Linux

$ update-java-alternatives --list
java-1.11.0-openjdk-amd64      1111       /usr/lib/jvm/java-1.11.0-openjdk-amd64
java-1.8.0-openjdk-amd64       1081       /usr/lib/jvm/java-1.8.0-openjdk-amd64

$ sudo update-java-alternatives --set /usr/lib/jvm/java-1.8.0-openjdk-amd64
update-alternatives: error: no alternatives for mozilla-javaplugin.so
update-java-alternatives: plugin alternative does not exist: /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/amd64/IcedTeaPlugin.so

macOS

$ /usr/libexec/java_home -V
Matching Java Virtual Machines (3):
    11.0.9, x86_64:     "OpenJDK 11.0.9"        /Library/Java/JavaVirtualMachines/jdk-11.0.9+11/Contents/Home
    1.8.0_282, x86_64:  "Zulu 8.52.0.23"        /Library/Java/JavaVirtualMachines/zulu-8.jdk/Contents/Home
    1.8.0_221, x86_64:  "Java SE 8"     /Library/Java/JavaVirtualMachines/jdk1.8.0_221.jdk/Contents/Home

/Library/Java/JavaVirtualMachines/jdk-11.0.9+11/Contents/Home

export JAVA_HOME=$(/usr/libexec/java_home -v 1.8.0_282)
# or
export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)

jenv

jenvを使っている場合はmacOSの↑の手順が動かない。rbenvっぽく使う。

IntelliJ IDEA

File > Project Structure > Project Settings > Project > Project SDK

を変更する。