Week 7 - Java Memory & GIT - Java Memory Management, JDK, JRE, JVM, Compilation, DevOps, Command Line and GIT intro

Question 1

A Java Development Kit (JDK) is used

Answer 1

used to create Java code

Question 2

The Java code is then compiled into bytecode for a particular

Answer 2

Java Runtime Environment (JRE)

Question 3

Since every machine can implement its own JREs and JVMs, Java source code does not have to be rewritten to be machine-specific. This is known as

Answer 3

Write Once, Run Anywhere (WORA)

Question 4

describe the terms stack

Answer 4

The stack is a region of memory that is used for storing local variables, function arguments, and return addresses during the execution of a program. The stack is a last-in, first-out (LIFO) data structure, which means that the most recently added item is the first one to be removed. The stack is managed automatically by the program’s runtime system, and memory is automatically allocated and deallocated as needed. Because the stack is limited in size, it is typically used for storing data that is of fixed size and is short-lived.

Stack Memory in Java is used for static memory allocation and the execution of a thread. It contains primitive values that are specific to a method and references to objects referred from the method that are in a heap.
Access to this memory is in Last-In-First-Out (LIFO) order. Whenever we call a new method, a new block is created on top of the stack which contains values specific to that method, like primitive variables and references to objects.
When the method finishes execution, its corresponding stack frame is flushed, the flow goes back to the calling method, and space becomes available for the next method.
Key Features of Stack Memory

It grows and shrinks as new methods are called and returned, respectively.
Variables inside the stack exist only as long as the method that created them is running.
It’s automatically allocated and deallocated when the method finishes execution.
If this memory is full, Java throws java.lang.StackOverFlowError.
Access to this memory is fast when compared to heap memory.
This memory is threadsafe, as each thread operates in its own stack

Question 5

Describe Heap space in java

Answer 5

The heap, on the other hand, is a region of memory that is used for storing data that is allocated dynamically during the execution of a program. The heap is managed explicitly by the program, which means that the program is responsible for allocating and deallocating memory as needed. Data stored on the heap can be of variable size and can persist for the lifetime of the program. Because the heap is not limited in size, it is typically used for storing data that is of variable size and is long-lived. However, because the program is responsible for managing the heap, it is also more prone to memory leaks and other errors if not managed properly.

Introduction
Heap space is used for the dynamic memory allocation of Java objects and JRE classes at runtime. New objects are always created in heap space, and the references to these objects are stored in stack memory.
These objects have global access and we can access them from anywhere in the application.
We can break this memory model down into smaller parts, called generations, which are:
Young Generation – this is where all new objects are allocated and aged. A minor Garbage collection occurs when this fills up.
Old or Tenured Generation – this is where long surviving objects are stored. When objects are stored in the Young Generation, a threshold for the object’s age is set, and when that threshold is reached, the object is moved to the old generation.
Permanent Generation – this consists of JVM metadata for the runtime classes and application methods.
Key Features of Java Heap Memory
Some other features of heap space include:
It’s accessed via complex memory management techniques that include the Young Generation, Old or Tenured Generation, and Permanent Generation.
If heap space is full, Java throws java.lang.OutOfMemoryError.
Access to this memory is comparatively slower than stack memory
This memory, in contrast to stack, isn’t automatically deallocated. It needs Garbage Collector to free up unused objects so as to keep the efficiency of the memory usage.
Unlike stack, a heap isn’t threadsafe and needs to be guarded by properly synchronizing the code.

Question 6

Access to stack memory can best be described as…?

Answer 6

LIFO; Last In First Out.

Question 7

When a method is finished executing, which of the following does <b>not</b> happen with the stack?

The flow goes back to the calling method.
Its corresponding stack frame is flushed.
The memory allocated for the stack is lost until the machine is reset.
Space becomes available for the next method.

Answer 7

The memory allocated for the stack is lost until the machine is reset.

Question 8

Which of the following is <b>not</b> true about heap space?

Heap space is used for the dynamic memory allocation of Java objects and JRE classes at runtime.
New objects are always created in heap space.
References to objects in heap space are stored in stack memory.
Heap space uses static memory allocation.

Answer 8

Heap space uses static memory allocation.

Question 9

Which is <b>not</b> a feature of heap memory?

If heap space is full, Java throws java.lang.OutOfMemoryError.
This memory, in contrast to stack, isn’t automatically deallocated.
Unlike stack, a heap isn’t threadsafe and needs to be guarded by properly synchronizing the code..
Access to this memory is faster compared to stack memory

Answer 9

Access to this memory is faster compared to stack memory.

Question 10

Define and describe how garbage collection works.

Answer 10

Garbage collection is the process of removing objects from the heap which have no references to them.
Java abstracts the details away from the developer by allowing the JVM to handle memory management itself.
Garbage collection is run in the background by the JVM. There is no way we can explicitly force garbage collection to happen, but we can request garbage collection to be run through the use of one of the following:
System.gc()
Runtime.getRuntime().gc()
System.runFinalize()

Question 11

Garbage collection can be forced by

Answer 11

It can never be forced.

