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  • How do you make your Java application memory efficient?

    - by Boune
    How do you optimize the heap size usage of an application that has a lot (millions) of long-lived objects? (big cache, loading lots of records from a db) Use the right data type Avoid java.lang.String to represent other data types Avoid duplicated objects Use enums if the values are known in advance Use object pools String.intern() (good idea?) Load/keep only the objects you need I am looking for general programming or Java specific answers. No funky compiler switch. Edit: Optimize the memory representation of a POJO that can appear millions of times in the heap. Use cases Load a huge csv file in memory (converted into POJOs) Use hibernate to retrieve million of records from a database Resume of answers: Use flyweight pattern Copy on write Instead of loading 10M objects with 3 properties, is it more efficient to have 3 arrays (or other data structure) of size 10M? (Could be a pain to manipulate data but if you are really short on memory...)

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  • How do I prevent JAXB from generating Java from imported schema?

    - by Mark
    I've got two Java projects, both generate Java classes based on a schema definition, I'm using xjc to create the classes. My second project depends on a class from the first project, and in particular, one of the classes I'd like to generate in my second project needs to use one of the types from the first project. To accomplish this dependency at the schema level, I'm using a simple xsd:import to map the namespace to a particular schema. JAXB works just fine with this condition, except it also generates the first project's types in the second project. So after running have something like this: Project A +-- com.foo.bar +-- TypeA Project B +-- com.foo.asdf +-- TypeB +-- com.foo.bar +-- TypeA The second "TypeA" is undesirable, and I'd like to never generate it in the first place. How do I instruct JAXB not to generate the classes for "TypeA" that it finds as a result of the import statement?

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  • Good simple C/C++ FTP and SFTP client library recommendation for embedded Linux

    - by Roman Nikitchenko
    Could anyone recommend FTP / SFTP client C/C++ library for Linux-based embedded system? I know about Curl library but I need something as simple as possible just to download files from FTP / SFTP servers. Is there any recommendation to look for? Yes, SFTP support is critical. Actually I can even sacrifice multi-threading because I need only one stream at a time. And I'd like it to be able to work through memory buffers but this should be not a problem. Thank you in advance.

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  • Can functions like sin() be redefined, in Fortran, C or Java?

    - by EOL
    Can a mathematical function like sin() be redefined, in Fortran, C or Java code, while preserving the default behavior of other mathematical functions like cos()? Or can another function named sin() but that accepts different argument types be defined in addition to the built-in sin()? I am interested in general features of these languages (I am thinking of applications like the implementation of non-usual number algebras). I tried to define a sin() function in a Fortran 95 program, but the intrinsic sin() function was called instead… Is there a way around this? what about C and Java?

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  • inconsistency between Sun JRE javac and Eclipse java compiler?

    - by Jason S
    This confuses me. The following compiles fine under Eclipse. package com.example.gotchas; public class GenericHelper1 { static <T> T fail() throws UnsupportedOperationException { throw new UnsupportedOperationException(); } /** * just calls fail() * @return something maybe */ public boolean argh() { return fail(); } public static void main(String[] args) { // TODO Auto-generated method stub } } But if I try to do a clean build with ant, or at the command line with javac, I get this: src\com\example\gotchas\GenericHelper1.java:14: type parameters of <T>T cannot be determined; no unique maximal instance exists for type variable T with upper bounds boolean,java.lang.Object public boolean argh() { return fail(); } ^ 1 error what gives, and how do I fix it?

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  • In java can i have more than one class/object in a file?

    - by David
    So the way i've been told to do things is you have your file and the file name is Classname.java and then the code is something like this: class ClassName { SOME METHODS main {} } and then thats all. I'd like to have two objects defined and used within the same .java file. (i don't want to have to put the other class in a difernt file just because i'd like to send this to someone and i want to avoid hasstle of atatching multiple files to an email [the lazy do make good programers though if you think about it]) is it possible to do this? do i have to do anything special and if so what? what are some mistakes i'm likely to make or that you have made in the past when doing this?

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  • How to leave out some statements while executing java code?

    - by iamrohitbanga
    In C++ if we do not want some statements to compile into code that ships like assert function calls, we control their compilation through #ifndef preprocessor directives. How do we do this in Java? I have some System.out.println() statements for debugging which I would like to remove for the final code. one way is to make them execute conditionally under the affect of a boolean variable. Is there a better way of doing this? As I have a java swing application I can turn off the System.out.println statements without affecting the output. What is the method of doing this?

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  • How to make my Java Swing application a Client-Server application?

    - by Jonas
    I have made a Java Swing application. Now I would like to make it a Client-Server application. All clients should be notified when data on the server is changed, so I'm not looking for a Web Service. The Client-Server application will be run on a single LAN, it's a business application. The Server will contain a database, JavaDB. What technology and library is easiest to start with? Should I implement it from scratch using Sockets, or should I use Java RMI, or maybe JMS? Or are there other alternatives that are easier to start with? And is there any server library that I should use? Is Jetty an alternative?

