Before you can use the SolrJ API, you need to add some dependencies to your "pom.xml" file.
Here's the minimal dependencies needed in order for the following examples to work.

<dependency>
    <groupId>org.apache.solr</groupId>
    <artifactId>solr-solrj</artifactId>
    <version>7.6.0</version>
</dependency>

<dependency>
    <groupId>org.apache.solr</groupId>
    <artifactId>solr-core</artifactId>
    <version>7.6.0</version>
</dependency>