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Idea Intellij: Can't import the libraries: package does not exist

I'm trying to make use of a jReddit library, which, in turn requires apache HttpComponents and Commons IO libraries.
I downloaded the sources, added them in Itellij Idea through File - Project Structure - Modules - Add Content Root.
All the classes from the libraries that my code makes use of are imported successfully. But the problem appears when compiling - it says that package com.github.jreddit.oauth does not exist and package org.apache.http.impl.client does not exist and that it cannot find symbol of those libraries' classes.
Why does this happen and how to fix it?

Don't use Maven or Gradle if you can't even manage adding a JAR to your project manually.
You should acquire those JARs (containing .class byte code files, not .java source), add them a folder in your project named /lib, and add that directory as a JAR source location in your project. They'll be in the CLASSPATH then.
You need to add the /lib folder to an artifact when you run. Be sure you know how to do that as well.

I ran into this error after upgrading IntelliJ to version 2019.1. These steps fixed it for me:
Click Run from the toolbar
Choose Edit Configurations
Make sure the Scratch file you want to run is selected on the left panel
In Use classpath of module dropdown, select the project module that contains the proper module

Import of excuting jar doesn't work in eclipse

I deleted my project permanently and I want to import my old executing .jar into eclipse.
Eclipse import function does not find anything it the jar file.
Can someone give me a tip or recover the file?
Here is a screenshot of eclipse
Thank you
Click here for the file to recover

You cannot recover the project. You did not set to export source files while exporting, there are only .class files.
For future reference select Export Java source file and resources while exporting the project.
Reference: http://www.albany.edu/faculty/jmower/geog/gog692/ImportExportJARFiles.htm

I am curious to know if it's jar why do you want to import as a project?
You can import external jar as a "APP Client Jar File" which is under Java EE.
I tried this with your attached jar and I am able to import it and able to see the content in it.

You are mixing a few things. If you deleted your project source you won't be able to recover it unless you decompile it, and it will be obfuscated even though.
What you have is the compiled project, resulting from taking your source (.java) and compiling it to .class files.
The Import option in Eclipse is made for importing .project files, that hold your project's information. Also you could create a new project from your existing code but as far as I know you just can't create/recover a project from it's compiled jar.

package com.mongodb.client does not exist

I am getting error in eclipse console while building application
package com.mongodb.client does not exist.
I am importing two packages :
import com.mongodb.client.AggregateIterable;
import org.bson.Document;

Download the jar files of the packages and add it to the build path of your project.
If you are using maven, add the dependency in pom.xml

How do I fully include the Batik library in my Java project?

I am trying to add the Batik library to my Java project. I added the unzipped binary to my lib folder, and added all of the JARs in the top level to the build path.
However, when I make the following import statement:
import org.apache.batik.dom.svg.SAXSVGDocumentFactory; I get the following error:
"The import org.apache.batik.dom.svg.SAXSVGDocumentFactory cannot be resolved"
Other import statements do not exhibit this problem, such as the following:
import org.apache.batik.util.XMLResourceDescriptor;
As far as I understand, the SAXSVGDocumentFactory class exist somewhere in the binary I added to the lib folder, so the issue must be with adding JARs to the build path. How can I make sure that all of the Batik source files are added to the build path correctly and are usable?

It cannot be resolved because it doesn't present in 1.8 jar. It exists in 1.7.
http://www.findjar.com/class/org/apache/batik/dom/svg/SAXSVGDocumentFactory.html
The easy way to find this out is to find the right jar and click to open it by the package: org.apache.batik.dom.svg.SAXSVGDocumentFactory. You wont see this class in 1.8 jar, but you will see it in 1.7 jar.
To fix, download a 1.7 jar and add it to your project again.
http://apache.mirrors.hoobly.com/xmlgraphics/batik/source/

If somebody can't find SAXSVGDocumentFactory after updating Batik in legacy project, just change the import:
import org.apache.batik.dom.svg.SAXSVGDocumentFactory;
with
import org.apache.batik.anim.dom.SAXSVGDocumentFactory;

How to solve com.sun.org.apache.xml.internal.security package does not exist in netbean javafx application

I want to build .jar in NetBean 8.0.2.
I have developed a small project based javafx application.
My project uses packages such as com.sun.org.apache.xml.internal.security, com.sun.org.apache.xml.internal.security.c14n and com.sun.org.apache.xml.internal.security.utils.
My project runs well at Netbean without warning or error.
But When I build this project as jar file, following error is occurred:
**error: package com.sun.org.apache.xml.internal.security does not exist**
**error: package com.sun.org.apache.xml.internal.security.c14n does not exist**
**error: package com.sun.org.apache.xml.internal.security.utils does not exist**
How to solve this problem?

If your project uses Maven, add Maven dependency com.sun.xml.security:xml-security-impl:1.0 to your Maven project file (pom.xml).

You have to add the xml-security-impl.jar to your classpath.

Downloading xmlsec-x.y.z.jar (eg. xmlsec-1.4.0.jar) and adding it to project libraries, resolved the issue.

You have used a package which should not be used. It has already been clarified here on SO and many places that any class within package starting from com.sun must not be used, as they can be removed in later versions of Java, without replacement.
More information on this : Package com.sun.org.apache.xml.internal.security.utils.Base64 does not exist
To resolve your problem, you should use some other library.