open your terminal first

At first check is java jdk already installed on your system or not

java --version
Unknown command: java
which java
which: no java in (/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl)

Install JRE in Arch Linux

sudo pacman -sS java | grep jre

Install the latest version of JRE

sudo pacman -S jre-openjdk

Install JDK in Arch Linux

sudo pacman -sS java | grep jdk

Install the latest version of the JDK

sudo pacman -S jdk-openjdk

Now check, is java installed or not on your system by running this command

java --version
openjdk 18.0.1.1 2022-04-22
OpenJDK Runtime Environment (build 18.0.1.1+2)
OpenJDK 64-Bit Server VM (build 18.0.1.1+2, mixed mode)