I installed openjdk8
In Eclipse (v3.8.1) -> window -> preferences -> Java -> installed JREs, I added the related standard VM.
In Eclipse -> Project -> Properties -> Java build path, I changed the JRE system library
then, some of my project errors, related to Java 8 disappeared; but some others remained which, as I suppose, they are related to Compiler.
In Eclipse -> window -> preferences -> Java -> Compiler, I cannot see any version more than 1.7!
So, where is my Java compiler 1.8?!