BE Study 12th

Why use VO?

there are some objects that are suitable for VO. For example, there is an 'application form'. If there are two different application form that contains exactly the same content. Should we accept both? It would be ridiculous. We should regard it a duplication.


JPA


JPA is a essential library that enables spring to manipulate the DB.

There's a critical concept called "entity manager".

Entity manager is an object offerred by JPA that enables the management of entities.


Dirty Checking

You don't have to actually em.persist. or em.update. When em is flush()ed, it makes an snapshot of the entities that are enrolled in the persistence context; thus the snapshot would be same with the current state of the DB. The next time when ex is flush()ed, the em compares the snapshot and the current state of the entities. Then it creates an UPDATE query which would make the snapshot same with the current state. Then it pushes the UPDATE query into the store query.

1. directly calls flush()

2. JPQL Query

3. Transaction committed

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Interface of Java

 You can define(declare + initialize) variable like interface_name variable_name = new class_name() In such cases, the variable can only use method overriding the interfaces methods. Also, the language does not recognized it as the class; the language recognizes it as interface. Still, it does have member variables of the class and there are some libraries that enables the programmer to tell what class it is constructed(i.e. Reflection) Interface can be used as parameter of functions or return types of functions. When interface is a parameter, the class that implements the interface should be given to the function. When interface is a return type, it can be used for polymorphism. Also, variable of java(in case its not primitive type like int, float), it is rather closer to pointer of C++; it refers to the value, not contains value itself. The important point is that, when the class implements the interface, all of its public methods should be the interface's methods. If not, client...
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Data Analytics Overview(OLTP vs OLAP)

You’ve likely heard about the lucrative careers of high-level Data Analysts, but have you ever wondered how they actually handle "Big Data"? Of course, they aren’t just using Microsoft Excel. To understand the technology behind it, we must first distinguish between the two primary ways we process information:  OLTP  and  OLAP . 1. OLTP OLTP (Online Transactional Processing)  is the DB used for company’s daily operations. Whenever you post on Reddit or place an order on Amazon, you are interacting with an OLTP database. It is triggered by the business's customer and handled by the backend server so that it can provide the business. Fast:  It handles millions of requests with millisecond response times. Small&Frequent:  It focuses on small, frequent transactions—usually just a few rows at a time. Write-intensive:  Its primary job is to record or update new data as it happens. 2. OLAP OLAP (Online Analytical Processing)  is the very technology we...
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Leetcode

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1. Two Sum: find a pair with specific sum   class Solution : def twoSum ( self , nums : List [ int ] , target : int ) - > List [ int ] : numMap = { } n = len ( nums ) # Build the hash table for i in range ( n ) : numMap [ nums [ i ] ] = i # Find the complement for i in range ( n ) : complement = target - nums [ i ] if complement in numMap and numMap [ complement ] != i : return [ i , numMap [ complement ] ] return [ ] # No solution found You might wonder how does the code deals with duplicates. As you can see, in the iteration of building the hash table, always the latter(with bigger index) will be saved in the hash table. In the latter iteration finding the complement, the former one(with smaller index) always goes through the iteration earlier, so the [i, numMap[complement]] can return two different indexes, carrying duplicate...
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