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  • Why does Java think my object is a variable?

    - by user2896898
    Ok so I'm trying to make a simple pong game. I have a paddle that follows the mouse and a ball that bounces around. I wrote a method collidesWith(Sprite s) inside of my Sprite class that checks if the ball collides with the paddle (this works and isn't the problem). I have two objects extending my sprite class, a ball and a paddle object. So inside of my ball class I'm trying to check if it collides with the paddle. So I've tried if(this.collidesWith(paddle) == true){ System.out.println("They touched"); } I've also tried ball.collidesWith(paddle) and other combinations but it always says the same thing about the paddle (and the ball when I use ball.collidesWith) "Cannot find symbol. Symbol: variable paddle(or ball). Location: class Ball" So if I'm reading this right, it thinks that the paddle (and ball) are variables and it's complaining because it can't find them. How can I make it understand I am passing in objects, not variables? For extra information, an earlier assignment had me make two boxes and for them to change colors when they were colliding. In that assignment I used very similar code to above with if(boxOne.collidesWith(boxTwo) == true){ System.out.println("yes"); } And in this code it worked just fine. The program knew that boxOne and boxTwo were child classes of my Sprite class. Anyone know why they wouldn't work the same?

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  • Difference between name != null and null != name in java?

    - by GK
    I have seen people using the following conditions interchangeably. String name; //some code if(name != null) //or if(null != name) So is there any difference in this? or both are same? I am asking this here because, i heard that there is some difference in these, but don't know what. Thanks

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  • how to get http responses continuously to a java application?

    - by senrulz
    I have the coding shown below where I have sheduled to get a http response from a php web server page. public static void stopPHPDataChecker() { canStop=true; } public static void main (String args[]) { // http request to the php page and get the response PHPDataChecker pdc = new PHPDataChecker(); ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1); final ScheduledFuture<?> pdcHandle = scheduler.scheduleAtFixedRate(pdc, 0L, 10L, TimeUnit.MILLISECONDS);// Start schedule scheduler.schedule(new Runnable() { public void run() { System.out.println(">> TRY TO STOP!!!"); pdcHandle.cancel(true); Sheduler.stopPHPDataChecker(); System.out.println("DONE"); } }, 1L, TimeUnit.MILLISECONDS); do { if (canStop) { scheduler.shutdown(); } } while (!canStop); System.out.println("END"); } this coding only returns one response but I want it to get responses continuously so i can do different tasks according to the returned value. how can i do it? Thank you in advance :)

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  • Java: If I overwrite the .equals method, can I still test for reference equality with ==?

    - by shots fired
    I have the following situation: I need to sort trees based by height, so I made the Tree's comparable using the height attribute. However, I was also told to overwrite the equals and hashCode methods to avoid unpredictable behaviour. Still, sometimes I may want to compare the references of the roots or something along those lines using ==. Is that still possible or does the == comparison call the equals method?

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  • Is Java class initialized by the thread which use it for the first time?

    - by oo_olo_oo
    Lets assume following classes definition: public class A { public final static String SOME_VALUE; static { SOME_VALUE = "some.value"; } } public class B { private final String value = A.SOME_VALUE; } Assuming that the class A hasn't been loaded yet, what does happen when object of the class B is instantiated by some thread T? The class A has to be loaded and instantiated first. But my question is: if it's done in context of the thread T, or rather in context of some other (special) "classloader" thread?

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  • How to write a cgi script in perl that accepts output (an image file) from a urlconnection in a Java applet and writes it to the server?

    - by Brad Rock
    I created a Java applet for my web page that creates an image file. I want users to be able to run the applet and create unique images, click a button, and have the image they created be saved to the web server. I think I have the code down for writing the image to a URLconnection in the Java applet itself after having successfully written a file while running the applet on my system rather than the web page and saving a file to the local disk, and then altering a few things to write to a URLConnection instead of to the local disk (although we shall see how that works too). I am now trying to write a cgi script in perl that takes the image output from the URLConnection and writes the image to a file on my web server (note: I am a newbie to perl). I have found many examples of how to do something similar with simple text coming from an applet and then writing it to a text file, but I want to know how to apply the same concept to an image. Particularly, how do I read in the image? I've seen text input get read by using read(STDIN, $some_variable)--does the same thing work with an image? Likewise, how do I write the image file? What function do I use? Thanks for your help. I know that I am rather naive about all of this.

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  • Can I pass an array as arguments to a method with variable arguments in Java?

