What is the difference between using JDK 7 vs using JDK 8 with compiler compliance level 1.7?

Question

I was wondering if there is any difference running/building a software under JDK 8 and using compiler compliance level 1.7 vs JDK 7 as system default? I am more interested in reference to Android building, building apps, Eclipse, Android Studio, etc.

Answer 1

Yes, there are loads of new classes in the JDK 1.8, for example, the java.time classes. You won't get those if you build in JDK 1.7; but you will be able to use them if you build in JDK 1.8 with compiler compliance level 1.7.

Answer 2

Yes there is a difference between running/building a software under JDK 8 and using compiler compliance level 1.7 vs JDK 7 as system default.

1. running a software under JDK 8 and using compiler compliance level : You compile in jdk 1.7 but run on 1.8. No problem, your programm will work as needed.

2. JDK 7 as system default : You compile in 1.7 version and run on the same version.

I am wondering in waht case you would like to use the first case ?

Answer 3

truck load of difference actually. With JDKs the the compliance level is a directive to the compiler to specifically use the optimizations and linking features for the version you specified. It has a lot more going under the hood but I don't think you want to know that. New JDK versions bring new features and the compilers in those version are able to understand and link those features when building class files or assembled code of your source Java files. Consequently the JVM runtime in those JDKS is also equipped to handle such optimizations and cases and process them. So without compliance levels the class file that you build with JDK8 would only run correctly with JDK8 based runtimes. They may not do so with JDK7 or 6. To counter this problem and thus allow your JDK8 compiled code to run on JDK8,7 and maybe even on 6, hyou need to add compliance level to compiler directives accordingly. Downside is that you may not be able to use some latest features which the compiler offers but such cases are far few and outweigh the need for interoperability and potability.