I have retrived some datas from DB and I have stored it in an ArrayList. The ArrayList contains some 50 rows returned each row containin 4 columns. How do I access a particular column of a particular object in ArrayList? Can someone help me with this?
Not sure what is the exact issue here. List is based on the index and hence you can access any data based on index. Another option is to convert use Map which allows you to refer to the data based on a key you desire.
Due to new data posted in comments on the question, this is now know to not be what OP wants. I'd delete it but, given what I've read from him/her so far, I'm afraid he/she may be forever flummoxed by the disappearance of an answer.
ORIGINAL ANSWER
If you really have an ArrayList and not a ResultSet then do this
myList.get( desiredRow*column_width /*4*/ + desiredCol);
This assumes row-major ordering.
Related
So I've found questions similar to this one, but none that have helped me with my problem. So I have an ArrayList< ArrayList < String > >. This basically creates a table of user inputs, so you can add columns and each column can have different amounts within them. I need to cycle through the combinations that can be created without comparing objects in the same column. Ideally I could send it through a nested for loop and access each element using an if statement to separate as needed, but since it is a dynamic size I haven't been able to find a way to do this that doesn't compare within the same column as well. Thank you in advance for your help.
If I'm understanding your problem correctly, it sounds like you have a List of Lists, where the first List is kind of like a key, where each slot is a list of the data you need. I ran into a very similar problem, and I was able to use a Map to hold the values. If order matters, then you'll want to use a TreeMap.
I mention the Maps, because you mention you want to manipulate (what sounds like the rows in a table), rather than the columns. If you use a TreeMap, then the keys stay in the same order, and the value for each key will be like the rows in the table. Then, the index in each List would be the column.
Without a solid example of your data, I'm not able to really go into how to compare the "combinations", which I assume can be handled by the Lists in the values of the Map, in this situation.
I have a SQLite database. Now I try to create a function, that fetches the entire table content and returns it as an Array or ArrayList or whatever.
How do I do that the best way?
Can I directly convert the cursor into an array of strings and add that to an arraylist? If so, how?
Should I create a map for each row the cursor traverses through, and add the strings into that, with their keys? I tried that, but it didn't work.
I haven't programmed anything in Java for many years.
Thanks for any help.
As given in the below code I am inserting data in table by simple hibernate code.
But when all the fields are then also it is saving data in table but only id is changed
(auto-incremented). I want to know is there any way to know that the same data exists in table & I can save redundant inserts. Please tell an easy way I know I can write queries anytime to figure out the same.
List<Route> listRoute = newList();
listRoute.get(0).setSource("DelhiTest");
listRoute.get(0).setDestination("KotaTest");
listRoute.get(1).setSource("DelhiTest");
listRoute.get(1).setDestination("KotaTest");
listRoute.get(2).setSource("DelhiTest");
listRoute.get(2).setDestination("KotaTest");
RowReferenceByEntity.setListRoute(listRoute);
new RouteDAOImpl().saveOnly(listRoute.get(0));
new RouteDAOImpl().saveOnly(listRoute.get(1));
new RouteDAOImpl().saveOnly(listRoute.get(2));
Thanks
Searching for duplicate rows already persisted can lead to performance issues. For each new item you ill need to search a match in that table.
To guarantee routes are unique you can create a unique index covering source and destination ids. Any attempt to duplicate data ill throw an exception. and this ill work fast.
But if all you want is to just find duplicate items in a list look at this post.
How to find duplicate items in list
This query counts how many ids have the same value:
SELECT COUNT(id), value
FROM table
GROUP BY value
I have the MYSQL database in the local machine where I'm running the java program from.
I plan create a array list of all the entries of a particular table. From this point on wards I will not access the database to get a particular entry in the table, instead I will use the array list created. Is this going to be faster or slower compared to accessing the database to grab a particular entry in the table?
Please note that the table I'm interested has about 2 million entries.
Thank you.
More info : I need only two fields. 1 of type Long and 1 of type String. The index of the table is Long , not int.
No, it's going to be much slower, because to find an element in an ArrayList, you've to scan sequentially the ArrayList until your element is found.
It can be faster, for a few hundreds entry, because you don't have the connection overhead, but with two millions entry, MySQL is going to win, provided that you create the correct indexes. Only retrieve the rows that you actually need each time.
Why are you thinking to do this? Are you experiencing slow queries?
To find out, in your my.cnf activate the slow query log, by uncommenting (or adding) the following lines.
