Single Responsibility principle

In this article you're going to master SOLID principles of object-oriented design and architecture. These principles embody more than 40 years of academic research and industry experience, passed to us from previous generations of software engineers. Therefore, by learning SOLID, you gain universal, time-proven tools to build high-quality software.

SOLID is an acronym that stands for five different principles:

• Single Responsibility Principle

• Open Closed Principle

• Liskov Substitution Principle

• Interface Segregation Principle

• Dependency Inversion Principle

In this course, you will learn about all five SOLID principles in detail and understand their importance. You will see how these principles manifest themselves in real-world software architecture and discover how they translate into actionable guidelines for writing clean and maintainable code. Speaking about code... SOLID code is flexible, extensible and readable. It is a joy to work with!

In addition to in-depth discussion of SOLID, in this course you will also find many interesting historical facts about the people behind these principles. These short historical references will allow you to see the bigger picture, and they will also make the course much more interesting and engaging for you.

So, if you're a professional software developer and you're serious about design, architecture and clean code, this course is for you!

What you’ll learn

• Discover the theory behind SOLID principles
• See common SOLID use cases
• Learn SOLID architecture practices
• Understand the scope of applicability of SOLID principles
• Discover the role of abstractions in software design
• Acquire pragmatic mindset and treat SOLID principles as tools

Are there any course requirements or prerequisites?

• Knowledge of any object-oriented language

• Developers who want to learn SOLID software architecture and write clean and maintainable applications

SRP:- Single Responsibility principle

Means the class should be only one reason to change, There can be more reason but at the pint of implementing we should keep this in mind as well.

If there is and , or in definition that may violate single responsibility principle. 

How to create thread pool executor in java

package kp;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class MyPooledThread extends Thread {

private BlockingQueue<Runnable> taskQueue = null;
private boolean isStopped = false;
CountDownLatch latch = null;

public MyPooledThread(BlockingQueue<Runnable> queue, CountDownLatch latch) {
taskQueue = queue;
this.latch = latch;
}

@Override
public void run() {
while (!isStopped()) {
try {
Runnable runnable = taskQueue.poll(5, TimeUnit.SECONDS);
if(runnable != null){
runnable.run();
doStop();
}
latch.countDown();
System.out.println("count down");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

public synchronized void doStop() {
this.isStopped = true;
//this.interrupt();
}

public synchronized boolean isStopped() {
return isStopped;
}
}

-----------------------------------------------------------------------------------------------------------------

package kp;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.BlockingQueue;

import java.util.concurrent.CountDownLatch;

public class  MyThreadPoolExecuter {

BlockingQueue<Runnable> queue = null;

List<Thread> threads = new ArrayList<Thread>();

CountDownLatch latch = null;

private boolean isStopped = false;

public MyThreadPoolExecuter(int noOfThreads, BlockingQueue<Runnable> queue, CountDownLatch latch) {

this.queue = queue;

this.latch = latch;

for (int i = 0; i < noOfThreads; i++) {

Thread t = new MyPooledThread(this.queue, latch);

threads.add(t);

t.start();

}

}

public synchronized void execute(Runnable r) {

if (this.isStopped) {

new IllegalStateException("Threadpool is stopped");

}

try {

queue.put(r);

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

public synchronized void stop() {

this.isStopped = true;

for (Thread thread : threads) {

((MyPooledThread)thread).doStop();

}

}

}

---------------------------------------------------------------------------------------------------------------------

package kp;

import java.util.concurrent.ArrayBlockingQueue;

import java.util.concurrent.CountDownLatch;

public class MyThreadPoolExecuterTestDrive {

public static void main(String[] args) {

CountDownLatch latch = new CountDownLatch(4);

MyThreadPoolExecuter executer = new MyThreadPoolExecuter(4,new ArrayBlockingQueue<Runnable>(4), latch);

executer.execute(()->{

System.out.println("I am thread 1");

});

executer.execute(()->{

System.out.println("I am thread 2");

});

executer.execute(()->{

System.out.println("I am thread 3");

