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Creating jar files from selected java files using eclipse

I have a project where i developed couple of java utility files and entire project is working good.
These utility files i placed in "utility" folder of my project.
Now i dont want everyone to see what i have coded in these utility files, so planning to create a jar file for these utility files.
Going forward i will place only this jar file in place of utility files.
So can some help how to create jar files for only selected java files in this utility folder? I saw many articles where they are converting the entire project into jar, but what i need is i want jar only to selected files.
So can some help me here?

Here is a handy guide: https://www.cs.utexas.edu/~scottm/cs307/handouts/Eclipse%20Help/jarInEclipse.htm
This link walks you through how to choose and pick which files you wish to have in the jar file you want to create.
Steps:
Right click the project you'll be converting into a jar > Export > Java > Jar File > Select the project, and view its contents by clicking the arrow left of its name >
Select or deselect the package/folder/etc you want to include or the parts of it by clicking it. The contents should appear on the right side of the menu which can be included or excluded from the jar build.

Add a .class file to a project in NetBeans

Our professor provided a .class file (Out.class) we should use in our Programm. When I create a new project in NetBeans, in what directory am I supposed to copy that file and/or how do I "import" it to my project so I can use it like Out.methodwhatever(var);?
I have already searched for that problem, but I just found cases where people have a .jar file, but I have a .class file.

Copy the Out.class into some directory (not your source folder). Make sure you replicate the package structure of that class, e.g. if that class is actually com.foobar.Out you need to put it into e.g. libs/com/foobar
The right click on your project in NetBeans, select the "Libraries" category, and click on the "Add JAR/Folder". The select the e.g. the libs folder from the above example.

How do I add .java/.class files to eclipse?

I know how to add .java/class files to a certain project, or import them into a package. So, when you're using eclipse you can import java.awt.; and java.util.; and the but those are all packages inside the JRE. So my real question is: is there any way to import a package containing the files i want into the JRE so that, if i create a new project, I can simply import them whenever I need them? Thanks!

Yes. Generally, you have more then one choice. You can link the project(s) in eclipse, and then have project B with a reference to project A. Or, you can create a build artifact like a jar file and add it to the project B build/run path. Simply add a jar file to your project, right click and select Add to Build Path. Another alternative is to use a build tool that manages dependencies like gradle, sbt, maven, or apache ivy. Note that there are a wide variety of third-party opensource libraries you can add as dependencies using these (and other) tools.

You seem to be asking how to create a shared library, which in Java is a .jar file. The steps to do this in Eclipse are:
In the Package Explorer, you can optionally pre-select one or more Java elements to export. (These will be automatically selected in the Opens the JAR export wizard JAR Package Specification wizard page, described in Step 4.)
Either from the context menu or from the menu bar's File menu, select Export.
Expand the Java node and select JAR file. Click Next.
In the JAR File Specification page, select the resources that you want to export in the Select the resources to export field.
Select the appropriate checkbox to specify whether you want to Export generated class files and resources or Export Java source files and resources. Note: Selected resources are exported in both cases.
In the Select the export destination field, either type or click Browse to select a location for the JAR file.
Select or clear the Compress the contents of the JAR file checkbox.
Select or clear the Overwrite existing files without warning checkbox. If you clear this checkbox, then you will be prompted to confirm the replacement of each file that will be overwritten.
Note: The overwrite option is applied when writing the JAR file, the JAR description, and the manifest file.
You have two options:
Click Finish to create the JAR file immediately.
Click Next to use the JAR Packaging Options page to set advanced options, create a JAR description, or change the default manifest.
Credit goes to the docs.

How to import a Java program into Eclipse?

I have some java source code. It an just an archive which is four folders.
Folders MAIN, DATA, TAGS, USER_INTERFACE. Each folder contains a few class files.
I see how everything works together, but can't see to get the program to import correctly or run. I did find a MANIFEST.MF.
What is the proper way to import such a project?

Create an Eclipse project. Under the project settings, select the "Java Build Path", and add each source folder (and remove the default src folder that Eclipse may add for you).
If necessary, you may set the output/build folders for each source directory, and clear Eclipse's default bin output directory. Check the "Allow output folders for source folders" to enable this, then set each source folder's output folder.
(You may or may not want to keep compiled classes in individual directories.)
If there are class files in the project that aren't generated from source in the project, in the same dialog, select the "Libraries" tab, and click the "Add Class Folder" button to add dependencies from within the project.

It doesn't seem that your source code is an eclipse-importable project (it would have to have a .classpath and .project file in the top level directory for that).
You should instead create a new Java project. For simplicity, set the project's location to be the parent directory of your MAIN, DATA, etc. directories. Then after the project is created, configure a source folder for each of MAIN, DATA, etc. (right-click the project, choose Properties > Java Build Path, then work in the Source tab). Your source folders should then show up in the navigator and the project should be able to be compiled.

Create a hello world project in eclipse and understand the project structure in eclipse.
And then manually import the files it is simple and avoids a lot of confusion especially for somebody new to the environment.

How to create a project from existing source in Eclipse and then find it?

