i want to compile child.java class an get child.class, child class extends parent class.
i want child.class include all necessary code from parent class, in fact i can use it independently.
is it possible?
second Question: suppose we have a .jar library containing a.class , b.class , c.class ... i have make an updated version of a.class, how can i replace the new one with the original a.class in jar? is it possible ?
sorry for bad English.
1) as Dmitry mentioned in the comment is impossible. Even if your parent class did absolutely nothing it's definition will still be checked, and if not found ClassNotFoundException (or similar) will be thrown
2) Is certainly possible and usefull in some situations (patching external library without recompiling everything from sources comes to mind) - see Is there are way to patch jar files?
Specifically the most usefull (IMO) is CoolBeans's answer:
jar uf test.jar com\test\Test.class
Note that you have to take care of correct packaging. And it will not work if the jar is signed.
I'm wondering if someone can tell me how to obfuscate a single file, or maybe just two files, inside a jar file using proguard. I'm hoping that keeping every single class but the one i want to obfuscate isn't my only option, where as that takes a ton of time and is very tedious.. So, Is it possible to only obfuscate a single class? If so, how.. Thanks in advance!
Obfuscating a single class is generally not very useful: it will be easy to find and easy to reverse-engineer, since other classes and references to them remain readable. ProGuard therefore obfuscates all classes (except the specified ones) by default.
That being said, this should work:
-keep class !mypackage.MySecretClass, !mypackage.MyOtherSecretClass {
*;
}
It preserves the class/field/method names of all classes except the specified one. In other words, it only obfuscates the specied classes.
You can exclude all the items except the ones you want to obfuscate; see How to keep/exclude a particular package path when using proguard?
If that doesn't seem practical, a workaround would be to package the files you want to obfuscate in their own jarfile, run ProGuard, then repackage those files with the other files you want to include in a new jarfile.
I would like to know What are the difference between folder-structure and package used in Eclipse IDE for Java EE development.
When do we use which one and why?.
Whats should be the practice
create a folder structure like src/com/utils and then create a class inside it
create a package like src.com.util and then create a class inside it
which option would be better and easy to deploy if i have to write a ant script later for deployment ?
if i go for the folder-structure will the deployment is as easy as copying files from development to deployment target ?
If you configured stuffs correctly. Adding a folder inside src, is same as adding a package from File > New Package.
So, it's up to you, whatever feels comfortable to you -- add a folder or create a package. Also, when you put stuffs under src the package name starts from subfolder. So, src/com/naishe/test will be package com.naishe.test.
Basically there is no difference, both are the same.
In both the cases, the folder structure will be src/com/utils.
and in both the cases, you will need to mention
package com.utils;
as first line in the class
Since it doesn't have any difference practically, it won't make any difference to ant script.
"Packaging helps us to avoid class name collision when we use the same class name as that of others. For example, if we have a class name called "Vector", its name would crash with the Vector class from JDK. However, this never happens because JDK use java.util as a package name for the Vector class (java.util.Vector). So our Vector class can be named as "Vector" or we can put it into another package like com.mycompany.Vector without fighting with anyone. The benefits of using package reflect the ease of maintenance, organization, and increase collaboration among developers. Understanding the concept of package will also help us manage and use files stored in jar files in more efficient ways."
check out http://www.jarticles.com/package/package_eng.html for more information on packages
create a package like 'src.com.util'
That sounds like a mistake. The package name should be 'com.util', and 'src' is the name of the source folder.
Other than that, I fail to see what the difference is between your two choices. The result is the same, right? Just different steps in the GUI to arrive at it. The wizard to create a new package in Eclipse is just a wrapper around creating the appropriate folder hierarchy within a source folder.
You don't need to create empty packages at all, you can directly create classes (the package will be created automatically if it does not already exist).
A package is automatically "source folder" where folder is just a normal folder.
When you compile an Eclipse project, all files in source folders are compiled but not in regular folders (unless those regular folders a)
folder structure or to be specific source folder in eclipse is meant just for eclipse but package is universal irrespective of any editor..
I am new to java.My friend asked me this question today And i am looking for an answer to it.
