Merge pull request #3352 from JCount/legacy-java-cask-requirement-support

java_requirement: support prompting users to install legacy Java casks

Library/Homebrew/extend/os/mac/requirements/java_requirement.rb

 class JavaRequirement < Requirement
-  cask "java"
-
   env do
     env_java_common
     env_oracle_jdk || env_apple
   end
 
+  # A strict Java 8 requirement (1.8) should prompt the user to install
+  # the legacy java8 cask because the current version, Java 9, is not
+  # completely backwards compatible, and contains breaking changes such as
+  # strong encapsulation of JDK-internal APIs and a modified version scheme
+  # (9.0 not 1.9).
+  def cask
+    if @version.nil? || @version.to_s.end_with?("+") ||
+       @version.to_f >= JAVA_CASK_MAP.keys.max.to_f
+      JAVA_CASK_MAP.fetch(JAVA_CASK_MAP.keys.max)
+    else
+      JAVA_CASK_MAP.fetch("1.8")
+    end
+  end
+
   private
 
+  JAVA_CASK_MAP = {
+    "1.8" => "caskroom/versions/java8",
+    "9.0" => "java",
+  }.freeze
+
   def possible_javas
     javas = []
     javas << Pathname.new(ENV["JAVA_HOME"])/"bin/java" if ENV["JAVA_HOME"]