});

executer.execute(()->{

System.out.println("I am thread 4");

});

executer.execute(()->{

System.out.println("I am thread 5");

});

try {

latch.await();

//executer.stop();

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

Custom ConcurrentHashMap Implementation in java

package kp;

import java.util.HashMap;
import java.util.concurrent.locks.ReentrantReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock.WriteLock;

public class MyConcurrentHashMap<K, V> extends HashMap<K, V> {

private static final long serialVersionUID = 1L;

private ReentrantReadWriteLock[] lock = new ReentrantReadWriteLock[16];

public MyConcurrentHashMap() {
super();

for (int i = 0; i < 16; i++) {
lock[i] = new ReentrantReadWriteLock();
}

}

@Override
public V get(Object key) {

ReentrantReadWriteLock lock = getLock(key);
WriteLock wl = lock.writeLock();

if (lock.isWriteLocked()) {
try {
wl.wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

lock.readLock().lock();
V value = super.get(key);
lock.readLock().unlock();
return value;
}

@Override
public V put(K key, V value) {

ReentrantReadWriteLock lock = getLock(key);
while (lock.getReadLockCount() > 0) {
try {
lock.readLock().wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
lock.writeLock().lock();
V val = super.put(key, value);
lock.writeLock().unlock();
return val;
}

private ReentrantReadWriteLock getLock(Object key) {
int hash = hashCode(key);
return lock[hash / 100];
}

private int hashCode(Object key) {

return (key == null ? 0 : key.hashCode() % 1600);
}

}

------------------------------------------------------------------------------------------------------------------

package kp;

public class MyConcurrentHashMapTestDrive {

public static void main(String[] args) {

MyConcurrentHashMap<Integer, String> myConcurrentHashMap = new MyConcurrentHashMap<Integer, String>();

WriteMap w1 = new WriteMap(myConcurrentHashMap);

WriteMap w2 = new WriteMap(myConcurrentHashMap);

ReadMap r1 = new ReadMap(myConcurrentHashMap);

w1.setName("First Thread");

w2.setName("Second Thread");

w1.start();

w2.start();

r1.start();

}

}

class WriteMap extends Thread {

MyConcurrentHashMap<Integer, String> cMap;

public WriteMap(MyConcurrentHashMap<Integer, String> cMap){

this.cMap = cMap;

}

public void run(){

int i=1;

while(true){

cMap.put(i, this.currentThread().getName());

try {

Thread.sleep(500);

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

}

class ReadMap extends Thread {

MyConcurrentHashMap<Integer, String> cMap;

public ReadMap(MyConcurrentHashMap<Integer, String> cMap){

this.cMap = cMap;

}

public void run(){

int i=1;

while(true){

System.out.println(cMap.get(i));

}

}

}

BlockingQueue Implementaion in java

package kp;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class BlockingQueueExample {

public static void main(String[] args) throws InterruptedException {
BlockingQueue<String> queue = new ArrayBlockingQueue<String>(1024);
     Producer producer = new Producer(queue);
     Consumer consumer = new Consumer(queue);
     new Thread(producer).start();
     new Thread(consumer).start();
     Thread.sleep(4000);
}
}

class Producer implements Runnable{

BlockingQueue< String> q=null;
public Producer(BlockingQueue<String> queue) {
this.q=queue;
}
public void run() {
try{
q.put("1");
Thread.sleep(1000);
q.put("2");
Thread.sleep(1000);
q.put("3");
Thread.sleep(1000);
q.put("4");
}
catch (Exception e) {
e.printStackTrace();
}
}

}

class Consumer implements Runnable {

BlockingQueue<String> queue=null;

public Consumer(BlockingQueue< String> b) {
this.queue=b;
}
public void run() {
try {
for (String string : queue) {
System.out.println(queue.take());
}
         

