An adoption guide typically provides the information that you need to migrate from JDK 7 to JDK 8. This information is available in the release notes.
For an overview of the new features and enhancements provided by JDK8, see What's New in JDK 8. Use this information to improve existing Java applications by making use of these additions to the JDK.
For information on changes in JDK 8 that are incompatible with JDK 7, see the Incompatibilities between Java SE 8 and Java SE 7 and Incompatibilities between JDK 8 and JDK 7. Use this information to determine the changes that are needed to migrate a JDK 7 application to JDK 8.
For information on features that are no longer available, see Features Removed from Java SE 8 and Features Removed from JDK 8. Use this information to modify existing Java applications that use these features.
For information on classes and methods that are deprecated in JDK 8, see Deprecated APIs. Use this information to remove dependencies on these APIs from existing applications before the APIs are removed from the JDK.