Question 12

Variable references are stored in the Stack. T/F?

Answer 12

True

Question 13

What is the easiest way to immediately force Java garbage collection?

Answer 13

You cannot force Java garbage collection

Question 14

A Java programming language developer must write memory management into their programs. T/F?

Answer 14

False

Question 15

Which is NOT a consideration when selecting an operating system to use for development?

Marketing of the final product
Speed
Platforms for the final product (local machine, cloud, etc)
Learning curve

Answer 15

Marketing of the final product

Question 16

gitbash: ls

Answer 16

This command lists directory contents.
It lists files and directories.
Some versions may support color-coding. The names in blue represent the names of directories.

Question 17

$ ls -l | more

Answer 17

This command helps paginate the output so you can view page by page. Otherwise the listing scrolls down rapidly. You can always use ctrl + c to return to the command line.
Note: more is not supported in GitBash for Windows.

Question 18

$ ls –I

Answer 18

This command shows more details of the contents in the directory. It lists the following:

Permissions associated with the fileThe owner of the file
The group associated with the file
The size of the file
The timestamp
The name of the file

Question 19

cd

Answer 19

This changes the current directory. Note that it uses a forward slash.
$ cd /var/log

Question 20

pwd

Answer 20

One way to identify the directory you are working in is the pwd command. It displays the current working directory path and is useful when directory changes are made frequently.

$ pwd

Question 21

$ mkdir myproject

Answer 21

The mkdir command makes a directory. The command is written as follows: mkdir [directory name]

Question 22

$ cat > newfilename

Answer 22

The cat command can be used to create, view, and concatenate files. The example below creates a new file called “newfilename”.

Question 23

$ cat sourcefilename > destinationfilename

Answer 23

Another way to create a file using cat:
The cat example below will copy the file “sourcefilename” into the file “destinationfilename”.

Question 24

$ touch filename

Answer 24

The touch command creates an empty file for editing later. The command below creates an empty file called “filename”. From there you can use a terminal-based editor like “vi” to edit the file.

Question 25

$ echo “Some line” > file1.txt

Answer 25

The echo command prints the strings that are passed as arguments to the standard output, which can be redirected to a file. To create a new file run the echo command followed by the text you want to print and use the redirection operator > (as explained with cat above) to write the output to the file you want to create.
The command below will create the file “file.txt” containing the text “Some line”.

Question 26

$ grep Aaron filename

Answer 26

The grep command can be used to search files and directories (and subdirectories) for a string. The example below searches the file “filename” for the string “Aaron”.

Question 27

$ diff file1.txt file2.txt

Answer 27

The diffcommand compares two files line by line to find differences. The output will be the lines that are different.

Question 28

mv

Answer 28

The mv command moves a file or renames it. Some examples; inline comments denoted with //

Question 29

$ mv file1 directory1

Answer 29

moves ‘file1’ to ‘directory1’

Question 30

$ mv file1 file2 file3 dir1

Answer 30

moves ‘file1’, ‘file2’, and ‘file3’ to ‘dir1’

Question 31

$ mv file1 file2

Answer 31

renames ‘file1’ with the new name ‘file2’. This can also be used to rename directories.

Question 32

$ mv -i file1 directory1

Answer 32

just as above, this will move ‘file1’ into ‘directory1’ but the -i flag will prompt the operator should the command result in overwriting an existing file

Question 33

$ mv -n file1 directory1

Answer 33

similar to the above, however the -n flag will not move ‘file1’ to ‘directory1’ if it causes an overwrite

Question 34

$ mv -u file1 directory1

Answer 34

The -u flag will only move ‘file1’ to ‘directory1’ if the source file is newer than the destination file

Question 35

$ mv -b file1 directory1

Answer 35

The -b flag will create a backup of any existing destination file overwritten by ‘file1’.

Question 36

cp

Answer 36

The cp command copies a file. Some examples; inline comments denoted with //

Question 37

$ cp second.txt third.txt

Answer 37

copies ‘second.txt’ into a file ‘third.txt’

Question 38

$ cp -i second.txt third.txt

Answer 38

-i stands for Interactive copying. With this option system first warns the user before overwriting the destination file

Question 39

$ cp -b second.txt third.txt

Answer 39

-b will create a backup of the destination file in the same folder

Question 40

$ cp -f second.txt third.txt

Answer 40

-f stands for force; if the system cannot open the destination file then the destination file is deleted first before the copying proceeds

Question 41

$ cp -r directory1 directory2

Answer 41

-r is for Recursive copying which is used for copying the contents of ‘directory1’, including all subdirectories, into ‘directory2’

Question 42

$ cp -p second.txt third.txt

Answer 42

-p stands for preserve. The command preserves some characteristics of ‘second.txt’ in the destination file ‘third.txt’ including times of last modification and access, ownership, and file permissions

Question 43

rm

Answer 43

The rm command deletes files/directories. Some examples; inline comments denoted with //

Question 44

$ rm file1

Answer 44

deletes ‘file1’ in the current working directory.

