How to fix Java Install error for minecraft forge? I am trying to install minecraft forge 1.12.2-14.23.0.2552 on a Mac Mojave with java "java version "14" 2020-03-17". However, upon running 

java -jar forge-1.12.2-14.23.0.2552-installer.jar


to install the client I see the following terminal output:

...
Checking "/Users/me/Library/Application Support/minecraft/libraries/com/typesafe/akka/akka-actor_2.11/2.3.3/akka-actor_2.11-2.3.3.jar" internal checksums
    checksums.sha1 was not found, validation failed
Downloading library com.typesafe.akka:akka-actor_2.11:2.3.3
Unpacking packed file akka-actor_2.11-2.3.3.jar.pack.xz
  Signed
  Checksum Length: 140942
  Total Length:    2606968
  Temp File:       /var/folders/0g/wwq2q6nn35n8vmrwhw5qfk601zlkw_/T/art9850699590461548829.pack
Exception in thread "main" java.lang.NoClassDefFoundError: java/util/jar/Pack200
    at net.minecraftforge.installer.DownloadUtils.unpackLibrary(DownloadUtils.java:253)
    at net.minecraftforge.installer.DownloadUtils.downloadInstalledLibraries(DownloadUtils.java:89)
    at net.minecraftforge.installer.ClientInstall.run(ClientInstall.java:131)
    at net.minecraftforge.installer.InstallerAction.run(InstallerAction.java:43)
    at net.minecraftforge.installer.InstallerPanel.run(InstallerPanel.java:406)
    at net.minecraftforge.installer.SimpleInstaller.launchGui(SimpleInstaller.java:165)
    at net.minecraftforge.installer.SimpleInstaller.main(SimpleInstaller.java:51)
Caused by: java.lang.ClassNotFoundException: java.util.jar.Pack200
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:602)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
    ... 7 more


How can I fix this problem?
The install is trying to use the Pack200 class, but this has deprecated for some time and has now been removed from Java 14 - see JEP 367

You will have to use an older version of Java to run the install.