How to make the number of class files in a package, constant?
i.e., even though one can access that package,they should not be able to add any new class to the exisiting package.
You want sealed packages. Once sealed, all classes from a package must come from the same JAR file. It basically boils down to adding the package to the manifest:
Name: myCompany/myPackage/
Sealed: true
See Sealing packages within a jar file
This is called sealing the package and works on the level of jar files.
From the official trail:
Packages within JAR files can be optionally sealed, which means that all classes defined in that package must be archived in the same JAR file. You might want to seal a package, for example, to ensure version consistency among the classes in your software.
To clarify: Since the classes must come from the same jar file, no one can add classes to your package, since the new classes wouldn't come from your jar file.
I come from a .NET background and am completely new to Java and am trying to get my head around the Java project structure.
My typical .NET solution structure contains projects that denote logically distinct components, usually named using the format:
MyCompany.SomeApplication.ProjectName
The project name usually equals the root namespace for the project. I might break the namespace down further if it's a large project, but more often than not I see no need to namespace any further.
Now in Java, you have applications consisting of projects, and then you have a new logical level - the package. What is a package? What should it contain? How do you namespace within this App.Project.Package structure? Where do JARs fit into all this? Basically, can someone provide a newbies intro to Java application structure?
Thanks!
Edit: Some really cracking answers thanks guys. A couple of followup questions then:
Do .JAR files contain compiled code? Or just compressed source code files?
Is there a good reason why package names are all lower case?
Can Packages have 'circular dependencies'? In other words, can Package.A use Package.B and vice versa?
Can anyone just show the typical syntax for declaring a class as being in a package and declaring that you wish to reference another package in a class (a using statement maybe?)
"Simple" J2SE projects
As cletus explained, source directory structure is directly equivalent to package structure, and that's essentially built into Java. Everything else is a bit less clear-cut.
A lot of simple projects are organized by hand, so people get to pick a structure they feel OK with. What's often done (and this is also reflected by the structure of projects in Eclipse, a very dominant Java tool) is to have your source tree begin in a directory called src. Your package-less source files would sit directly in src, and your package hierarchy, typically starting with a com directory, would likewise be contained in src. If you CD to the src directory before firing up the javac compiler, your compiled .class files will end up in the same directory structure, with each .class file sitting in the same directory and next to its .java file.
If you have a lot of source and class files, you'll want to separate them out from each other to reduce clutter. Manual and Eclipse organization often place a bin or classes directory parallel to src so the .class files end up in a hierarchy that mirrors that of src.
If your project has a set of .jar files to deliver capability from third-party libraries, then a third directory, typically lib, is placed parallel to src and bin. Everything in lib needs to be put on the classpath for compilation and execution.
Finally, there's a bunch of this and that which is more or less optional:
docs in doc
resources in resources
data in data
configuration in conf...
You get the idea. The compiler doesn't care about these directories, they're just ways for you to organize (or confuse) yourself.
J2EE projects
J2EE is roughly equivalent to ASP.NET, it's a massive (standard) framework for organizing Web applications. While you can develop your code for J2EE projects any way you like, there is a firm standard for the structure that a Web container will expect your application delivered in. And that structure tends to reflect back a bit to the source layout as well.
Here is a page that details project structures for Java projects in general (they don't agree very much with what I wrote above) and for J2EE projects in particular:
http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html
Maven projects
Maven is a very versatile project build tool. Personally, my build needs are nicely met by ant, which roughly compares with nmake. Maven, on the other hand, is complete-lifecyle build management with dependency management bolted on. The libs and source for most of the code in the Java world is freely available in the 'net, and maven, if asked nicely, will go crawling it for you and bring home everything your project needs without you needing to even tell it to. It manages a little repository for you, too.
The downside to this highly industrious critter is the fact that it's highly fascist about project structure. You do it the Maven way or not at all. By forcing its standard down your throat, Maven manages to make projects worldwide a bit more similar in structure, easier to manage and easier to build automatically with a minimum of input.
