Develop Reference

How do I use another java project in my code?

I am currently using the Eclipse IDE and want to import an external java project so that I can use it in something I am working on. I downloaded the directory of icePDF, found here, and would like to use the java class file "SwingViewBuilder.java" that is found in the directory I linked at "/icepdf/branches/icepdf-6.2.5/icepdf/viewer/src/org/icepdf/ri/common". I have tried some basic things like importing the folder itself, but it did not work.

I would highly suggest using a dependency management tool like Maven or Gradle to download this dependency and any other dependencies you may have into your project for you.
Here is the maven getting started guide: https://maven.apache.org/guides/getting-started/
And here is the maven dependency for you to add to your pom.xml file: https://mvnrepository.com/artifact/org.icepdf.os/icepdf-viewer/6.2.2
If you specifically need version 6.2.5, you will need to build the icePDF project yourself and configure your class path to use it as a dependency.

GWT maven build eclipse

I had a GWT app, and I wanted to automate its build and deploy system, since I do it manually. But I did not find a way how to build the app from command line, so it can than be automated. I had to click the Google button, then compile GWT project and then click Compile.
I found out that it is possible to create a GWT maven project and that it should then be possible to compile my project from commandline with mvn gwt:compile.
So I created a new project using this plugin. Copied my sources from the old project to this new one.
Now the structure is like this:
/src
---/main
------/java -> here are all my sources including my Project.gwt.xml file.
------/webapp
---/test
pom.xml
Now I have 2 problems.
1. I thought that I add dependencies to the pom.xml, and then when I build the app, it will create the jars and I can use those libraries in my GWT app. I guess 'mvn clean install' should do this, but so far I'm getting compile errors.
2. I did not get mvn clean install to work, so I added all the jars manually again... And then yes! I was able to build the app using the plugin GWT button! So I was thinking that now I can use 'mvn gwt:compile', but it fails with:
Unable to find: "com/company/project/Project.gwt.xml" on your classpath; could be a typo, or maybe you forgot to include a classpath entry for source?
EDIT:
So I fixed my <moduleName> element in pom.xml, so now it finds the Project.gwt.xml. I'm trying to run:
mvn clean install gwt:compile
But I am getting compile errors. I think, it tries to build my project without the actual dependecies because it tells few classes don't exist, but those classes are part of an external library. (specifically this one). But I have it in the dependencies, so I don't know what more to do.
<dependency>
<groupId>com.github.tdesjardins</groupId>
<artifactId>gwt-ol3</artifactId>
<version>3.0.0</version>
</dependency>
Also in eclipse I had to manually add the jars to my project, so that was why it worked there and not in the command line. So I would also like to ask how to tell eclipse to get those jars and include them to the project, because otherwise eclipse is missing those dependencies and displays many errors.

First I had a problem with <moduleName> in my pom.xml was missing com.company.project prefix before the actual module name.
Then I had errors in my Java files, which was caused by RELEASE version of GWT-OpenLayers 3 library missing some of the features that I previously used by building the JAR from the GitHub repository.

How to properly use any Java API

I have been trying to use the vget library/api to make my own youtube video downloader. The vget library can be found here: https://github.com/axet/vget
I have downloaded the zip on github and I imported the project into eclipse. However, I am confused to how I am supposed to properly use this API. Should I make a completely new project, and import the classes that I need or do I put my own source files in the project of the api?
I have read other threads concerning this problem. However, they all mention how a api is typically packaged in a JAR file, but in my case it is just files and classes. So I am confused to how I should properly use this api.

