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How to add a jar to the classpath of a project using Eclipse

I'm using Eclipse as IDE, and I would like to add a jar to the classpath of a Java project.
However, I can find only a way to add the jar to the project buildpath.
How can I add a jar to a project classpath using Eclipse?

You can use user library in Eclipse.
Eclipse version for steps: Luna Service Release 2 (4.4.2)
Steps for adding an User Library:
Windows -> Preferences -> Java -> Build Path -> User Libraries -> New -> input library name like "testUserLib" -> Add JARs / Add External JARs -> OK
Steps for applying an User Library to Java Project:
Properties -> Java Build Path -> Libraries -> Add Library -> User Library -> choose "testUserLib" -> Finish -> OK
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Using jar file in another project

I have one jar file, named pwd.jar and I want to use that in another project named sample.
How can I do that? What are all the steps I should perform?
Note: I am using Eclipse IDE

Right click your project-> properties->Java Build Path -> Libraries tab -> Add External jar, and browse your jar
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why this eclipse error showing and what should be the solution for it

Missing library: xdoclet-1.2.1.jar. Select the home directory for XDoclet. 1.2.1
why this eclipse error showing and what should be the solution for it alz

It is causing possibly because that jar is not added to your project build path. Follow this steps:
Right click on project(in the package explorer)
-> Buid Path
-> Configure Build Path
-> Java Build Path
-> Libraries tab
-> Add JARs button
Then locate the folder where the jar resides. You can filter your search by typing *.jar in the search field. And then click ok.

I downloaded the xdoclet-lib-1.2.3.zip file from:
http://sourceforge.net/projects/xdoclet/files/xdoclet/1.2.3/
unziped it to its folder name, xdoclet-1.2.3, and then from the Eclipse (I was using 4.5) preferences JavaEE -> XDoclet tab selected the Version 1.2.3 and then the Browse button to the xdoclet-1.2.3 folder. Apply and OK and Eclipse error goes away.
Saying all that, I suspect you don't really need it unless you know that you are using xdoclet. If that's the case and it was intentionally left out then the error message should be fixed in Eclipse. I only went around fixing it because of the message.

If have maven version of the project, can also try by updating the project. I also had this XDoclet. 1.2.1.jar complaint, which got resolved once I update the project.
Right Click Project > Maven > Update Project ... > Check the projects > OK.

Please add maven dependency in your project . Steps are -
1- Right Click on Project
2- select configure build path
3- select library tab
4- add library and select Maven Dependencies
5- apply and Apply and Close .

Including a Jar in Android App

I want to include a jar into my Android package.
Unfortunately, I can't get it to work. I followed several explanations, such as this one, but I still get NoClassDefFoundErrors - at runtime, building, compiling, installing the project worked without errors.
Most answers seem to be outdated. How do I solve this issue using the current Eclipse, ADT and Android versions? Adding them to the Java Build Path like in plain Java projects didn't help.
All help appreciated.
What I have tried
Putting them into a folder and including into the project [Screenshot]

Put the jar in a folder entitled 'libs' (should be in the root of your project. Then add it to the java build path.
If that doesn't work try this:
Your Project -> right click -> Import -> File System -> yourjar.jar
Your Project -> right click -> Properties -> Java Build Path -> Libraries -> Add Jar -> yourjar.ja

Add your jars to libs folder. Automatically add's those JARs to your compile-time build path. More importantly, it will put the contents of the JARs into your APK file, so they will be part of your run-time build path.

You two things and the error will be fixed
Right click on Project-->Select Properties-->Go to Java Build Path--> Select 3rd tab from top i.e "Libraries" -->Click Add Jar--> Select your Jar
Right click on Project-->Select Properties-->Go to Java Build Path--> Select 4th tab from top i.e "Order and Export" --> Select(chekbox) your Jar.
For more help check this link

How to reflect another project's class info

there are two projects in eclipse! how to reflect one class in other project? I put other project's class path into system's property. but still popup class not found exception. Is there any good solution for it?
System.out.println(System.getProperty("java.class.path"));
System.getProperties().put("java.class.path",
System.getProperty("java.class.path") +
";C:\\Users\\fu.jian\\workspace\\Study\\bin");
System.out.println(System.getProperty("java.class.path"));
Class<?> forName2 = Class.forName("study.Activator");
System.out.println(forName2);

Right click the project to bring up the context menu.
Build Path->Configure Build Path
Under the Java Build Path dialog, select the Projects tab and add the project.

Try to add the project B to classpath of project A by:
In Eclipse click on project A -> mouse right click and select Properties -> Select Java Build Path -> Select Projects tab -> Click on Add... and check project B
The problem will be resolved inside Eclipse, if you use external tools for compilation and building the project - your problem will still appear.
You can export the project B as jar and copy the jar into the project A libs. This way you won't depend on Eclipse.