If I'm understanding you correctly, you wish to use other projects/libraries in your project. You should look into Java build tools (ev. Maven, Gradle and others). Among other things, they let you specify a list of dependencies for your project.
Maven example
First of all, you should set up your project to use Maven.
For instance, if you wish to use joda-time (a popular date and time library for Java), you could go to http://mvnrepository.com/ and look for joda-time. From there select your desired version and copy-paste the Maven dependency to your pom.xml
file.
<?xml version="1.0" encoding="UTF-8"?>
<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.myproject</groupId>
<artifactId>myproject</artifactId>
<version>0.1-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.9.1</version>
</dependency>
</dependencies>
</project>
Now when you call mvn clean install
from your commandline or use the Maven Eclipse plugin, your project is built and the dependencies you specified are downloaded and added to your class path.
Keep in mind, the source code for the dependencies is never added to your project, only the jar
files are added to your class path.