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Link an integer to two people [closed]

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I'm currently coding a roleplay bot that should save relationships between two people as an integer.
The problem is that when I use a define normal integer: int relationship = 0;
and add something to it as soon as someone uses a specific command:
if (event.getMessage().getContentStripped().startsWith("r!kiss")) {
relationship = relationship + 3;
it adds 3 if anyone uses the command with anyone. I'd like to make the command so if userA uses the command r!kiss #userB it adds 3 to the relationship integer of userA and userB.
In a comment I got told that I would need more "structure" and I then would be able to use maps to link it to 2 people. How can I get the needed structure and how exactly can I use maps to link the int to two people then?

What is the most efficient way to use database? [closed]

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I'm new to using a database, specifically MySQL. I'm creating a web application for class in which you can look up the name of a book and it'll display the summary of the book. My question is should I send a query to the database that collects all of the books' data on initialization and put them into a HashMap inside a manager class for lookup or should I use a query each time to lookup a specific book information?

It depends on the data transport time I would say. If your average query time times the number of request goes faster than a script to put everything into a HashMap, use queries. Otherwise, use a script that collects everything and puts it into a HashMap.
But if you have thousands of rows, you should use queries, because otherwise you will use too much RAM.

How to set a restriction for each phone/user on my app? [closed]

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I'm starting to build an app where people vote certain stuff just by clicking on happy/sad faces (kind of like grading it). The thing is that I dont want to make a log-in nor registration for my app (the reason is a long story). So is there any way that I can limit people's vote to 2 per day? Every phone/user could only vote twice a day, and that count will reset after 24h from the first vote.

You have tow options:
Limit the number of votes on the device, by saving the number of votes into persistant storage on the device with a timestamp and act accordingly.
Or (more secure, but also more difficult)
Track the installations as described here identifying app installation
And send the id with every request and validate on the server side.

Storing large number of objects in list? [closed]

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I have a three million rows in a database and I need to get all the values in a table as object and operate on those objects ? what is the best possible solution ?

The best solution is to not load all of them.
Why would need to load them all and operate on them?
Maybe you can do a SP (stored procedure) and work on these rows on the DB server.
If you still need to load them all, try to not load all columns of these rows.
Maybe you can use something like paging (if that is applicable to your case).
My answer is maybe too general but so is your question.

As Peter said, don't load them all. Instead, use an iterator, like a database cursor (ResultSet for the rows in a SQL query) to keep track of your place in the data. For any more specific answer, you'll need to give more detail, but you should also consider whether you can use SQL aggregation functions (COUNT, GROUP BY, etc.) to reduce the number of rows your application needs to process.

Is it good idea to store store List<100000> Pojo objects in memory [closed]

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for some test data scenario i need to read file containing 100000 lines of row and process each row with some condition and then based on condition need to output the data in text format
for this i am planning to store all lines of data in some pojo then pojo to List
My worry is to having pojo of 100000 row in memory . this is just for testing case .

I think using InputSteam to read the file will be better since you still fetch rows one by one. You can read one line per time, and process your confition and then output.
Storing too much Objects in List may encounter an Out of Memory Error.

In any case, its a bad design to store all 100000 rows as POJO in memory. Some of the possible solutions are:
Read one row at a time and process it.
Rather than reading from a file one record at a time and processing it using java, use some scripting language to populate a database table, and then from your java code you can process the records from the table.