimeTest() { } @Test public void testIsPrime() throws IllegalArgumentException, IllegalAccessException, InvocationTargetException { Prime prime = new Prime(); TreeMap<Long, String> methodMap = new TreeMap<Long, String>(); for (Method method : Prime.class.getDeclaredMethods()) { long startTime = System.currentTimeMillis(); int primeCount = 0; for (int i = 0; i < 1000000; i++) { if ((Boolean) method.invoke(prime, i)) { primeCount++; } } long endTime = System.currentTimeMillis(); Assert.assertEquals(method.getName() + " failed ", 78498, primeCount); methodMap.put(endTime - startTime, method.getName()); } for (Entry<Long, String> entry : methodMap.entrySet()) { System.out.println(entry.getValue() + " " + entry.getKey() + " Milli seconds "); } } } I am trying to find the fastest way to check whether the given number is prime or not. This is what is finally came up with. Is there any better way than the second implementation(isPrime2).

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  • Java language convention; getters/setters

    - by Skogen
    Public class Example { private int number; public Example(int number){ this.number = number; } public int getNumber(){ return number; } public void setNumber(int number){ this.number = number; } public static void main(String[] args){ Example e = new Example(5); What is preffered when accessing a variable within its own class; "e.number" or "e.getNumber()" ?

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  • What's wrong with my logic (Java syntax)

    - by soda
    I'm trying to make a simple program that picks a random number and takes input from the user. The program should tell the user if the guess was hot (-/+ 5 units) or cold, but I never reach the else condition. Here's the section of code: public static void giveHint (int guess) { int min = guess - 5; int max = guess + 5; if ((guess > min) && (guess < max)) { System.out.println("Hot.."); } else { System.out.println("Cold.."); } }

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  • How to access files in Java ME

    - by cancelledout
    I want to access the files encircled in this screenshot. What path should I use? This is for my JavaME application. file:///ParseExample/service1.xml doesn't seem to be the right one. http://www.fileden.com/files/2010/5/28/2872707//projectfiles.JPG

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  • Are there any libraries for parsing "number expressions" like 1,2-9,33- in Java

    - by mihi
    Hi, I don't think it is hard, just tedious to write: Some small free (as in beer) library where I can put in a String like 1,2-9,33- and it can tell me whether a given number matches that expression. Just like most programs have in their print range dialogs. Special functions for matching odd or even numbers only, or matching every number that is 2 mod 5 (or something like that) would be nice, but not needed. The only operation I have to perform on this list is whether the range contains a given (nonnegative) integer value; more operations like max/min value (if they exist) or an iterator would be nice, of course. What would be needed that it does not occupy lots of RAM if anyone enters 1-10000000 but the only number I will ever query is 12345 :-) (To implement it, I would parse a list into several (min/max/value/mod) pairs, like 1,10,0,1 for 1-10 or 11,33,1,2 for 1-33odd, or 12,62,2,10 for 12-62/10 (i. e. 12, 22, 32, ..., 62) and then check each number for all the intervals. Open intervals by using Integer.MaxValue etc. If there are no libs, any ideas to do it better/more efficient?)

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  • Java HashMap with Int Array

    - by Sunil
    Hello I am using this code to check that array is present in the HashMap. public class Test { public static void main(String[]arg) { HashMap<int[],String> map= new HashMap<int[],String>(); map.put(new int[]{1,2}, "sun"); System.out.println(map.containsKey((new int[]{1,2}))); } } But this prints False. How can I check that array is present in the HashMap. Thanks in advance.

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  • Java: charAt convert to int?

    - by sling
    Hi, I would like to key in my nirc number eg.S1234567I and then put 1234567 individualy as a integer as indiv1 as charAt(1), indiv2 as charAt(2), indiv as charAt(3), etc. However, when I do as the codes below, I cant seem to get even the first the number out? Any idea? Scanner console = new Scanner(System.in); System.out.println("Enter your NRIC number: "); String nric = console.nextLine(); int indiv1 = nric.charAt(1); System.out.println(indiv1);

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  • Java hashcode based on identity

    - by hjfreyer
    The default behavior of Object.hashCode() is to return essentially the "address" of the object so that a.hashCode() == b.hashCode() if and only if a == b. How can I get this behavior in a user-defined class if a superclass already defines hashCode()? For instance: class A { public int hashCode() { return 0; } } class B extends A { public int hashCode() { // Now I want to return a unique hashcode for each object. // In pythonic terms, it'd look something like: return Object.hashCode(this); } } Ideas?

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  • Java OS X - No app icon in dock

    - by Jacob
    I am using Jar Bundler to create a .app file out of my .jar file. When I launch the app, I do not want a dock icon to show at all. I have already tried modifying my .plist file to include: <string>NSUIElement</string> <key>1</key> But it does not work... Any help?

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  • How to Create Own HashMap in Java?

    - by Taranfx
    I know about hashing algorithm and hashCode() to convert "key" into an equivalent integer (using some mathematically random expression) that is then compressed and stored into buckets. But can someone point me to an implementation or at least data structure that should be used as baseline? I haven't found it anywhere on the web.

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  • Java ternary operator and boxing Integer/int?

    - by Markus
    I tripped across a really strange NullPointerException the other day caused by an unexpected type-cast in the ternary operator. Given this (useless exemplary) function: Integer getNumber() { return null; } I was expecting the following two code segments to be exactly identical after compilation: Integer number; if (condition) { number = getNumber(); } else { number = 0; } vs. Integer number = (condition) ? getNumber() : 0; . Turns out, if condition is true, the if-statement works fine, while the ternary opration in the second code segment throws a NullPointerException. It seems as though the ternary operation has decided to type-cast both choices to int before auto-boxing the result back into an Integer!?! In fact, if I explicitly cast the 0 to Integer, the exception goes away. In other words: Integer number = (condition) ? getNumber() : 0; is not the same as: Integer number = (condition) ? getNumber() : (Integer) 0; . So, it seems that there is a byte-code difference between the ternary operator and an equivalent if-else-statement (something I didn't expect). Which raises three questions: Why is there a difference? Is this a bug in the ternary implementation or is there a reason for the type cast? Given there is a difference, is the ternary operation more or less performant than an equivalent if-statement (I know, the difference can't be huge, but still)?

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  • Implementing Iterable in Java

    - by Artium
    I have the following code public class A extends Iterable<Integer> { ... public Iterator<Integer> iterator() { return new Iterator<Integer>() { A a; public boolean hasNext() { ... } public Integer next() { ... } public void remove(){ ... } }; I would like to initialize the "a" field in the anonymous class with the instance of A that iterator method was called on. Is it possible? Thank you.

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