    - by user352382
    I'd like to be able to create a function like: class A { private String extraVar; public String myFormat(String format, Object ... args){ return String.format(format, extraVar, args); } } The problem here is that args is treated as Object[] in the method myFormat, and thus is a single argument to String.format, while I'd like every single Object in args to be passed as a new argument. Since String.format is also a method with variable arguments, this should be possible. If this is not possible, is there a method like String.format(String format, Object[] args)? In that case I could prepend extraVar to args using a new array and pass it to that method.

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  • Why does this while terminate before receiving a value? (java)

    - by David
    Here's the relevant code snippet. public static Territory[] assignTerri (Territory[] board, String[] colors) { for (int i = 0; i<board.length; i++) { // so a problem is that Territory.translate is void fix this. System.out.print ("What team controls ") ; Territory.translate (i) ; System.out.println (" ?") ; boolean a = false ; while (a = false) { String s = getIns () ; if ((checkColor (s, colors))) { board[i].team = (returnIndex (s, colors)) ; a =true ; } else System.out.println ("error try again") ; } System.out.print ("How many unites are on ") ; Territory.translate (i) ; System.out.println (" ?") ; int n = getInt () ; board[i].population = n ; } return board ; } As an additional piece of information, checkColor just checks to make sure that its first argument, a string, is a string in one of the indexes of its second argument, an array. It seems to me that when the while the method gets a string from the keyboard and then only if that string checks out is a true and the while allowed to terminate. The output I get though is this: What team controls Alaska ? How many unites are on Alaska ? (there is space at the end to type in an input) This would seem to suggest that the while terminates before an input is ever typed in since the first line of text is within the while while the second line of text comes after it outside of it. Why is this happening?

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  • Why does this while terminate before recieving a value? (java)

    - by David
    here's the relevant code snippet. public static Territory[] assignTerri (Territory[] board, String[] colors) { for (int i = 0; i<board.length; i++) { // so a problem is that Territory.translate is void fix this. System.out.print ("What team controls ") ; Territory.translate (i) ; System.out.println (" ?") ; boolean a = false ; while (a = false) { String s = getIns () ; if ((checkColor (s, colors))) { board[i].team = (returnIndex (s, colors)) ; a =true ; } else System.out.println ("error try again") ; } System.out.print ("How many unites are on ") ; Territory.translate (i) ; System.out.println (" ?") ; int n = getInt () ; board[i].population = n ; } return board ; } as an additional piece of information, checkColor just checks to make sure that its first argument, a string, is a string in one of the indexes of its second argument, an array. it seems to me that when the while the method gets a string from the keyboard and then only if that string checks out is a true and the while allowed to terminate. The output i get though is this: What team controls Alaska ? How many unites are on Alaska ? (there is space at the end to type in an input) This would seem to suggest that the while terminates before an input is ever typed in since the first line of text is within the while while the second line of text comes after it outside of it. why is this happening?

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  • how to print numbers using for loop in java?

    - by Balkrushn Viroja
    I have one text box in which I take the value of how many number do you want to print. Now My question is that how can I use for loop so that the number which I want to print is equal to the number that I got from textbox.One more thing is that i want to print only three numbers in one line. i.e. If I got 14 in my text box the result will look like below. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

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  • (This is for a project, so yes it is homework) How would I finish this java code?

    - by user2924318
    The task is to create arrays using user input (which I was able to do), then for the second part, use a separate method to sort the array in ascending order then output it. I have gotten it to do everything I need except I don't know how I would get it to sort. The directions say to use a while loop from 0 to the length to find the minimum value then swap that with the 1st, but I don't know how to do this. This is what I have so far: public static void main(String[] args) { Scanner in = new Scanner(System.in); int storage = getNumDigits(in); if(storage == 0){ System.out.print("No digits to store? OK, goodbye!"); System.exit(0); } int []a = new int [storage]; a = getDigits(a, in); displayDigits(a); selectionSort(a); } private static int getNumDigits(Scanner inScanner) { System.out.print("Please enter the number of digits to be stored: "); int stored = inScanner.nextInt(); while(stored < 0){ System.out.println("ERROR! You must enter a non-negative number of digits!"); System.out.println(); System.out.print("Please enter the number of digits to be stored: "); stored = inScanner.nextInt(); } return stored; } private static int[] getDigits(int[] digits, Scanner inScanner) { int length = digits.length; int count = 0; int toBeStored = 0; while(count < length){ System.out.print("Enter integer " +count +": "); toBeStored = inScanner.nextInt(); digits[count] = toBeStored; count++; } return digits; } private static void displayDigits(int[] digits) { int len = digits.length; System.out.println(); System.out.println("Array before sorting:"); System.out.println("Number of digits in array: " +len); System.out.print("Digits in array: "); for(int cnt = 0; cnt < len-1; cnt++){ System.out.print(digits[cnt] + ", "); } System.out.println(digits[len-1]); } private static void selectionSort(int[] digits) { int l = digits.length; System.out.println(); System.out.println("Array after sorting:"); System.out.println("Number of digits in array: " +l); System.out.print("Digits in array: "); int index = 0; int value = digits[0]; int indVal = digits[index]; while(index < l){ indVal = digits[index]; if(indVal <= value){ indVal = value; digits[index] = value; index++; } else if(value < indVal){ index++; } System.out.print(value); //This is where I don't know what to do. } }