Should you ever opt for Maven, you can stop worrying about project structure, because there can only be one. This is it: http://maven.apache.org/guides/introduction/introduction-to-the-standard-directory-layout.html
A package in Java is very similar to a namespace in .Net. The name of the package essentially creates a path to the classes that live inside it. This path can be thought of as the class's namespace (in .Net terms) because it is the unique identifier for the specific class you want to use. For example if you have a package named:
org.myapp.myProject
And inside it you had a bunch of classes:
MyClass1
MyClass2
To specifically refer to those classes you would use:
org.myapp.myProject.MyClass1
org.myapp.myProject.MyClass2
The only real difference between this and .Net (that I know of) is that Java organizes its "namespaces" structurally (each package is a distinct folder) whereas .Net allows you to scope classes using the namespace keyword and ignores where the document actually lives.
A JAR file is roughly analogous to a DLL in most cases. It is a compressed file (you can open them with 7zip) that contains source code from other projects that can be added as dependencies in your application. Libraries are generally contained in JARs.
The thing to remember about Java is that is is very structural; WHERE files live is important. Of course there is more to the story then what I posted but I think this should get you started.
A package is much like a .Net namespace. The general convention in Java is to use your reversed domain name as a package prefix so if your company is example.com your packages will probably be:
com.example.projectname.etc...
It can be broken down to many levels rather than just one (projectname) but usually one is sufficient.
Inside your project structure classes are usually divided into logical areas: controllers, models, views, etc. It depends on the type of project.
There are two dominant build systems in Java: Ant and Maven.
Ant is basically a domain-specific scripting language and quite flexible but you end up writing a lot of boilerplate stuff yourself (build, deploy, test, etc tasks). It's quick and convenient though.
Maven is more modern and more complete and is worth using (imho). Maven is different to Ant in that Maven declares that this project is a "Web application project" (called an archetype). Once that is declared the directory structure is mandated once you specify your groupId (com.example) and artifactId (project name).
You get a lot of stuff for free this way. The real bonus of Maven is that it manages your project dependencies for you so with a pom.xml (Maven project file) and correctly configured Maven you can give that to someone else (with your source code) and they can build, deploy, test and run your project with libraries being downloaded automatically.
Ant gets something like this with Ivy.
Here are some notes about Java packages that should get you started:
The best practice with Java package names is to use the domain name of the organisation as the start of the package, but in reverse, e.g. if your company owns the domain "bobswidgets.com", you would start your package off with "com.bobswidgets".
The next level down will often be the application or library level, so if it's your ecommerce libraries, it could be something like "com.bobswidgets.ecommerce".
Further down than that often represents the architecture of your application. Classes and interfaces that are core to the project reside in the "root" e.g. com.bobswidgets.ecommerce.InvalidRequestException.
Using packages to subdivide functionality further is common. usually the pattern is to put interfaces and exceptions into whatever the root of the subdivision is and the implementation into sub packages e.g.
com.bobswidgets.ecommerce.payment.PaymentAuthoriser (interface)
com.bobswidgets.ecommerce.payment.PaymentException
com.bobswidgets.ecommerce.payment.paypal.PaypalPaymentAuthoriser (implementation)
This makes it pretty easy to pull the "payment" classes and packages into their own project.
Some other notes:
Java packages are tightly coupled to directory structure. So, within a project, a class with a Package of com.example.MyClass will invariably be in com/example/MyClass.java. This is because when it is packaged up into a Jar, the class file will definitely be in com/example/MyClass.class.
Java packages are loosely coupled to projects. It is quite common that projects will have their own distinct package names e.g. com.bobswidgets.ecommerce for ecommerce, com.bobswidgets.intranet for the intranet project.
Jar files will container the class files that are the result of compiling your .java code into bytecodes. They are just zip files with .jar extension. The root of the Jar file is the root of the namespace hierarchy e.g. com.bobswidgets.ecommerce will be /com/bobswidgets/ecommerce/ in the Jar file. Jar files can also container resources e.g. property files etc.
A package is a grouping of source files that lets them see each others' package-private methods and variables, so that that group of classes can access things in each other that other classes can't.
