Some time back I have written one article Java Lambda Expression Explained with Example but it was easy for me to explore and Java 8 because I was using it in my project and I was allowed to install and use it. But in my current project, we are still using Java 8 and now I want to upgrade myself and learn Java 11 but can not install it.

And this happens with a lot of Java developers, Sometimes you want to learn and explore latest version of Java e.g. Java 11 but you can not install it on your machine because you and your team is working on some older version e.g. Java 8, and you do not want to break your project.

Or suppose you are working on multiple projects some of which use Java 8 and some new projects are using Java 11. So in order to work on these projects parallelly, you will need to install multiple JDK on your machine and you should be able to switch between them.

What if there is a way and what if you are enabled to install multiple versions of Java and turn off and turn on them according to your need.

Well there is a tool called SDKMan which allow us to do the same and according to the official website:

SDKMan! is a tool for managing parallel versions of multiple Software Development Kits on most Unix based systems. It provides a convenient Command Line Interface (CLI) and API for installing, switching, removing and listing Candidates.

Some points about SDKMan are as following:
  1. SDKMan is free to use and it is developed by the open source community.
  2. SDKMan is written in bash and it only requires curl and zip/unzip programs to be present on your system.
  3. SDKMan can install around 29 Software Development Kits for the JVM such as Java, Groovy, Scala, Kotlin and Ceylon. Ant, Gradle, Grails, Maven, SBT, Spark, Spring Boot, Vert.x.
  4. We do not need to worry about setting the _HOME and PATH environment variables because SDKMan handles it automatically.

Installing SDKMan

SDKMan can run on any UNIX based platforms such as Mac OSX, Linux, Cygwin, Solaris and FreeBSD and we can install it using following commands:

Simply open a new terminal and enter:

$ curl -s "https://get.sdkman.io" | bash
$ source "$HOME/.sdkman/bin/sdkman-init.sh"

We can verify the installation using sdk version and sdk help command will us complete help about the usage of sdk command.

Because SDKMan is written in bash and only requires curl and zip/unzip to be present on your system. You can install SDKMan on windows as well either by first installing Cygwin or Git Bash for Windows environment and then running above commands.

Installing Java Using SDKMan

SDKMan supports installations of 29 Software Development Kits for the JVM e.g. Java, Groovy, Scala, Kotlin and Ceylon, Grails, SBT, Spark, Spring Boot. We can get the complete list using sdk list command.

SDKMan also helps us install build tools such as Ant, Maven and Gradle, You can read more about these build tools on Java Build Tools Comparisons: Ant vs Maven vs Gradle.

Command sdk list java will give us a list of java versions which we can install using SDKMan like below:

$ sdk list java

================================================================================
Available Java Versions
================================================================================
     13.ea.02-open       1.0.0-rc-10-grl                                        
     12.ea.26-open       1.0.0-rc-9-grl                                         
     11.0.2-zulu         1.0.0-rc-8-grl                                         
     11.0.2-open                                                                
     11.0.1-zulufx                                                              
     10.0.2-zulu                                                                
     10.0.2-open                                                                
     9.0.7-zulu                                                                 
     9.0.4-open                                                                 
     8.0.202-amzn                                                               
     8.0.201-zulu                                                               
     8.0.201-oracle                                                             
     8.0.192-zulufx                                                             
     7.0.181-zulu                                                               
     1.0.0-rc-11-grl                                                            
================================================================================
+ - local version
* - installed
> - currently in use
================================================================================

Now Suppose we want to install OpenJDK version of Java v9 then we will execute below command in our terminal which will also update the PATH and JAVA_HOME variables:

$ sdk install java 9.0.4-open

The command will take some time to execute because it will be downloading the JDK on our machine and the command will output something like:

Downloading: java 9.0.4-open
In progress...
######################################################################## 100.0%

Repackaging Java 9.0.4-open...

Done repackaging...
Cleaning up residual files...

Installing: java 9.0.4-open
Done installing!

Setting java 9.0.4-open as default.


Now if we check our Java version and JAVA_HOME, we can see Java has been updated to 9.0.4

$ java -version
openjdk version "9.0.4"
OpenJDK Runtime Environment (build 9.0.4+11)
OpenJDK 64-Bit Server VM (build 9.0.4+11, mixed mode)

$ echo $JAVA_HOME
/Users/xbbnrde/.sdkman/candidates/java/current

We can follow the same process to install any version which we can see in the above list, let us suppose we want to install Java V11 version then we can execute the command:

$ sdk install java 11.0.2-open
Downloading: java 11.0.2-open
In progress...
######################################################################## 100.0%

Repackaging Java 11.0.2-open...

