Integer b = new Integer(8);
b=b+1;
System.out.println(b);

here I added 1 to b and I got 9 on the console, I could change the Integer object within the main method

You're not actually "changing" the Integer object here. What you're doing is creating a new Integer, with a value of 9, and then storing it in the b variable, throwing away the old object which had the value 8.

i.e. you're changing the variable to point to a new object, rather than modifying the object in-place.

Modifying / mutating the original object would look something like this - it's not something valid which you can do:

Integer b = new Integer(8);
b.increaseBy(1);
System.out.println(b);

The difference is subtle, but it also explains why the code in 'main' couldn't "see" the changes made in the 'add' method.

I also used valueOf and parseInt methods, valueOf method returns an Integer object but I assigned an int primitive type to it and it worked. Similarly, I declared an Integer object and assigned it to the result of parseInt method which returns a primitive type and it also worked.

Basically Java will automatically convert between Integer and int for you, based on what's required in any given context.

This is called "automatic boxing / unboxing".

The main reason Integer exists is:

• It's possible to have a 'null' Integer, but not a null int - if you want to allow a null value, you need to use the object wrapper type

• Collections like List, Set, Map etc do not support primitive types, only objects - so these object wrapper / boxed versions need to exist to allow you to store primitive values in collections

More on reddit.com

Please ensure that: Your code is properly formatted as code block - see the sidebar (About on mobile) for instructions You include any and all error messages in full - best also formatted as code block You ask clear questions You demonstrate effort in solving your question/problem - plain posting your assignments is forbidden (and such posts will be removed) as is asking for or giving solutions. If any of the above points is not met, your post can and will be removed without further warning. Code is to be formatted as code block (old reddit/markdown editor: empty line before the code, each code line indented by 4 spaces, new reddit: https://i.imgur.com/EJ7tqek.png ) or linked via an external code hoster, like pastebin.com, github gist, github, bitbucket, gitlab, etc. Please, do not use triple backticks (```) as they will only render properly on new reddit, not on old reddit. Code blocks look like this: public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } } You do not need to repost unless your post has been removed by a moderator. Just use the edit function of reddit to make sure your post complies with the above. If your post has remained in violation of these rules for a prolonged period of time (at least an hour), a moderator may remove it at their discretion. In this case, they will comment with an explanation on why it has been removed, and you will be required to resubmit the entire post following the proper procedures. To potential helpers Please, do not help if any of the above points are not met, rather report the post. We are trying to improve the quality of posts here. In helping people who can't be bothered to comply with the above points, you are doing the community a disservice. I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns. More on reddit.com

If you want exactly this (List of primitives) there are few libraries that offer it. These collections are faster and with lower memory footprint, but there is real advantage to use them when your collections are huge. HPPC Trove COLT ... More on reddit.com

If you search for "java jdbc simple DAO" you might be able to borrow some ideas from that. You can blow allot of time with it though so I would set a time limit on how much your research.

I am assuming your are using jdbc. One thing that might be useful is to test database calls outside of the web app so you get all the bugs out first before using it in a web app. ie make a simple java Application using your database classes to call the database directly.

More on reddit.com