}
catch (Exception e) {
e.printStackTrace();
}
}
}

LRU Implementation in Java

package kp;

import java.util.LinkedHashMap;

public class LRUCache<K, V> extends LinkedHashMap<K, V> {
private static final long serialVersionUID = 1L;
private int size;

public LRUCache(int size) {
super(size, 0.75f, true);
this.size = size;
}

@Override
protected boolean removeEldestEntry(java.util.Map.Entry<K, V> paramEntry) {
return size() > size;
}

public static void main(String args[]) {

LRUCache<String, String> lruCache = new LRUCache<String, String>(7);

lruCache.put("2", "2");
lruCache.put("1", "1");
lruCache.put("3", "3");
lruCache.put("4", "4");
lruCache.put("5", "5");
lruCache.put("6", "6");
lruCache.put("7", "7");



//System.out.println("---" + lruCache.get("1"));
//System.out.println("---" + lruCache.get("2"));
//System.out.println("---" + lruCache.get("3"));

System.out.println(lruCache);
}

}

How to print hello world sequentially using thread in java.

package kp;

public class HelloWorldByWaitNotify {

public static void main(String[] args) {
Object o = new Object();
World worldObj = new World(o);
Hello hello = new Hello(worldObj);
Thread helloThread = new Thread(hello);
Thread worldThread = new Thread(worldObj);
helloThread.start();
worldThread.start();
Thread helloThread1 = new Thread(hello);
Thread worldThread1 = new Thread(worldObj);
helloThread1.start();
worldThread1.start();
}
}

class Hello implements Runnable {
World worldObj;

public Hello(World worldObj) {
this.worldObj = worldObj;
}

@Override
public void run() {
while (true) {
synchronized (worldObj) {
if (!this.worldObj.isWorld) {
System.out.println("Hello");
this.worldObj.isWorld = true;
worldObj.notify();
}
}
}
}
}

class World implements Runnable {
public boolean isWorld = false;
Object o;

public World(Object o) {
this.o = o;
}

@Override
public void run() {
while (true) {
synchronized (this) {
try {
if (!this.isWorld) {
this.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("WORLD!!!!!!");
this.isWorld = false;
}
}
}
}

What do you mean by analysis and design

What do you mean by analysis and design?

Systems Analysis. It is a process of collecting and interpreting facts, identifying the problems, and decomposition of a system into its components. ... It is a problem solving technique that improves the system and ensures that all the components of the system work efficiently to accomplish their purpose.

Analysis:

1. Basically, it is the process of determining what needs to be done before how it should be done. In order to accomplish this, the developer refers the existing systems and documents. So, simply it is an art of discovery.
Design:
It is the process of adopting/choosing the one among the many, which best accomplishes the users needs. So, simply, it is compromising mechanism.

2. What are the steps involved in designing?
Before getting into the design the designer should go through the SRS prepared by the System Analyst.
The main tasks of design are Architectural Design and Detailed Design.
In Architectural Design we find what are the main modules in the problem domain.
In Detailed Design we find what should be done within each module.

3. What are the main underlying concepts of object orientation?
Objects, messages, class, inheritance and polymorphism are the main concepts of object orientation.

4. What do u meant by "SBI" of an object?
SBI stands for State, Behavior and Identity. Since every object has the above three.

State:
It is just a value to the attribute of an object at a particular time.
Behaviour:
It describes the actions and their reactions of that object.
Identity:
An object has an identity that characterizes its own existence. The identity makes it possible to distinguish any object in an unambiguous way, and independently from its state.

5. Differentiate persistent &amp; non-persistent objects?
Persistent refers to an object's ability to transcend time or space. A persistent object stores/saves its state in a permanent storage system with out losing the information represented by the object.
A non-persistent object is said to be transient or ephemeral. By default objects are considered as non-persistent.

6. What do you meant by active and passive objects?
Active objects are one which instigate an interaction which owns a thread and they are responsible for handling control to other objects. In simple words it can be referred as client.
Passive objects are one, which passively waits for the message to be processed. It waits for another object that requires its services. In simple words it can be referred as server.