Done repackaging...
Cleaning up residual files...

Installing: java 11.0.2-open
Done installing!

Do you want java 11.0.2-open to be set as default? (Y/n): Y

Setting java 11.0.2-open as default.

Now if we again check our Java version and JAVA_HOME, we can see Java has been updated to 11.0.2:

$ java -version
openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)

$ echo $JAVA_HOME
/Users/xbbnrde/.sdkman/candidates/java/current

Note: Some times you will require to close and reopen your terminal in order to see the changes.

sdkman-installing-and-running-multiple-java-versions-java11-and-java-9-simultaneously

Pointing SDKMan to your existing installed Java version

First, we need to find out where on your machine Java is installed. On my machine, it is installed in the folder jdk1.8.0_172.jdk which lies under the folder /Library/Java/JavaVirtualMachines, let's refer to this folder by <java-folder>.

The second thing we want to do is to set up a symbolic link between our installed <java-folder> and SDKMan, we can do it by running below commands:

$ ln -s /Library/Java/JavaVirtualMachines/<java-folder>  ~/.sdkman/candidates/java/<java-folder>

$ sudo ln -s /Library/Java/JavaVirtualMachines/<java-folder>/Contents/Home/bin  ~/.sdkman/candidates/java/<java-folder>/bin


Now if we again execute sdk list java command, we will get:

================================================================================
Available Java Versions
================================================================================
   + jdk1.8.0_172.jd     8.0.201-zulu                                           
     13.ea.02-open       8.0.201-oracle                                         
     12.ea.26-open       8.0.192-zulufx                                         
     11.0.2-zulu         7.0.181-zulu                                           
 > * 11.0.2-open         1.0.0-rc-12-grl                                        
     11.0.2.j9-adpt      1.0.0-rc-11-grl                                        
     11.0.2.hs-adpt      1.0.0-rc-10-grl                                        
     11.0.1-zulufx       1.0.0-rc-9-grl                                         
     10.0.2-zulu         1.0.0-rc-8-grl                                         
     10.0.2-open                                                                
     9.0.7-zulu                                                                 
   * 9.0.4-open                                                                 
     8.0.202-amzn                                                               
     8.0.202.j9-adpt                                                            
     8.0.202.hs-adpt                                                            

================================================================================
+ - local version
* - installed
> - currently in use
================================================================================


We can clearly see that we have 3 JDK installed on our machine and JDK 11.0.2-open is in use as of now. However, if we want to switch between them or install a new one we can do that very easily by using sdk use java <version_which_you_want_to_use> like the following:

Using Java 9

$ sdk use java 9.0.4-open
Using java version 9.0.4-open in this shell.

$ java -version
openjdk version "9.0.4"
OpenJDK Runtime Environment (build 9.0.4+11)
OpenJDK 64-Bit Server VM (build 9.0.4+11, mixed mode)

Using locally installed Java 8

$ sdk use java jdk1.8.0_172.jdk
Using java version jdk1.8.0_172.jdk in this shell.

$ java -version
java version "1.8.0_172"
Java(TM) SE Runtime Environment (build 1.8.0_172-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.172-b11, mixed mode)

Using Java 11

$ sdk use java 11.0.2-open
Using java version 11.0.2-open in this shell.

$ java -version
openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)


Using sdk use command will only enable you to use a specified version of JDK in the same terminal shell where you are executing the sdk use command. And if you close the terminal and open it again you will be on the previously installed version.

But if you want to activate one version of JDK for all terminals and applications, you can use the command sdk default java <your_version>

$ sdk default java 11.0.2-open
Default java version set to 11.0.2-open

$ java -version
openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)

Or you can uninstall that version and install it again like below which will set that version to the current JDK.

Uninstalling a JDK Version

In case you want to uninstall any JDK version e.g., 11.0.2-open you can do that as follows:

$ sdk uninstall java 11.0.2-open

And in case you want to install the uninstalled version back again, you can install it again and this time SDKMan will not download it because it already has that on your machine (unless you do not delete that manually).

$ sdk install java 11.0.2-open

Found a previously downloaded java 11.0.2-open archive. Not downloading it again...

Installing: java 11.0.2-open
Done installing!

Do you want java 11.0.2-open to be set as default? (Y/n): y

Setting java 11.0.2-open as default.