I have created several .java files. All of them are located in one directory. I used a text editor to write these files. Now I want to switch to Eclipse. How can I do it? I have tried many ways. None of them works.
ADDED:
I think the common way is to have source in the eclipse folder called "workspace". So, how do I get my files in these directory. Should I use eclipse to create a new project from existing source and Eclipse will put all file to the workspace? Or I should manually copy all my files to the workspace? Where should I put my class files than? Should I create a subdirectory? With which name?
ADDED 2:
When I try to create a project with name "game", the eclipse writes me that a project with such name already exist. But how can I open this project?
ADDED 3:
In my "workspace" I have created a subdirectory called "game". I copied all my .java file into this subdirectory. Then with Eclipse I created a new project with the name game. As a result, Eclipse created .classpath and .project files in the directory "game". It also created bin and src subdirectories. And now I think it is not the correct way to go. The source files are supposed to be in the "src" directory. Right? And at the moment all my .java files are in the "workspace\game".
ADDED 4:
I did it other way around. With Eclipse I have created a new project with the name "game". As the result, Eclipse created a folder called "game" into folder "workspace". In "game" the folder I found "src" folder. I copied all my .java files into this folder. But now in the "Package Explorer" I cannot open "src" folder. So, how can I access my source files from Eclipse? Why Eclipse does not want to open the "src" folder?

Easiest Method:
Put all source files into one directory named after your project. i.e. "ProjectName" You can keep this directory in your workspace or it can be somewhere else.
Start a new project in eclipse and name it using that same project name.
Uncheck the "use default location" box and find the directory where your project is unless your project is already in the workspace - then you must not uncheck the "use default location" box
Click 'next'.
Eclipse should be smart enough to figure out what's going on. After clicking next, it will show you all of the files it found in that directory. It will just automatically add those files to your project. Voilà!

Right-click in the package explorer and select New - Java Project
Create the new project Game
Open the new project in the package explorer - you should see only the source folder called src (there's nothing inside yet)
Open a file Explorer (e.g. Windows Explorer) and drag your sources
Drag them to Eclipse and drop them inside the new src folder - if asked select "Copy files"
Eclipse should put the files into the default package, if that's not correct you can edit the offending files (marked with a red cross) by opening them in Eclipse, selecting the package declaration (usually line 1), pressing Ctrl + 1 and selecting the appropriate option (e.g. "Move xy to package com.game"

This answer is going to be for the question
How to create a new eclipse project and add a folder or a new package into the project,
or how to build a new project for existing java files.
Create a new project from the menu
File->New-> Java Project
If you are going to add a new pakcage, then create the same package name here by
File->New-> Package
Click the name of the package in project navigator, and right click, and import...
Import->General->File system (choose your file or package)
this worked for me I hope it helps others.
Thank you.

The easiest method is really good but you don't get a standard Java project, i.e., the .java and .class files separated in different folders.
To get this very easily:
Create a folder called "ProjectName" on the workspace of Eclipse.
Copy or move your folder with the .java files to the "ProjectName" folder.
Create a new Java Project called "ProjectName" (with the Use default location marked).
Press <Enter> and that's it.

There are two things
1- If its already a Eclipse Project, then simply go to File->Import->General->Existing Project into Workplace
2- Otherwise define project type e.g. Java, Web etc
Create a new project of type you define into your workplace. Copy Paste source , lib and other necessary files. refresh, compile and run project in eclipse.

In the package explorer and the navigation screen you should now see the project you created. Note that eclipse will not copy your files, it will just allow you to use the existing source and edit it from eclipse.

There are several ways to add files to an existing Java project in Eclipse. So lets assume you have already created the Java project in Eclipse (e.g. using File -> New -> Project... - and select Java project).
To get Java files into the new project you can do any of the following. Note that there are other ways as well. The sequence is my preference.
Drag the files into the Navigator view directly from the native file manager. You must create any needed Java packages first. This method is best for a few files in an existing Java package.
Use File -> Import... - select File System. Here you can then select exactly which files to import into the new project and in which Java package to put them. This is extremely handy if you want to import many files or there are multiple Java packages.
Copy the fires directly to the folder/directory in the workspace and then use File -> Refresh to refresh the Eclipse view of the native system. Remember to select the new project before the refresh.
The last one is what you did - minus the refresh...

While creating a project from a full folder may or may not work within the workspace, there's a condition outside of the workspace that prevents starting a new project with a full folder.
This is relevant if you use numerous folder locations for sources, for example an htdocs or www folder for web projects, and a different location for desktop Java applications.
The condition mentioned occurs when Eclipse is told to create a new project, and given a full folder outside the workspace. Eclipse will say the folder isn't empty, and prevent creating a new project within the given folder. I haven't found a way around this, and any solution requires extra steps.
My favorite solution is as follows
Rename the full folder with an appended "Original" or "Backup.
Create the Eclipse project with the name of the full folder before the folder was renamed.
Copy all the relabeled full folders contents into the new project folder.
Eclipse should make a new project, and update that project with the new folder contents as it scans for changes. The existing sources are now part of the new project.
Although you had to perform three extra steps, you now have a backup with the original sources available, and are also able to use a copy of them in an existing project. If storage space is a concern, simply move/cut the source rather than fully copy the original folder contents.

If you creating a new project based on an existing Maven structure :
Create the project using a general project wizard and give the project the same name as just created.
If you try to create the project as a Maven project via m2e will receive an error that project/pom already exists.

Create a new project..
Right Click on your project..
Select Build path --> Configure Build Path
Under source tab choose link source, your .java files containing folder..
I am suggesting this since none of the methods that you tried have worked ---FYI

Follow this instructions from standard eclipse docs.
From the main menu bar, select command link File > Import.... The Import wizard opens.
Select General > Existing Project into Workspace and click Next.
Choose either Select root directory or Select archive file and click the associated Browse to locate the directory or file containing the projects.
Under Projects select the project or projects which you would like to import.
Click Finish to start the import.