Looking for tutorials on Java how it's used in AEM

General Java

For basic Java tutorials and documentation :

• https://docs.oracle.com/en/java/
• https://docs.oracle.com/javase/tutorial/ 

For Apache Sling API

• https://sling.apache.org/old-stuff/sling-api.html

General AEM Java

https://developer.adobe.com/experience-manager/reference-materials/ 
https://developer.adobe.com/experience-manager/reference-materials/6-5/javadoc/index.html 

For Sling API in AEM

• https://experienceleague.adobe.com/docs/experience-manager-learn/foundation/development/understand-java-api-best-practices.html?lang=en 
• https://experienceleague.adobe.com/docs/experience-manager-65/developing/platform/sling-adapters.html?lang=en 

For JCR API in AEM

• https://experienceleague.adobe.com/docs/experience-manager-65/developing/platform/access-jcr.html?lang=en 

For QueryBuilder API in AEM

• https://experienceleague.adobe.com/docs/experience-manager-64/developing/platform/query-builder/querybuilder-api.html?lang=en 

For TagMaganer API in AEM

• https://experienceleague.adobe.com/docs/experience-manager-65/developing/platform/tagging/building.html?lang=en 

That should cover the somewhat most important parts, but you will need to check the general AEM Java documentation of the classes you are using. It can help to take a look at the WKND project samples to get you started. You will also need to lookup the documentation of the packages specific to your implementation, e.g. if you use GSON, check Google GSON documentation etc.