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  • How do I do "nested if" in JSTL for Java JSP?

    - by user678616
    I want to do something like the following: <c:choose> <c:when test="${empty example1}"> </c:when> <c:otherwise> <c:when test="${empty example2}"> </c:when> <c:otherwise> </c:otherwise> </c:otherwise> </c:choose> Is this even possible? I get an exception thrown when trying to run. Thank you.

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  • SQLite - executeUpdate exception not caught when database does not exist? (Java)

    - by giant91
    So I was purposely trying to break my program, and I've succeeded. I deleted the sqlite database the program uses, while the program was running, after I already created the connection. Then I attempted to update the database as seen below. Statement stmt; try { stmt = Foo.con.createStatement(); stmt.executeUpdate("INSERT INTO "+table+" VALUES (\'" + itemToAdd + "\')"); } catch(SQLException e) { System.out.println("Error: " + e.toString()); } The problem is, it didn't catch the exception, and continued to run as if the database was updated successfully. Meanwhile the database didn't even exist at that point since this was after I deleted it. Doesn't it check if the database still exists when updating? Do I have to check the database connection manually, every time I update to ensure that the database wasn't corrupted/deleted? Is this the way it is normally done, or is there a simpler/more robust approach? Thank you.

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  • WebSocket and Java EE 7 - Getting Ready for JSR 356 (TOTD #181)

