Download the latest JAR from https://search.maven.org/remote_content?g=com.squareup.retrofit2&a=retrofit&v=LATEST.

Alternatively, you can inject the dependency via Maven with the following code:

<dependency>
    <groupId>com.squareup.retrofit2</groupId>
    <artifactId>retrofit</artifactId>
    <version>2.4.0</version>
</dependency>

Alternatively, you can use Gradle, as per the following code:

implementation 'com.squareup.retrofit2:retrofit:2.4.0'
implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
compile 'com.jakewharton.picasso:picasso2-okhttp3-downloader:1.1.0'