AdoptOpenJDK is an open community who are advocates of OpenJDK and produce prebuilt binaries from OpenJDK.
Visit AdoptOpenJDK Website to download and install OpenJDK for your operating system.
Zulu offers community builds of OpenJDK free of cost.
Red Hat’s implementation of OpenJDK is available free for development.
In good old days, you could simply visit java.com to download and install JDK but starting April 16, 2019
Oracle has completely changed the License and it is no longer advisable to download and use JDK from Oracle (specially if you are a business/enterprise).
If you want to buy, Oracle is Selling Subscription to OracleJDK.