    - by arungupta
    WebSocket is developed as part of HTML 5 specification and provides a bi-directional, full-duplex communication channel over a single TCP socket. It provides dramatic improvement over the traditional approaches of Polling, Long-Polling, and Streaming for two-way communication. There is no latency from establishing new TCP connections for each HTTP message. There is a WebSocket API and the WebSocket Protocol. The Protocol defines "handshake" and "framing". The handshake defines how a normal HTTP connection can be upgraded to a WebSocket connection. The framing defines wire format of the message. The design philosophy is to keep the framing minimum to avoid the overhead. Both text and binary data can be sent using the API. WebSocket may look like a competing technology to Server-Sent Events (SSE), but they are not. Here are the key differences: WebSocket can send and receive data from a client. A typical example of WebSocket is a two-player game or a chat application. Server-Sent Events can only push data data to the client. A typical example of SSE is stock ticker or news feed. With SSE, XMLHttpRequest can be used to send data to the server. For server-only updates, WebSockets has an extra overhead and programming can be unecessarily complex. SSE provides a simple and easy-to-use model that is much better suited. SSEs are sent over traditional HTTP and so no modification is required on the server-side. WebSocket require servers that understand the protocol. SSE have several features that are missing from WebSocket such as automatic reconnection, event IDs, and the ability to send arbitrary events. The client automatically tries to reconnect if the connection is closed. The default wait before trying to reconnect is 3 seconds and can be configured by including "retry: XXXX\n" header where XXXX is the milliseconds to wait before trying to reconnect. Event stream can include a unique event identifier. This allows the server to determine which events need to be fired to each client in case the connection is dropped in between. The data can span multiple lines and can be of any text format as long as EventSource message handler can process it. WebSockets provide true real-time updates, SSE can be configured to provide close to real-time by setting appropriate timeouts. OK, so all excited about WebSocket ? Want to convert your POJOs into WebSockets endpoint ? websocket-sdk and GlassFish 4.0 is here to help! The complete source code shown in this project can be downloaded here. On the server-side, the WebSocket SDK converts a POJO into a WebSocket endpoint using simple annotations. Here is how a WebSocket endpoint will look like: @WebSocket(path="/echo")public class EchoBean { @WebSocketMessage public String echo(String message) { return message + " (from your server)"; }} In this code "@WebSocket" is a class-level annotation that declares a POJO to accept WebSocket messages. The path at which the messages are accepted is specified in this annotation. "@WebSocketMessage" indicates the Java method that is invoked when the endpoint receives a message. This method implementation echoes the received message concatenated with an additional string. The client-side HTML page looks like <div style="text-align: center;"> <form action=""> <input onclick="send_echo()" value="Press me" type="button"> <input id="textID" name="message" value="Hello WebSocket!" type="text"><br> </form></div><div id="output"></div> WebSocket allows a full-duplex communication. So the client, a browser in this case, can send a message to a server, a WebSocket endpoint in this case. And the server can send a message to the client at the same time. This is unlike HTTP which follows a "request" followed by a "response". In this code, the "send_echo" method in the JavaScript is invoked on the button click. There is also a <div> placeholder to display the response from the WebSocket endpoint. The JavaScript looks like: <script language="javascript" type="text/javascript"> var wsUri = "ws://localhost:8080/websockets/echo"; var websocket = new WebSocket(wsUri); websocket.onopen = function(evt) { onOpen(evt) }; websocket.onmessage = function(evt) { onMessage(evt) }; websocket.onerror = function(evt) { onError(evt) }; function init() { output = document.getElementById("output"); } function send_echo() { websocket.send(textID.value); writeToScreen("SENT: " + textID.value); } function onOpen(evt) { writeToScreen("CONNECTED"); } function onMessage(evt) { writeToScreen("RECEIVED: " + evt.data); } function onError(evt) { writeToScreen('<span style="color: red;">ERROR:</span> ' + evt.data); } function writeToScreen(message) { var pre = document.createElement("p"); pre.style.wordWrap = "break-word"; pre.innerHTML = message; output.appendChild(pre); } window.addEventListener("load", init, false);</script> In this code The URI to connect to on the server side is of the format ws://<HOST>:<PORT>/websockets/<PATH> "ws" is a new URI scheme introduced by the WebSocket protocol. <PATH> is the path on the endpoint where the WebSocket messages are accepted. In our case, it is ws://localhost:8080/websockets/echo WEBSOCKET_SDK-1 will ensure that context root is included in the URI as well. WebSocket is created as a global object so that the connection is created only once. This object establishes a connection with the given host, port and the path at which the endpoint is listening. The WebSocket API defines several callbacks that can be registered on specific events. The "onopen", "onmessage", and "onerror" callbacks are registered in this case. The callbacks print a message on the browser indicating which one is called and additionally also prints the data sent/received. On the button click, the WebSocket object is used to transmit text data to the endpoint. Binary data can be sent as one blob or using buffering. The HTTP request headers sent for the WebSocket call are: GET ws://localhost:8080/websockets/echo HTTP/1.1Origin: http://localhost:8080Connection: UpgradeSec-WebSocket-Extensions: x-webkit-deflate-frameHost: localhost:8080Sec-WebSocket-Key: mDbnYkAUi0b5Rnal9/cMvQ==Upgrade: websocketSec-WebSocket-Version: 13 And the response headers received are Connection:UpgradeSec-WebSocket-Accept:q4nmgFl/lEtU2ocyKZ64dtQvx10=Upgrade:websocket(Challenge Response):00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00 The headers are shown in Chrome as shown below: The complete source code shown in this project can be downloaded here. The builds from websocket-sdk are integrated in GlassFish 4.0 builds. Would you like to live on the bleeding edge ? Then follow the instructions below to check out the workspace and install the latest SDK: Check out the source code svn checkout https://svn.java.net/svn/websocket-sdk~source-code-repository Build and install the trunk in your local repository as: mvn install Copy "./bundles/websocket-osgi/target/websocket-osgi-0.3-SNAPSHOT.jar" to "glassfish3/glassfish/modules/websocket-osgi.jar" in your GlassFish 4 latest promoted build. Notice, you need to overwrite the JAR file. Anybody interested in building a cool application using WebSocket and get it running on GlassFish ? :-) This work will also feed into JSR 356 - Java API for WebSocket. On a lighter side, there seems to be less agreement on the name. Here are some of the options that are prevalent: WebSocket (W3C API, the URL is www.w3.org/TR/websockets though) Web Socket (HTML5 Demos - html5demos.com/web-socket) Websocket (Jenkins Plugin - wiki.jenkins-ci.org/display/JENKINS/Websocket%2BPlugin) WebSockets (Used by Mozilla - developer.mozilla.org/en/WebSockets, but use WebSocket as well) Web sockets (HTML5 Working Group - www.whatwg.org/specs/web-apps/current-work/multipage/network.html) Web Sockets (Chrome Blog - blog.chromium.org/2009/12/web-sockets-now-available-in-google.html) I prefer "WebSocket" as that seems to be most common usage and used by the W3C API as well. What do you use ?

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