The Java Platform, Standard Edition(Java SE) is strongly compatible with previous versions of the Java platform. Each version of Java SE has upwards binary-compatibility with the previous version, except for the incompatibilities noted in the documentation. Java SE does not support downward source compatibility, however: If source files use new language features or Java SE platform APIs, they will not be usable with an earlier version of the Java platform.
The documentation describes any compatibility issues between the current version and the version immediately preceding it.
Confused about Java SE, J2SE, and 1.5.0? Read about Java Naming and Versioning.