The expectation is that all java classes have a package that is used to disambiguate them. So if you open a jar file in your project, like spring, every package starts with org.springframework. The classloaders don't know about the jarfile name, they use only the package.
There's a common practice of breaking things down by type of object or function, not everybody agrees about this. Like Cletus posted here, there's a tendency to group web controllers, domain objects, services, and data access objects into their own packages. I think some Domain-Driven Design people do not think this is a good thing. It does have the advantage that typically everything in your package shares the same kind of dependencies (controllers might depend on services and domain objects, services depend on domain objects and data access objects, etc.) so that can be convenient.
Okay so in java you have three different types of access to a classes member functions and variables
public
protected
package-private
and private
All classes in the same package can see each others public, protected, and package-private elements.
Packages are not hierarchical in the system. Usually they are organized in a hierarchical way, but as far as runtime is concerned com.example.widgets is a completely different package from com.example.widgets.cogs
Packages are arranged as directories, which helps keep things organized: your file structure is always similar to your package structure.
They are planning on adding a module system to Java in JDK7 (called Project Jigsaw) and there is an existing module system called OSGi. These module systems will/can give you a lot more flexibility and power then the simple package system.
Also, package names are usually all lower case. :)
To answer the example sub-question:
package com.smotricz.goodfornaught;
import java.util.HashMap;
import javax.swing.*;
public class MyFrame extends JFrame {
private HashMap myMap = new HashMap();
public MyFrame() {
setTitle("My very own frame");
}
}
Do .JAR files contain compiled code? Or just compressed source code files?
They might contain both, or even totally different kinds of files like pictures. It's a zip archive first of all. Most often you would see JARs that contain class files, and those which contain source files (handy for debugging in your IDE if you use third party code) or those that contain javadoc (sourcecode documentatin), also handy if your IDE supports tooltipping the documentation when you access the lib's functions.
Is there a good reason why package names are all lower case?
Yes there is a good reason for package names to be written in lowercase letters: There is a guideline which says that only classnames are written with a capital letter in front.
Can Packages have 'circular dependencies'? In other words, can Package.A use Package.B and vice versa?
Packages do not use each other. Only classes do. And yes that might be possible but bad practice.
Can anyone just show the typical syntax for declaring a class as being in a package and declaring that you wish to reference another package in a class (a using statement maybe?)
Let's assume you want to use the ArrayList class from package java.util, either use
import java.util.ArrayList;
ArrayList myList = new ArrayList();
or use without import (say you use two different classes named ArrayList from different packages)
java.util.ArrayList myList = new java.util.ArrayList();
your.package.ArrayList mySecondList = new your.package.ArrayList();
From Wikipedia:
A Java package is a mechanism for
organizing Java classes into
namespaces
and
Java packages can be stored in
compressed files called JAR files
So for package a.b.c, you could have Java classes in the a, a.b, and a.b.c packages. Generally you group classes inside the same package when they represent related functionality. Functionally, the only difference between classes in the same package and classes in different package is that the default access level for members in Java is "package-protected", which means that other classes in the same package have access.
For a class a.b.c.MyClass, if you want to use MyClass in your project you would import a.b.c.MyClass or, less recommended, import a.b.c.* Also, for MyClass to reside in package a.b.c in the first place, you would declare it in the first line of MyClass.java: package a.b.c;.
To do this you could JAR up the whole package (including packages b and c and class MyClass) and put this JAR into your $CLASSPATH; this would make it accessible for your other source code to use (via the aforementioned import statement).
While it is not as easy to make circular dependent classes work, it may not be impossible. I did get it to work in one case. class A and class B depended on each other and wouldn't compile from scratch. but realizing that a part of class A didn't need class B, and that part was what class B needed to compile completely, I rem'd out that part of class A, not needed by class B, and the remaining part of class A was able to compile, then I was able to compile class B. I was then able to un-rem that section of class A that needed class B, and was able to compile the full class A. Both classes then functioned properly. While it is not typical, if the classes are tied together like this, it is kosher and at times possibly necessary. Just make sure you leave yourself special compile instructions for future updates.