Using SDMan installed JDK from IntelliJ IDEA

SDKMan installs all JDK under .sdkman/candidates/java/ folder and you can find .sdkman folder in your home directory.

After opening any Java project in IntelliJ, you can press Command + : to open the project structure window. In that window you will need to click on New... under Project SDK section and there you can enter .sdkman/candidates/java/ in browse window and can select any JDK from .sdkman/candidates/java/.

Because .sdkman is a hidden folder and in case you face some problem in finding it, you can always create a symbolic link to it with a non-hidden folder using below command.

$ ln -s ~/.sdkman ~/sdkman

You can find the complete source code for my articles on this Github Repository and please feel free to provide your valuable feedback.

Git is a most widely used and powerful version control system for tracking changes in computer files and coordinating work on those files among multiple people. It is primarily used for source code management in software development, but it can be used to keep track of changes in any set of files.

Git was developed by Linus Torvalds in 2005 as an distributed open source software version control software and of course it is free to use. As a distributed revision control system it is aimed at speed, data integrity, and support for distributed, non-linear work flows.

While other version control systems e.g. CVS, SVN keeps most of their data like commit logs on central server, every git repository on every computer is a full-fledged repository with complete history and full version tracking abilities, independent of network access or a central server.

However almost all IDEs support git out of the box and we do not require submit the git commands manually but it is always good to understand these commands. Below is a list of some git commands to work efficiently with git.

Git Help

The most useful command in git is git help which provides us all the help we require. If we type git help in terminal, we will get:
usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p | --paginate | --no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           <command> [<args>]

These are common Git commands used in various situations:

start a working area (see also: git help tutorial)
   clone      Clone a repository into a new directory
   init       Create an empty Git repository or reinitialize an existing one

work on the current change (see also: git help everyday)
   add        Add file contents to the index
   mv         Move or rename a file, a directory, or a symlink
   reset      Reset current HEAD to the specified state
   rm         Remove files from the working tree and from the index

examine the history and state (see also: git help revisions)
   bisect     Use binary search to find the commit that introduced a bug
   grep       Print lines matching a pattern
   log        Show commit logs
   show       Show various types of objects
   status     Show the working tree status

grow, mark and tweak your common history
   branch     List, create, or delete branches
   checkout   Switch branches or restore working tree files
   commit     Record changes to the repository
   diff       Show changes between commits, commit and working tree, etc
   merge      Join two or more development histories together
   rebase     Reapply commits on top of another base tip
   tag        Create, list, delete or verify a tag object signed with GPG

collaborate (see also: git help workflows)
   fetch      Download objects and refs from another repository
   pull       Fetch from and integrate with another repository or a local branch
   push       Update remote refs along with associated objects

'git help -a' and 'git help -g' list available sub-commands and some concept guides.
See 'git help <command>' or 'git help <concept>' to read about a specific sub-command or concept.

Command git help -a will give us complete list of git commands:
Available git commands in '/usr/local/git/libexec/git-core'
  add                     gc                      receive-pack
  add--interactive        get-tar-commit-id       reflog
  am                      grep                    remote
  annotate                gui                     remote-ext
  apply                   gui--askpass            remote-fd
  archimport              gui--askyesno           remote-ftp
  archive                 gui.tcl                 remote-ftps
  askpass                 hash-object             remote-http
  bisect                  help                    remote-https
  bisect--helper          http-backend            repack
  blame                   http-fetch              replace
  branch                  http-push               request-pull
  bundle                  imap-send               rerere
  cat-file                index-pack              reset
  check-attr              init                    rev-list
  check-ignore            init-db                 rev-parse
  check-mailmap           instaweb                revert
  check-ref-format        interpret-trailers      rm
  checkout                log                     send-email
  checkout-index          ls-files                send-pack
  cherry                  ls-remote               sh-i18n--envsubst
  cherry-pick             ls-tree                 shortlog
  citool                  mailinfo                show
  clean                   mailsplit               show-branch
  clone                   merge                   show-index
  column                  merge-base              show-ref
  commit                  merge-file              stage
  commit-tree             merge-index             stash
  config                  merge-octopus           status
  count-objects           merge-one-file          stripspace
  credential              merge-ours              submodule
  credential-manager      merge-recursive         submodule--helper
  credential-store        merge-resolve           subtree
  credential-wincred      merge-subtree           svn
  cvsexportcommit         merge-tree              symbolic-ref
  cvsimport               mergetool               tag
  daemon                  mktag                   unpack-file
  describe                mktree                  unpack-objects
  diff                    mv                      update
  diff-files              name-rev                update-git-for-windows
  diff-index              notes                   update-index
  diff-tree               p4                      update-ref
  difftool                pack-objects            update-server-info
  difftool--helper        pack-redundant          upload-archive
  fast-export             pack-refs               upload-pack
  fast-import             patch-id                var
  fetch                   prune                   verify-commit
  fetch-pack              prune-packed            verify-pack
  filter-branch           pull                    verify-tag
  fmt-merge-msg           push                    web--browse
  for-each-ref            quiltimport             whatchanged
  format-patch            read-tree               worktree
  fsck                    rebase                  write-tree
  fsck-objects            rebase--helper