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  • What is the use of Method Overloading in Java when it is achieved by changing the sequence of parameters in the argument list?

    - by MediumOne
    I was reading a Java training manual and it said that Method Overloading in Java can be achieved by having a different argument list. It also said that the argument list could differ in (i). Number of parameters (ii). Datatype of parameters (iii). Sequence of parameters My concern is about (iii). What is the use of trying to overload a method just by changing the sequence of parameters? I am unable to think of any benefits by this way.

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  • OpenGL embedded in gtk has colour badly displayed

    - by Sardathrion
    Note that this is a re-write now that I have more clues as to where the problem could be... I am creating a GTK GUI which contains two embedded OpenGL displays. Both use the same shader code (complied once for each). On my normal hardware, this works fine. On a virtual machine running on the same hardware, I get horrible colours -- see images. I suspect that the shader code is at fault -- certainly dropping a simpler shader does make the problem moot. However, I do need both diffuse and spot lights in my shader thus making it non-trivial. Anyone has seen this before?

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  • Is there a Java unit-test framework that auto-tests getters and setters?

    - by Michael Easter
    There is a well-known debate in Java (and other communities, I'm sure) whether or not trivial getter/setter methods should be tested. Usually, this is with respect to code coverage. Let's agree that this is an open debate, and not try to answer it here. There have been several blog posts on using Java reflection to auto-test such methods. Does any framework (e.g. jUnit) provide such a feature? e.g. An annotation that says "this test T should auto-test all the getters/setters on class C, because I assert that they are standard". It seems to me that it would add value, and if it were configurable, the 'debate' would be left as an option to the user.

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  • Best approach to creating a database driven Java website?

    - by Craig Whitley
    I'm fairly new to programming and new to java, but I'd like to jump in the deep end with a little database driven website project. I've read quite a lot about requirements in the 'real world' requesting experience with Spring and Hibernate, so I have those installed on netbeans and a project created (if I hit run I get the default spring page). Now I just need a little guidance as to where to start designing my app (please tell me if I'm getting in a bit too over my head for a beginner!). Should I start off with my classes? - create all my classes as they map to my database tables and decide which attributes and methods each will require? Can anyone suggest any good books for maybe.. making a java based website from scratch (i.e. from design right through to deployment) that might be useful for a beginner? Any help appreciated thanks.

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  • In Java, how do I set a return type if an exception occurs?

    - by beagleguy
    hey all, I'm new to Java and was wondering if I define a method to return a database object like import java.sql.*; public class DbConn { public Connection getConn() { Connection conn; try { Class.forName("com.mysql.jdbc.Driver").newInstance(); if(System.getenv("MY_ENVIRONMENT") == "development") { String hostname = "localhost"; String username = "root"; String password = "root"; } conn = DriverManager.getConnection("jdbc:mysql:///mydb", username, password); return conn; } catch(Exception e) { throw new Exception(e.getMessage()); } } } if the connection fails when I try to create it what should I return? eclipse is telling me I have to return a Connection object but if it fails I'm not sure what to do. thanks!

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  • Why do Java and C# not have implicit conversions to boolean?

    - by Shaun
    Since I started Java it's been very aggravating for me that it doesn't support implicit conversions from numeric types to booleans, so you can't do things like: if (flags & 0x80) { ... } instead you have to go through this lunacy: if ((flags & 0x80) != 0) { ... } It's the same with null and objects. Every other C-like language I know including JavaScript allows it, so I thought Java was just moronic, but I've just discovered that C# is the same (at least for numbers, don't know about null/objects): http://msdn.microsoft.com/en-us/library/c8f5xwh7(VS.71).aspx Microsoft changed it on purpose from C++, so why? Clearly I'm missing something. Why change (what I thought was) the most natural thing in the world to make it longer to type? What on Earth is wrong with it?

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  • What is the most efficient algorithm for reversing a String in Java?

    - by Hultner
    I am wondering which way to reverse a string in Java that is most efficient. Should I use some sort of xor method? The easy way would be to put all the chars in a stack and put them back into a string again but I doubt that's a very efficient way to do it. And please do not tell me to use some built in function in Java. I am interested in learning how to do it not to use an efficient function but not knowing why it's efficient or how it's built up.

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  • Convert a string representation of a hex dump to a byte array using Java?

    - by ravigad
    I am looking for a way to convert a long string (from a dump), that represents hex values into a byte array. I couldn't have phrased it better than the person that posted the same question here: http://www.experts-exchange.com/Programming/Programming_Languages/Java/Q_21062554.html But to keep it original, I'll phrase it my own way: suppose I have a string "00A0BF" that I would like interpreted as the byte[] {0x00,0xA0,0xBf} what should I do? I am a Java novice and ended up using BigInteger and watching out for leading hex zeros. But I think it is ugly and I am sure I am missing something simple...

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