Question 45

$ rm -i file1

Answer 45

-i stands for interactive; this prompts the operator before deleting each file

Question 46

$ rm -f file1

Answer 46

-f forces deletion if a file is write protected. It will not remove a write-protected directory

Question 47

$ rm -r directory1

Answer 47

-r (recursive) will delete all files in ‘directory1’ and all subdirectories of ‘directory1’ and their contents

Question 48

what is git?

Answer 48

Git is a distributed version control system used for tracking changes in source code during software development. Git is designed to be fast and efficient, and it can handle projects of any size, from small personal projects to large enterprise software development.

With Git, developers can create a local repository on their computer to work on code changes and track those changes over time. They can then push those changes to a remote repository, which can be shared with other developers or integrated into a larger project. Git uses a branching and merging system that allows multiple developers to work on different features or parts of a project simultaneously, and then merge their changes together when they’re ready.

Git also provides features such as version history, blame (to see who made changes to specific lines of code), and diff (to see differences between different versions of a file). It is a command-line tool, but there are also many GUI clients available for those who prefer a visual interface.

Question 49

What is version control?

Answer 49

Version control, also known as source control, is the practice of tracking and managing changes to software code. Version control systems (VCS) are software tools that help software teams manage changes to source code over time. As development environments have accelerated, version control systems help software teams work faster and smarter.

Version control software keeps track of every modification to the code in a special kind of database. If a mistake is made, developers can turn back the clock and compare earlier versions of the code to help fix the mistake while minimizing disruption to all team members.

Question 50

Git has three main states in that your files can reside:

Answer 50

modified, staged, and committed:

Question 51

Modified

Answer 51

means that you have changed the file but have not committed it to your database yet

Question 52

Staged

Answer 52

means that you have marked a modified file in its current version to go into your next commit snapshot.

Question 53

Committed

Answer 53

means that the data is safely stored in your local database.

Question 54

What is Git Bash?

Answer 54

Git Bash is an application for Microsoft Windows environments which provides an emulation layer for a Git command line experience. Bash is an acronym for Bourne Again Shell. A shell is a terminal application used to interface with an operating system through written commands. Bash is a popular default shell on Linux and macOS. Git Bash is a package that installs Bash, some common bash utilities, and Git on a Windows operating system.

Question 55

The basic Git workflow goes something like this:

Answer 55

You modify files in your working directory.

You selectively stage just those changes you want to be part of your next commit, which adds only those changes to the staging area. (See “git add” in the diagram below)

You commit the changes, which takes the files as they are in the staging area and stores that snapshot permanently to your Git directory. (See “git commit” and “git push” in the diagram below)

Question 56

What is a Git Repository?

Answer 56

A Git repository tracks and saves the history of all changes made to the files in a Git project. It saves this data in a directory called .git, also known as the repository folder.

Question 57

Initialize a Repository
git

Answer 57

To initialize a Git repository in an existing directory, start by using the Git Bash terminal window to go to your project’s directory:

cd [directory path]
Where [directory path]: The path to your project directory.

Question 58

Once you navigate to the project directory, initialize a Git repository by using:

Answer 58

git init

Question 59

git clone [url] [directory]

Answer 59

Use the git clone command to clone an existing repository and copy it to your system:

Question 60

$ git add .

Answer 60

Use git add . in your bash to add all the files to the given folder.

Question 61

$ git status

Answer 61

Use git status in your bash to view all the files which are going to be staged to the first commit.

Question 62

git commit -m ‘your message”

Answer 62

You can create a commit message by git commit -m ‘your message’, which adds the change to the local repository.
git commit uses ‘-m’ as a flag for a message to set the commits with the content where the full description is included. It is recommended that you write in double-quotes an imperative sentence in 50 characters that describes what changed and why. For example, “Corrected println error in helloworld.java”

Question 63

git push

Answer 63

git push is used for pushing local content to GitHub.

Question 64

What is the git command for downloading files from a remote repository?

Answer 64

git pull

Question 65

What is the git command for integrating changes?

Answer 65

git merge

Question 66

git branch

Answer 66

list all current branches

Question 67

git branch branch_name

Answer 67

create new branch from current

Question 68

git checkout branch_name

Answer 68

checkout another branch

Question 69

git checkout -b branch_name

Answer 69

checkout branch and create it if branch doesnt already exist

Question 70

git merge branch_name

Answer 70

merge branch_name to the current branch you are in

Question 71

git add file_name

Answer 71

add specific file to staging

Question 72

git remote add origin url_to_remote_repo

Answer 72

connect remote repository origin to local repository

Question 73

git log

Answer 73

display all local commit IDs

Question 74

git clone remote_repo_url

Answer 74

retrieve code base from remote repository to your local machine for the first time