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How to resolve import in Eclipse?

Im new in Java development and not familiar with various kinds of import (Maven, Git, etc), so I make it simple:
import com.google.common.collect.*;
import com.google.gson.*;
These two is not resolved in code Im inspecting, and I have no idea what kind of actions I should take neither what I should import to resolve it but it is probably some popular library.
Is there complete guide how developers import packages in eclipse (for example C# developers use Nuget, despite there is a ton of hand made ones), or they really use all this enormous import selector?

First of all Mavenise your current project and add the following dependency to it:
Goto: POM.XML after converting your current project to Maven project.
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1.0-SNAPSHOT</version>
<properties>
------Properties Here----
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
</dependency>
<dependency>
<groupId>com.google.collections</groupId>
<artifactId>google-collections</artifactId>
<version>1.0-rc2</version>
</dependency>
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.8.5</version>
</dependency>
--------Add your Dependencies here, remember you have to add dependencies under <dependencies> here </dependencies> ----------
</dependencies>
</project>
Search for all dependencies here: https://mvnrepository.com/ , incase you need more dependencies to import.
How to mavenise your current Java Project:
In your eclipse just right click on Java Project and click Configure and you should see “ Convert to Maven Project ” option.
What is POM.XML
A Project Object Model or POM is the fundamental unit of work in Maven. It is an XML file that contains information about the project and configuration details used by Maven to build the project. It contains default values for most projects. Now, you can add a dependency in pom.xml.

If you use no dependency management tool like maven the simplest way is just download corresponding JARs and add them manually: http://www.oxfordmathcenter.com/drupal7/node/44
For GSON use this: https://mvnrepository.com/artifact/com.google.code.gson/gson/2.3.1
For the second dependency I suppose you should use GUAVA:
https://github.com/google/guava/wiki/UseGuavaInYourBuild
or this: https://mvnrepository.com/artifact/com.google.collections/google-collections/1.0-rc2
But actually better to convert your project to Maven project (as described for example here: https://crunchify.com/how-to-convert-existing-java-project-to-maven-in-eclipse/) so that you'll be able to use pom.xml for dependency management or Gradle (see configuring existing eclipse java project to build using gradle) and avoid manual JAR download

Download project via Maven

All I want is the project file/jar for this project: http://thiagolocatelli.github.io/parse4j/
It says I need to do the following to obtain it:
Getting Started
Download the library manually
Maven
<project ...>
...
<dependencies>
<dependency>
<groupId>com.github.thiagolocatelli</groupId>
<artifactId>parse4j</artifactId>
<version>1.3</version>
</dependency>
</dependencies>
...
</project>
I have never used Maven, do not really know what it is. Can someone advice me how I obtain the project file?

Maven is a dependency manager. Lots of information about it if you're interested - just use your favorite search engine.
You can also download the jar file directly from http://search.maven.org/remotecontent?filepath=com/github/thiagolocatelli/parse4j/1.3/parse4j-1.3.jar

Adding a war dependency with appassembler-maven-plugin

I am using appassembler-maven-plugin to create the script to run an application. That application depends on several projects and solves this dependencies using Maven. One of the project is packeged as a war.
<dependency>
<groupId>my.war.dependency</groupId>
<artifactId>project</artifactId>
<type>war</type>
<version>1.0</version>
</dependency>
The problem I have is that the appassembler is not extracting the war dependency, only the jar dependencies. So I cannot refer to the classes in this war in order to create the script calling to one of the classes inside.
Can you help me?
Thanks in advance.

Retrieve Jetty from Maven Central Repository as project dependency in IntelliJ IDEA Project

I want to use Jetty as an embedded library in a Java project I'm working on in IntelliJ IDEA. However, there are many different packages for Jetty available from the Maven Central Repository. The JAR available for direct download from here is named as jetty-distribution-9.0.3.v20130506.tar.gz, so I assumed the best complete package available from the Maven Central Repo was org.eclipse.jetty:jetty-distribution:9.0.3.v20130506. But IntelliJ returns this error when attempting to use that coordinate to retrieve the library:
No files were downloaded for org.eclipse.jetty:jetty-distribution:9.0.3.v20130506
Why can't that package be found? And if it's not usable, which packages should I download?
Edit: I now realise that the coordinate I should have been using is org.eclipse.jetty.aggregate:jetty-all:9.0.3.v20130506. I can locate this at search.maven.org, but IntelliJ cannot find anything newer than version 7. Can anyone reproduce or explain this issue? Moved to new question.

