There are several implementation of Java, one currently controlled by Oracle and it can be downloaded from Oracle.com but as far as I know there are various problematic licensing restrictions. A better choice is to download the Open JDK.
On MS Windows install it by unipping the .zip file and then configuring the PATH to include the bin/ directory.
Before we even write a line of code we would like to make sure that both the java compiler and the java runtime is accessible for us. So we open a command line window or a Power Shell window and we type in javac --version and java --version respectively.
On MS Windows
> javac --version javac 14.0.2
> java --version openjdk 14.0.2 2020-07-14 OpenJDK Runtime Environment (build 14.0.2+12-46) OpenJDK 64-Bit Server VM (build 14.0.2+12-46, mixed mode, sharing)
On Ubuntu Linux:
$ javac -version javac 1.8.0_252 $ java -version openjdk version "1.8.0_252" OpenJDK Runtime Environment (build 1.8.0_252-8u252-b09-1ubuntu1-b09) OpenJDK 64-Bit Server VM (build 25.252-b09, mixed mode)