And command git help -g will give us a list git concepts which git think is good for us:
The common Git guides are:

   attributes   Defining attributes per path
   everyday     Everyday Git With 20 Commands Or So
   glossary     A Git glossary
   ignore       Specifies intentionally untracked files to ignore
   modules      Defining submodule properties
   revisions    Specifying revisions and ranges for Git
   tutorial     A tutorial introduction to Git (for version 1.5.1 or newer)
   workflows    An overview of recommended workflows with Git

We can use git help <command> or git help <concept> command to know more about about a specific command or concept.

Git Configuration


DescriptionGit Command
Configure the author name to be used with your commits.git config --global user.name "Sam Smith"
Configure the author email address to be used with your commitsgit config --global user.email sam@example.com
Will remove user credential details from the repositorygit config --local credential.helper ""
List all currently configured remote repository URLsgit remote -v
If you haven't connected your local repository to a remote server, To add a remote server to a local repositorygit remote add origin <repo_url>


Git Commit and Push


DescriptionGit Command
Create a file name README.md with Readme content contentecho "Readme content" >> README.md
List the files you've changed and those you still need to add or commitgit status
Add all or one file to staginggit add . OR git add file_name
Commit changes to head with messagegit commit -m 'message'
Commit any files you've added with git add, and also commit any files you've changed since thengit commit -a
Send all commits from local repository to remote repositorygit push
Do a git push and sets the default remote branch for the current local branch. So any future git pull command will attempt to bring in commits from the <remote-branch>into the current local branchgit push -u <remote-branch>
Send changes to the master branch of your remote repositorygit push origin master
Push a specific branch to your remote repositorygit push origin <branch_name>
Push all branches to your remote repositorygit push --all origin


Git Checkout And Pull


DescriptionGit Command
To create a new local repositorygit init
Clone a repository into a new directorygit clone repo_url
Clone a repository into a new directory and point to mentioned branch_namegit clone --branch branch_name repo_url
To create a working copy of a local repositorygit clone /path/to/repository
Download objects and refs from remote repository for master branchgit fetch origin master
To merge a different branch into your active branchgit merge <branch_name>
Fetch and merge changes on the remote server to your working directory:git pull
View all the merge conflicts, View the conflicts against the base file, Preview changes, before merginggit diff, git diff --base <filename>, git diff <sourcebranch> <targetbranch>


Git Branch


DescriptionGit Command
List all the branches in your repository, and also tell you what branch you're currently ingit branch
Switch from one branch to anothergit checkout branch_name
Create a new branch and switch to itgit checkout -b branch_name
Create a new branch from master branch and switch to itgit checkout -b branch_name master
Delete the feature branch from local repositorygit branch -d <branch_name>
Delete a branch on your remote repositorygit push origin :<branch_name>


Git Cleaning


DescriptionGit Command
Fetch the latest history (objects & refs) from the remote server for master branchgit fetch origin master
Clean repository to initial stagegit clean -x -d -f
Reset local repository and point your local master branch to latest history fetched from remote servergit reset --hard origin/master
To bring all changes from remote repository to local repositorygit pull origin master


Other Git commands


DescriptionGit Command
You can use tagging to mark a significant change set, such as a releasegit tag 1.0.0 <commitID>
Commit Id is the leading characters of the change set ID, up to 10, but must be unique. Get the ID usinggit log
Push all tags to remote repositorygit push --tags origin
If you mess up, you can replace the changes in your working tree with the last content in head:Changes already added to the index, as well as new files, will be keptgit checkout -- <filename>
Search the working directory for foo()git grep "foo()"

I keep these commands as notes for my future reference and you can find more on this Github Repository and please feel free to provide your valuable feedback.
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