Check the dependency type.
There are so called pom type of dependencies, which act as a list of other dependencies. To be able to fetch them, you have to mark them as pom dependencies in your pom.xml
If you only need the server component, try searching for this string
'org.eclipse.jetty:jetty-server:9.0.3.v20130506'

Maven dependencies have a type, which by default is jar. The jetty distribution package is not a jar, and as you can see in the central repository, you can download either a .zip or a .tar.gz, so you'll have to declare the dependency as:
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-distribution</artifactId>
<version>${jetty.version}</version>
<type>zip</type>
</dependency>
If you build now, it will download the zip and the build will probably succeed. But, a zip is different from a jar, so depending on what you're actually doing in that build, you will have to do more things to actually make use of that zip.
You probably don't want to use the distribution package unless you're also building a standalone distribution (.zip) for your project as well, in which case you should probably use the maven-assembly-plugin which can unzip the jetty distribution and rezip your whole project.
What you should do is decide what exactly you're going to need and build a custom jetty. Here's the starting point, enough to be able to deploy a simple servlet-based application:
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-server</artifactId>
<version>${jetty.version}</version>
</dependency>
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-util</artifactId>
<version>${jetty.version}</version>
</dependency>
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-xml</artifactId>
<version>${jetty.version}</version>
</dependency>
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-deploy</artifactId>
<version>${jetty.version}</version>
</dependency>
You're probably going to need this one as well, since this is how you can start Jetty:
<dependency>
<groupId>org.eclipse.jetty</groupId>
<artifactId>jetty-start</artifactId>
<version>${jetty.version}</version>
</dependency>
Look at the list of modules to see what else you might need, for example jetty-ajp, jetty-websocket, or jetty-jsp.

Maven, package does not exist

I have a module whose pom file is:
<groupId>com.mycompany.Common</groupId>
<artifactId>common</artifactId>
<packaging>pom</packaging>
<version>1.0-SNAPSHOT</version>
<name>common module</name>
In that artifact ('common'), I have a package named com.mycompany.common.objects. In the consuming package, my pom file is:
<dependency>
<groupId>com.mycompany.Common</groupId>
<artifactId>common</artifactId>
<version>1.0-SNAPSHOT</version>
<type>pom</type>
</dependency>
When I run mvn install it always complain: package com.mycompany.common.objects does not exist.
I tried explicit importing in the class where the error was:
import com.mycompany.common.objects
No luck. I tried in both the IDE (IntelliJ) and in commandline. Any idea? thanks

While working with IntellijIDEA, generated files can cause this issue. Writing
mvn idea:idea
on IntellijIDEA Maven console to reset those files did the trick for me. Also, see:
Package doesn't exist error in intelliJ

From your sample, we cannot see any artifact containing the package com.mycompany.common.objects you are using.
You are adding dependency com.mycompany.Common:common as a POM (and you are declaring the packaging of com.mycompany.Common:common as POM too). If it is actually a JAR artifact that contains the package you need to use, then remove the packaging from the POM and dependency (which means, using default which is JAR).

For anyone struggling with this and not familiar with java, make sure that the said package exists in your local repository. Maven has a local repository ~/.m2 where the packages are installed for local access, so even if your dependency package is correctly declared as a dependency in pom.xml and is compiled and exists in your project, if it does not exist in the local repository, the mvn compile will trigger a "package does not exist" error.
To fix this:
In the missing package folder, do:
mvn install //--> this will package and install your missing package in the local repo
Then in your project that you wanted to compile:
mvn compile // --> now that the missing package is in the local repo it should work

Please correct me, If I'm wrong. I understand that the common is a POM that defines several dependencies which intents to be used by other modules. The Importing Dependencies may meet your requirement.
For example
<dependencyManagement>
<dependencies>
<dependency>
<groupId>com.mycompany.Common</groupId>
<artifactId>common</artifactId>
<version>1.0-SNAPSHOT</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
I hope this may help.

I had the same problem recently. Everything in my project was setup correctly with dependencies etc. I tried removing /target dirs but nothing worked.
Finally, I solved it by removing the Module Dependency from my dependent project and then readding the dependency. Not sure what is going on in the background, but some sort of refresh of the classpath must have been made. Perhaps the problem was due to the Maven setup.
Hope it helps someone who reaches this question from a search engine.

Not sure if there was file corruption or what, but after confirming proper pom configuration I was able to resolve this issue by deleting the jar from my local m2 repository, forcing Maven to download it again when I ran the tests.

For me the problem was with the sourceDirectory and testSourceDirectory nodes in my pom.xml.
I was using
<sourceDirectory>${basedir}/src/test</sourceDirectory>
<testSourceDirectory>${basedir}/test</testSourceDirectory>
and changed it to
<sourceDirectory>../src/test/java</sourceDirectory>
<testSourceDirectory>../src/test/java</testSourceDirectory>

Your IDE (Eclipse in my case) may not distinguish between compile and runtime scope. This means that the IDE will let you use runtime scope dependencies in your code, but maven won't. In such a such change the dependency scope from runtime to compile.

you need to add the maven-plugin into (each) child module (for compiling main and test source)
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<plugins>
and then you add the plugin-management into the parent pom, for centralizing the plugin config (version...)
<pluginManagement>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</pluginManagement>
Then you can add your dependency into the dependent module pom
<dependency>
<groupId>com.mycompany.Common</groupId>
<artifactId>common</artifactId>
<version>1.0-SNAPSHOT</version>
<type>pom</type>
</dependency>
http://www.jouvinio.net/wiki/index.php/Projet_Maven_multi-modules