java - What is "compiler compliance level" in Eclipse?

Question

For some time I tried to understand, but I still don't get exactly what "compiler compliance level" for a project in Eclipse means. I looked all over this site and Google and couldn't find an answer I could understand.

Let's say I want my program to be able to run on JRE 6.

I can do: Project > Preferences > Java Build Path > Libraries, and set the JRE library I use to JRE 6.

Why isn't this enough?

I never understood why I also need to set the compiler compliance setting to JRE 6.

I'd like to understand the difference between using JRE 6 in a project, and setting the project's compiler compliance setting to JRE 6.

What exactly does compiler compliance level mean?

Answer 1

The compiler compliance setting tells the compiler to pretend it's a different version of Java.

The Java 8 compiler will produce class files in the Java 8 version of the class file format, and accept Java 8 source files. JRE 6 can't load this version, because it was created after JRE 6 was.

If you set the compliance level to "JRE 6", it will instead compile Java 6 source files into Java 6 class files.

It's like saving a Word document as "Word 97-2003 format" - so that Word 97-2003 can read your document. You're saving the class files in Java 6 format so that Java 6 can read them.