Diagram:
client server
(Active) (Passive)

7. What is meant by software development method?
Software development method describes how to model and build software systems in a reliable and reproducible way. To put it simple, methods that are used to represent ones' thinking using graphical notations.

8. What are models and meta models?
Model:
It is a complete description of something (i.e. system).
Meta model:
It describes the model elements, syntax and semantics of the notation that allows their manipulation.

9. What do you meant by static and dynamic modeling?
Static modeling is used to specify structure of the objects that exist in the problem domain. These are expressed using class, object and USECASE diagrams.
But Dynamic modeling refers representing the object interactions during runtime. It is represented by sequence, activity, collaboration and statechart diagrams.

10. How to represent the interaction between the modeling elements?
Model element is just a notation to represent (Graphically) the entities that exist in the problem domain. e.g. for modeling element is class notation, object notation etc.

Relationships are used to represent the interaction between the modeling elements.
The following are the Relationships.

Association: Its' just a semantic connection two classes.

e.g.:

Aggregation: Its' the relationship between two classes which are related in the fashion that master and slave. The master takes full rights than the slave. Since the slave works under the master. It is represented as line with diamond in the master area.
ex:
car contains wheels, etc.

car

Containment(Composition): This relationship is applied when the part contained with in the whole part, dies when the whole part dies.
It is represented as darked diamond at the whole part.
example:
class A{
//some code
};

class B
{
A aa; // an object of class A;
// some code for class B;
};
In the above example we see that an object of class A is instantiated with in the class B. so the object class A dies when the object class B dies.we can represnt it in diagram like this.

Generalization: This relationship used when we want represents a class, which captures the common states of objects of different classes. It is represented as arrow line pointed at the class, which has captured the common states.

Dependency: It is the relationship between dependent and independent classes. Any change in the independent class will affect the states of the dependent class.
DIAGRAM:
class A class B

11. Why generalization is very strong?
Even though Generalization satisfies Structural, Interface, Behaviour properties. It is mathematically very strong, as it is Antisymmetric and Transitive.
Antisymmetric: employee is a person, but not all persons are employees. Mathematically all As’ are B, but all Bs’ not A.
Transitive: A=&gt;B, B=&gt;c then A=&gt;c.
A. Salesman.
B. Employee.
C. Person.
Note: All the other relationships satisfy all the properties like Structural properties, Interface properties, Behaviour properties.

12. Differentiate Aggregation and containment(Composition)?
Aggregation is the relationship between the whole and a part. We can add/subtract some properties in the part (slave) side. It won't affect the whole part.
Best example is Car, which contains the wheels and some extra parts. Even though the parts are not there we can call it as car.
But, in the case of containment the whole part is affected when the part within that got affected. The human body is an apt example for this relationship. When the whole body dies the parts (heart etc) are died.

13. Can link and Association applied interchangeably?
No, You cannot apply the link and Association interchangeably. Since link is used represent the relationship between the two objects.
But Association is used represent the relationship between the two classes.
link :: student:Abhilash course:MCA
Association:: student course

14. what is meant by "method-wars"?
Before 1994 there were different methodologies like Rumbaugh, Booch, Jacobson, Meyer etc who followed their own notations to model the systems. The developers were in a dilemma to choose the method which best accomplishes their needs. This particular span was called as "method-wars"


These terms signify the relationships between classes. These are the building blocks of object oriented programming and very basic stuff. But still for some, these terms look like Latin and Greek. Just wanted to refresh these terms and explain in simpler terms.

Association

Association is a relationship between two objects. In other words, association defines the multiplicity between objects. You may be aware of one-to-one, one-to-many, many-to-one, many-to-many all these words define an association between objects. Aggregation is a special form of association. Composition is a special form of aggregation.

Example: A Student and a Faculty are having an association.

Aggregation

Aggregation is a special case of association. A directional association between objects. When an object ‘has-a’ another object, then you have got an aggregation between them. Direction between them specified which object contains the other object. Aggregation is also called a “Has-a” relationship.

Composition

Composition is a special case of aggregation. In a more specific manner, a restricted aggregation is called composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition.

Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.


Difference between aggregation and composition

Composition is more restrictive. When there is a composition between two objects, the composed object cannot exist without the other object. This restriction is not there in aggregation. Though one object can contain the other object, there is no condition that the composed object must exist. The existence of the composed object is entirely optional. In both aggregation and composition, direction is must. The direction specifies, which object contains the other object.

Example: A Library contains students and books. Relationship between library and student is aggregation. Relationship between library and book is composition. A student can exist without a library and therefore it is aggregation. A book cannot exist without a library and therefore its a composition. For easy understanding I am picking this example. Don’t go deeper into example and justify relationships!

Abstraction

Abstraction is specifying the framework and hiding the implementation level information. Concreteness will be built on top of the abstraction. It gives you a blueprint to follow to while implementing the details. Abstraction reduces the complexity by hiding low level details.

Example: A wire frame model of a car.

Generalization

Generalization uses a “is-a” relationship from a specialization to the generalization class. Common structure and behaviour are used from the specializtion to the generalized class. At a very broader level you can understand this as inheritance. Why I take the term inheritance is, you can relate this term very well. Generalization is also called a “Is-a” relationship.

Example: Consider there exists a class named Person. A student is a person. A faculty is a person. Therefore here the relationship between student and person, similarly faculty and person is generalization.

Realization

Realization is a relationship between the blueprint class and the object containing its respective implementation level details. This object is said to realize the blueprint class. In other words, you can understand this as the relationship between the interface and the implementing class.

Example: A particular model of a car ‘GTB Fiorano’ that implements the blueprint of a car realizes the abstraction.

Dependency

Change in structure or behaviour of a class affects the other related class, then there is a dependency between those two classes. It need not be the same vice-versa. When one class contains the other class it this happens.

Example: Relationship between shape and circle is dependency.

java spring boot tutorial

Explanation on how Spring boot will be used:

Spring Boot aims to make it easy to create Spring-powered, production-grade applications and services with minimum fuss. It takes an opinionated view of the Spring platform so that new and existing users can quickly get to the bits they need. You can use it to create stand-alone Java applications that can be started using ‘java -jar’ or more traditional WAR deployments. We also provide a command line tool that runs ‘spring scripts’.

The diagram below shows Spring Boot as a point of focus on the larger Spring ecosystem. It presents a small surface area for users to approach and extract value from the rest of Spring

How to run spring boot without a web container?

Simply start your spring boot app in a non-web environment:

new SpringApplicationBuilder()

.sources(SpringBootApp.class)

.web(false)

.run(args);

Also, you obviously should not add the spring-boot-starter-web dependency.

By default, spring boot launches a web container if it finds one in the classpath. Using web(false)ensures that it does not happen. Tomcat could be included by another dependency without your knowledge, so it's better to disable the web environment if that is your goal.

What is Spring?

Simply put, the Spring framework provides comprehensive infrastructure support for developing Java applications.

It’s packed with some nice features like Dependency Injection and out of the box modules like:

• Spring JDBC
• Spring MVC
• Spring Security
• Spring AOP
• Spring ORM
• Spring Test

These modules can drastically reduce the development time of an application.

For example, in the early days of Java web development, we needed to write a lot of boilerplate code to insert a record into a data source. But by using the JDBCTemplate of the Spring JDBC module we can reduce it to a few lines of code with only a few configurations.

What is Spring Boot?

Spring Boot is basically an extension of the Spring framework which eliminated the boilerplate configurations required for setting up a Spring application.

It takes an opinionated view of the Spring platform which paved the way for a faster and more efficient development eco-system.