# Here you can see queries with especially long duration
log_slow_queries = /var/log/mysql/mysql-slow.log
long_query_time = 1
Then see which queries take a long time, and run them with EXPLAIN in front, consider to add index where the explain command tells you that is not using indexes, or just post a new question with your CREATE TABLE statement and your example query to optimize.
This question is too vague, and can easily go either way depending on:
How many fields in each record, how big are the fields?
What kind of access are you going to perform? Text search? Sequential?
For example, if each records consists of a couple bytes of data it's much faster to store them all in-memory (not necessarily an ArrayList though). You may want to put them into a TreeSet for example.
It depends on what you will do with the data. If you just wanted a few rows, only those should be fetched from the DB. If you know that you need ALL the data, go ahead and load the whole table into java if it can fit in memory. What will you do with it after? Sequencial or random reading? Will data be changed? A Map or Set could be a faster alternative depending on how the collection will be used.
Whether it is faster or slower is measurable. Time it. It is definitely faster to work with structures stored in memory than it is to work with data tables located on the disk. That is if you have enough memory and if you do not have 20 users running the same process at the same time.
How do you access the data? Do you have an integer index?
First, accessing an array list is much much faster than accessing a data base. Accessing memory is much more faster than accessing a hard disk.
If the number of entries in the array is big and I guess it is, then you need to consider using a "direct access" data structure such as a HashMap which will act as a database table where you have values referenced by their keys
I have a piece of code from an old project.
The logic (in a high level) is as follows:
The user sends a series of {id,Xi} where id is the primary key of the object in the database.
The aim is that the database is updated but the series of Xi values is always unique.
I.e. if the user sends {1,X1} and in the database we have {1,X2},{2,X1} the input should be rejected otherwise we end up with duplicates i.e. {1,X1},{2,X1} i.e. we have X1 twice in different rows.
In lower level the user sends a series of custom objects that encapsulate this information.
Currently the implementation for this uses "brute-force" i.e. continuous for-loops over input and jdbc resultset to ensure uniqueness.
I do not like this approach and moreover the actual implementation has subtle bugs but this is another story.
I am searching for a better approach, both in terms of coding and performance.
What I was thinking is the following:
Create a Set from the user's input list. If the Set has different size than list, then user's input has duplicates.Stop there.
Load data from jdbc.
Create a HashMap<Long,String> with the user's input. The key is the primary key.
Loop over result set. If HashMap does not contain a key with the same value as ResultSet's row id then add it to HashMap
In the end get HashMap's values as a List.If it contains duplicates reject input.
This is the algorithm I came up.
Is there a better approach than this? (I assume that I am not erroneous on the algorithm it self)
Purely from performance point of view , why not let the database figure out that there are duplicates ( like {1,X1},{2,X1} ) ? Have a unique constraint in place in the table and then when the update statement fails by throwing the exception , catch it and deal with what you would want to do under these input conditions. You may also want to run this as a single transaction just if you need to rollback any partial updates. Ofcourse this is assuming that you dont have any other business rules driving the updates that you havent mentioned here.
With your algorithm , you are spending too much time iterating over HashMaps and Lists to remove duplicates IMHO.
Since you can't change the database, as stated in the comments. I would probably extend out your Set idea. Create a HashMap<Long, String> and put all of the items from the database in it, then also create a HashSet<String> with all of the values from your database in it.
Then as you go through the user input, check the key against the hashmap and see if the values are the same, if they are, then great you don't have to do anything because that exact input is already in your database.
If they aren't the same then check the value against the HashSet to see if it already exists. If it does then you have a duplicate.
Should perform much better than a loop.
Edit:
For multiple updates perform all of the updates on the HashMap created from your database then once again check the Map's value set to see if its' size is different from the key set.
There might be a better way to do this, but this is the best I got.
I'd opt for a database-side solution. Assuming a table with the columns id and value, you should make a list with all the "values", and use the following SQL:
select count(*) from tbl where value in (:values);
binding the :values parameter to the list of values however is appropriate for your environment. (Trivial when using Spring JDBC and a database that supports the in operator, less so for lesser setups. As a last resort you can generate the SQL dynamically.) You will get a result set with one row and one column of a numeric type. If it's 0, you can then insert the new data; if it's 1, report a constraint violation. (If it's anything else you have a whole new problem.)
If you need to check for every item in the user input, change the query to:
select value from tbl where value in (:values)
store the result in a set (called e.g. duplicates), and then loop over the user input items and check whether the value of the current item is in duplicates.
This should perform better than snarfing the entire dataset into memory.