Why are there many folders for Java in Ubuntu? -

i'm newbie @ java development, if made me confused jdk x jre difference.

now want understand installed in each folder contains java code

for example, there folder:

/usr/lib/jvm/java-8-oracle/ 

and there folder called jre inside of it. jre?

if so, jdk installed? there folder that, or source is split in existing folders?

if run whereis java prints

/usr/bin/java /usr/share/java /usr/share/man/man1/java.1.gz 

and if run which java says

/usr/bin/java 

can give me short explanation this?

it common in unux install application in own directory. makes management easier. downside shell won't find there unless add path or add link in common directory in case of java.

in case can find jdk with

which javac 

however unless have installed don't have it.

there 2 ways check while file or directory has been configured. can do

ls -ld /usr/bin/java 

or

file /usr/bin/java 

note: java name of package , name of program in package why can appear more once.

btw /usr/share/man/man1/java.1.gz manual or "man" page java.