Here are just a few of the features in Spring Boot:

• Opinionated ‘starter’ dependencies to simplify build and application configuration
• Embedded server to avoid complexity in application deployment
• Metrics, Helth check, and externalized configuration
• Automatic config for Spring functionality – whenever possible

Maven Dependencies

First of all, let’s look at the minimum dependencies required to create a web application using Spring:

<dependency>

    <groupId>org.springframework</groupId>

    <artifactId>spring-web</artifactId>

    <version>5.1.0.RELEASE</version>

</dependency>

<dependency>

    <groupId>org.springframework</groupId>

    <artifactId>spring-webmvc</artifactId>

    <version>5.1.0.RELEASE</version>

</dependency>

Unlike Spring, Spring Boot requires only one dependency to get a web application up and running:

<dependency>

    <groupId>org.springframework.boot</groupId>

    <artifactId>spring-boot-starter-web</artifactId>

    <version>2.0.5.RELEASE</version>

</dependency>

Spring Boot provides a number of starter dependencies for different Spring modules.Some of the most commonly used ones are:

• spring-boot-starter-data-jpa
• spring-boot-starter-security
• spring-boot-starter-test
• spring-boot-starter-web
• spring-boot-starter-thymeleaf

Application Configuration with Spring Boot:

Configuration for applications vary from one environment to another

·        You would want to connect to a different database or queues

·        You would want to connect with different services

·        You would want to configure less logging in production

·        You might want to have different custom configuration

Application Configuration with Spring Boot application.properties

Spring Boot allows you to configure your application configuration using a file named application.properties

application.properties can reside anywhere in the classpath of the application.

In application.properties, we can use the “logging.level” prefix to set logging levels.

logging.level.some.package.path=DEBUG

logging.level.some.other.package.path=ERROR

You can configure a log file by using logging.file property in application.properties. The logging here would be in addition to the logging in console.

logging.file=\path_to\logfile.log

You can also configure the port to run the server on using server.port

server.port = 9080

For complete list of options refer spring documentation - https://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html

How application configuration will point common location:

Spring Boot looks at Frameworks available on the CLASSPATH b) Existing configuration for the application. Based on these, Spring Boot provides basic configuration needed to configure the application with these frameworks. This is called Auto Configuration.

For external configuration follow the below link:

https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html

How to load external property files into Spring Boot application?

It is a standard practice that during our production deployments, our application loads property files from external locations. This helps us to change our configurations without changing our code. In this page, we will come to know how to load external property files into Spring Boot application.

By default, Spring Boot look for externalized default property file application.properties into given below predetermined locations:

In the classpath root.

In the package "/config" in classpath.

In the current directory.

In the "/config" directory of current folder.

Now lets say, your application requires externalized properties like application.properties and another property file myapp.properties. The both properties can be in the same folder or they can be in different folder. There are 3 ways of doing it.

Command line arguments

In the first approach, all you need to do is pass folder names and property names as part of command line arguments as shown below:

Terminal:

java -jar myapp.jar --spring.config.name=application,myapp --spring.config.location=classpath:/data/myapp/config,classpath:/data/myapp/external/config

In the above command, we are passing property file name as part of "--spring.config.name" variable and folder location as part of "--spring.config.location" variable.

Environment variables

In the second approach, you can configure your externalized configuration details into environment variables and your Spring Boot application will read it from your environment as shown below:

Terminal:

set SPRING_CONFIG_NAME=application,myapp

set SPRING_CONFIG_LOCATION=classpath:/data/myapp/config,classpath:/data/myapp/external/config

java -jar myapp.jar

Programatically loding configurations:

ort org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.boot.builder.SpringApplicationBuilder;

import org.springframework.context.ConfigurableApplicationContext;

import org.springframework.core.env.ConfigurableEnvironment;

@SpringBootApplication

public class SpringBootWebApplication {

    private static Logger logger = LoggerFactory.getLogger(SpringBootWebApplication.class);

public static void main(String[] args) throws Exception {

ConfigurableApplicationContext applicationContext = new SpringApplicationBuilder(SpringBootWebApplication.class).properties("spring.config.name:application,myapp",          spring.config.location:classpath:/data/myapp/config,classpath:/data/myapp/external/config").build().run(args);

 ConfigurableEnvironment environment = applicationContext.getEnvironment();

     logger.info(environment.getProperty("cmdb.resource-url"));

    }

}