– Earth is a software application where human and other thing is software objects.For creating object we need to create a human class. Humans have basic behaviors as well as some basic attributes age height inches etc.
Java variables, Array, 2d array Method Different method invoking and return type Class
– Class is actually a blueprint/specification/design
– A class can create as many as objects it needs to create Object
– Earth is a software application where human and other thing is software objects.For creating object we need to create a human class
– Objects do particular behavior in a class
– Object exist only in application runtime actually when application is running
– Objects have identity,state and behavior
– this keyword – it represent current object Instance Constructor – The costructor is a special method with the same name as the class and it’s used to instantiate objects from that class
– Every class has a by default constructor if we don’t put it manually
– The constructor is special method containing instructions for object creation. it can be said it is so called birth method. It has instructions for how the object will be born when the application starts up Static keyword Stack and heap memory Garbage collection Object oriented Programming = better organizing of code, it is a way to organize our code from multiple files by creating objects Local variable Reference variable Instance variable Inheritance
– Inheritance is the approach to get/transfer behaviour of one class to another
– We use extends keyword for inheriting, in inheritance we inherit from parent class which known as base class and the class which needs inheritation is known as child class or sub class/derived class
– super keyword it extract constructor and it’s variable from parent class but reverse is not possible Overrides
– Same method name with different purpose
– Same method name in parent class but in child class the method name is same but with different purpose
– Overrides means replace
– Override is not a good idea sometimes so we go for interface Interfaces
– Interface is a contract with a class. The class needs to implements that method of the interface according to contract
– Interface has a method which does not need body, so it is just only to define, it is called as abstract method
– one parent class
– We use implements keyword while putting interface in a class Abstract class
– When you don’t need all methods to invoke you can use abstract method, you can not make object from abstract class. Abstract method needs to be in abstract class. We use abstract keyword for both method and object to define abstract class and methods
– you can only extend abstract class but can not instantiate abstract class
– We can define abstract class as a type while creating object
Loops Exception Handling
Collection Framework Wrapper class Threading Concurrency Database
Jshell Functional Interfaces Lambda expressions
-Lambda helps us to use separate from associated object
– it can run without class
– We need to use lot of interfaces for this
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Italic kora jaygagula motamuti clear asey In Sha Allah, Alhamdulillah
The source is : Imtiaz Ahmed’s complete Java course
Master Object Oriented Design in Java course note
Association: It defines relationship between classes, it defines how the software will behave
Dependency Association: For example from code:
A driver receives a vehicle reference only as driver then only he can drive or accelerate
Composition: A particular object . It imply ownership
for example in this code:
Aggregation Association: It does not imply ownership
Here Department to SpanishCourse relation is composite
But Course to Student relationship is not composite because it does not imply ownership a student can take many courses or not
We find this relationship from above diagram, the block diamond and white diamond is the part of UML diagram
Overview of software design:
For example: Hospital Employee Management System Task of the software:
Hire and Terminate Employees
Print Reports(XML,CSV Formats)
Current status of the software
Tips for design:
– You must draw design in paper or whiteboard don’t go to code directly
– Don’t overdesign or overdraw
– We have to develop iteratively but early steps should be kept simple
– Class names should be nouns based on their intention
Single Responsibility Priniciple(SRP): Not all the things in the same class
Don’t repeat yourself(DRY):
Arrow sign always describe that it is depends on that dependency class:
DAO=Data Access Object
Single Responsibility Principle Special Notes:
A class should have only one single reason to change
Open closed principle sayings about class design:
Classes should be open for extension but closed for modification.
Open Closed Principle and Strategy pattern:
Software modules should be open for extension and closed for modification
In development lifecycle software requirements needs to be constantly pour in. If developed need to change the already made codes the design is fragile. Fragile means broken.
So open closed principle save engineers from fragile design
instanceof operator we used here in the code:
System.out.println("cleaning patient Area");
Here it is like a junk drawer
So we will make it changes because it is violating OCP principle. This chamging process is known as Strategy pattern. So let’s do it.
We could also do it with interface class except abstract class
Example after applying Strategy pattern:
We depend on something in driving like car is stopping by traffic light
Imagine a life every car has different mechanism
Violation of Dependency Inversion Principle
When higher level modules depend on lower level ones succeptible to change Dangers of coupling: Tight coupling :
Well design software has single responsibility
This method says high level modules should not depend on lowlevel modules. It should depend on abstractions
Abstract classes and Interfaces don’t change as often as concrete classes/ derivatives
Cohesion means related things together.
This code I tried to run in class inside class but did not worked. Then I downloaded course code and run worked then again I tried to do with my directory structure then it worked.
Quizes from OOP course:
Q2: The Dependency Inversion Principle states
Interfaces and abstract classes change far less often the concrete derivatives. Concrete classes should depend on Interfaces and abstract classes because they change less often and are higher level modules
Q3: In general the dependency inversion principle states that code should only depend on things that don’t change often
True Liskov Substitution Principle: Subtypes must be substitutable for their basetypes
From Quiz: Main Kotha about Interface Segregation Principle:
This principle does not have to do with the number of dependencies that may exist in an application. The principle simply states that modules should not have dependencies on code they do not use.
If a module does not use functionality from another module, there is no reason to have a direct dependency between them. There should be an abstraction in between to segregate the 2 modules
When the application is not running there is no object because onbject only available in runtime, you will find only some classes
A very popular dependency injection implementation is spring framework bundled with spring container.
Dependency injection is
Dependency Injection Using Spring: Observer Pattern: The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
One basic UML diagram from wikipedia:
Created one for the course:
When there is so many constructors and tough to handle it. Example codes with some bugs. Will do the solution later.
User websiteUser=newUser.Builder("bobMax","[email protected]").firstName("bob").lastName("max").build();
The books which you have mentioned are a very good background when it comes to Java language. The problem with being a “backend developer” is that usually even if you don’t work on frontend you are still expected to know basics of frontend stuff.
When I’m interviewing a backend developer or rather web developer with focus on backend, I expect him to know:
Framework which we use which is Spring 3-4
Database: SQL and JPQL
Knowledge of HTML
Some familiarity with CSS
Knowledge how web works: Get and post requests, rest services, json, sessions
When it comes to frameworks most popular one are Spring and Java EE6, of course there are much more but if company is using something less commonly known they less expect you to know it.
Basically if you can write from scratch a simple to-do list application all by yourself, you are ok to be junior level web dev.
First, Try To Understand the Problem Statement. Second, Solve Code with Pen and Paper. Third, Then Write code and submit in the OJ to justify test cases. Fourth, If failed to AC then optimize your code to the better version. Fifth, After failed in 3rd time see my solution. Understnad the logic and implement by your own.
Please, don’t just copy-paste the code. It will kill your creativity 🙂