The vget project is a maven project. You can see that because it has a pom.xml file in the root folder of the project.
To use it, you don't even need to download the source, because the compiled jar files are already stored in the central maven repository. You can find more information about this here:
http://mvnrepository.com/artifact/com.github.axet/vget/1.1.23
(in general, you can use the http://mvnrepository.com/ site to search whether your library is available on the maven central repository. If it's even a mildly popular library, then chances are that it is)
What you need to do is to make your own project a maven project.
Here's a "5 minutes" starter guide that describes how to do that.
When you've done that, you just add the dependency on vget to your pom.xml file in the <dependencies> section:
<dependency>
<groupId>com.github.axet</groupId>
<artifactId>vget</artifactId>
<version>1.1.23</version>
</dependency>

Since you are making use of a 3rd party software, and not extending it with your own logic, the way to go is to create a new project, which references the 3rd party software.
You then construct your application and make it do whatever you need it to do. When it comes to using logic which is available within the 3rd party logic, you would then simply delegate that call to the 3rd party library.

I have seen on the link you have provided, that this is a maven project. You have to execute a maven package command, or maven install, so that the jar file will be generated.
With this jar follow the Bill's instructions, and add it as external library to your claspath.
When you do this, you will be able to invoke methods of that api.
Let us know if you need some help doing this in eclipse.
If your project is a maven project, you can solve dependencies problems just adding the dependency written on Readme file to your pom file.

The easiest and most automatic way is to use something like maven, ant, or gradle, that will automatically download and put the jars in to your classpath if they are in the central repositories. For example, in the maven configuration file(pom.xml) you can add this to the dependency list:
VGet Maven Repository
These build tools also allow you to add external jars if needed.

If
I would suggest you get familiar with Maven. At the bottom there is a Maven dependency you just have to include into your pom.xml, and then you can use the extension immediately.
Maven is a build platform which organizes your project in a technical way (convention over configuration, e.g. code is in /src/main/java, tests are in /src/test/java). The proper way is it to create a Maven project in Eclipse (you have to install the plugin and download Maven as well) and put the dependency
<dependency>
<groupId>com.github.axet</groupId>
<artifactId>vget</artifactId>
<version>1.1.23</version>
</dependency>
into your <dependencies> inside your pom.xml. After adding it, you project recognizes the additional package automatically.
Nobody tinkers by adding libraries manually. It's actually not professional to work without a build platform like Maven or Gradle.

How to add external jar files to maven

How do I add external jar files to maven web app project using Eclipse M2 Plugin?

The most correct way is probably to install them to the repo.
However, for an alternate, see the top answer to:
Can I add jars to maven 2 build classpath without installing them?

You want to use mvn install:install-file. See here for the instructions in the Maven guide:
http://maven.apache.org/guides/mini/guide-3rd-party-jars-local.html

JAR in project-specific local Maven repo, how to see library in Eclipse project?

I've been trying to add a custom .jar (ftp://ftp.ncbi.nlm.nih.gov/pub/eutils/soap/v2.0/java/axis2_1.5.2_jdk_6.0.12/eutils_axis2.jar) to a project that doesn't have a central corporate maven repository and that instead will have the custom JARs checked into SCM in the project directory tree. I was following this post to make it happen: Maven: add a dependency to a jar by relative path (awesome post btw).
What I did was:
Add local repository to pom.xml
install the file into the local repository
Add dependency to pom.xml
Based on what I see in m2eclipse, the library has been successfully recognized by Maven and added to the dependency list (or it'd be called ? : ? or something similar)
The problem is that Eclipse still doesn't see the referenced lib, so this still fails:
import gov.nih.nlm.ncbi.www.soap.eutils.*;
Pardon my maven newbiness, but what are changes / next steps I need to make to get to:
Have Eclipse see the library so that autocomplete works (and the import can be resolved)
Be able to compile the project
Be able to execute the jar produced by mvn package?
Thanks!

If you see the JAR under "Maven Dependencies" in your project, Eclipse should be able to see and use it. If it's not there, then m2eclipse wasn't able to resolve the dependency properly.
If it is missing, m2eclipse was unable to download the dependency from your local repository for some reason. Check the Maven 2 Console for errors and the Problem View.
Lastly, the JAR itself might be corrupt. Maven doesn't check for that, it simply adds the file to the classpath. If Eclipse can't open the JAR, you can also get the errors you mentioned.