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Using swing GUI in java

I have a few questions about Java swing GUI, FYI this is my first time using a GUI so I really don't know much about this stuff
I have code that is basically a user class with name and number attributes which I plan to use as a login page. So what I did was create a new GUI class for this user class completely separate from the user class (as in one class for functions and another for the GUI). Was this wrong? as in should I have placed the user methods and the GUI in one class?
I have 2 GUIs, a welcome GUI and a login GUI. How do I ensure that one leads to the next? I tried to make the welcome frame invisible (the login frame is made visible in its own GUI) but that didn't work.
The login GUI is similar to the one above and has frame.setVisible(true);

1- I have a code that is basically a user class with name and number
attributes which I plan to use as a login page, so what I did was
create a new GUI class for this user class completely separate from
the user class (as in one class for functions and another for the GUI)
, was this wrong? as in should I have placed the user methods and the
GUI in one class?
A common concept in UI development is "model-view-controller" (don't worry about controller just yet). This means that your "data" is modelled in some way (ie User) and your "view" (UI) takes that model and makes decisions about how the model should be presented to the user. It also helps manage the interaction between the user and the model.
So, yes, keeping your "data" independent from your "ui" is the right strategy. Always remember, it's the UI's responsibility to determine "how" the data is formatted, the model is just a means to manage the data in some meaningful way.
2- I have 2 GUIs, a welcome GUI and a login GUI, how do I ensure that
one leads to the next? I tried to make the welcome frame invisible(the
login frame is made visible in its own GUI) but that didn't work
This is a little broader in concept. Typically, we might recommend using a CardLayout to "flip" between views, but this would assume you want to revisit those views at some point.
On a more "abstract" point of view, you would use some kind of "controller" to make decisions about what should happen based on the current state.
This means, if "welcome" has not been presented, you'd present the "welcome" view. When the user is ready to move beyond it, "welcome" would notify the "controller" and the controller would then decide what to do next.
ie. Do you have previously saved credentials or not? If so, you could auto login the user and move on, otherwise you'd need to present the "login" view in order to get the credentials and allow the controller(s) to authenticate the user.
This moves you onto the "observer pattern" (aka listeners), where an interest party registers interest in been notified when some state has changed.
When trying to design these kind of systems, always be asking yourself some basic questions
Just how much do I need to expose to other parts of the program? ie You're welcome view doesn't need to know about the login view, as it could do things to the login view which are out side of it's scope of responsibility
How hard or how much work would I need to do to change any part of it?! So you get the welcome screen to open the login screen, but now you want to add in "auto login", just how much work are you going to have to go to make that work? Would it have been easier if the welcome and login views were independent of each other and controlled through some other mechanism?
Take a look at Java and GUI - Where do ActionListeners belong according to MVC pattern? for simple implementation example

How to loop a GUI using java?

im creating a game using java language. I am new to GUI. I want to be able to somehow loop a frame. For example, if the user pressed 2 players, the second frame should be able to show twice like "Player 1 name: " then after pressing enter "player 2 name: " should be the next one and then after pressing next another frame will show. I want to be able to update my GUI every time the enter button is clicked, and limit the update depending on how many players was chosen. The images shown below have different .java files
while(i < game.getnPlayers()) {
if (action.getActionCommand().equals("enter")) {
// codes
}
i++;
}
im trying to start with this code, but when i do. it doesnt get updated

I see you have tagged the post with IntelliJ-idea, so I am assuming you are using the GUI designer. Graphical user interfaces use listeners to listen for certain input (OnMouseClick, ActionListener and so on), and then performs methods you assign to the listener. I would suggest reading some documents or watching some videos on the topic before moving on with your game.
A source from oracle on writing Event listeners
A video on making Java Swing GUIs using IntelliJ
Hope this helps! Good luck!

On your GUI class you must have the button function. There you can create an function for when the button is pressed (in this case the "ENTER"), then you will see how many players were selected.

using DefaultListMethod to make custom buttons via showOptionDialog

I am just learning AWT / Swift / JavaFx recently and I feel like I have learned a lot but have barely scratched the surface. There might be a much easier way to do this BUT, I am trying to make a GUI button in eclipse that calculates the distance between two objects that the user creates. Lets call them Robots for now. So, I have one button that allows the user to create the Robots and it stores them in a DefaultListModel (listModel) and displays them in a Jlist (list) below the buttons. When the user then clicks on a robot, another button becomes actice and allows them to calculate the distance between them (one of the parameters of the robots is their location on a grid). I have all that worked out but my problem is that I am trying to make it to where they have to select two different Robots. At first I thought I could let them select two Robots and then make the computeDistance button becomes active, but I am not really sure how to do that because the only way I can select more than one object in the JList is to cntrl click and I don't want the user to have to know that trick.
My next idea was to allow the user to have one Robot selected and then give them a popup window displaying the other Robots and have them select one. Via showOptionsDialog, I have discovered how to make a custom JOptionPane, so I thought, why not make them buttons (probably will look awful, but I don't know how to make anything other than JOptionPane.showXxx at this point (like I said, only skin deep so far). Have tried consulting the javadocs, but right now that is a LOTTT to take in and have read a decent amount, I thought.
Ok, sorry if this is long, but is there a way, using my DefaultListMethod to make custom buttons? I tried a bunch of ways by creating Object[] options = {list.elements()}; etc but that doesn't work. Any help would be much appreciated!

Retrieve Values from multiple JFrame to a single JFrame

I have 5 JFrames in my application and I want the values from all 5 JFrames to be sent to a single JFrame. And it is a process where I have to go one frame to another and the value entered previously should not be lost and must be visible at the end of the process.
Easy example is,
I key in my name in the first frame,
then I key in my Address in the second frame,
then my mobile number in the third frame
and so on till the last frame where I want my keyed in details in the previous forms to be in the final frame to display my data in JTextfields. Is this possible? Because if it is a single form, I know how to do it. But when it is multiple forms in this situation I am lost. Please help.

This has nothing to do with Swing or JFrames and all to do with the general issue of getting information from one object into another. Yes it's possible -- give the classes that you wish to extract information from "getter" methods, and then call them when you want the information. If you want to gather this information in an event-dependent fashion, then you will need to have one class listening for state changes brought on by events in the other classes. A PropertyChangeListener can work well for this.
Or if you use modal JDialog windows instead of JFrames, you will always be notified when the dialog has returned and is no longer visible, since the calling code's program flow resumes from right after where it told the dialog to become visible.
Next we can discuss whether having 5 separate JFrames is a good idea or not. I'm guessing you know my opinion on this, else I wouldn't have mentioned the subject.

JOptionPane vs. JDialog

This is a crosspost to the thread in Javaranch (includes some images): http://www.coderanch.com/t/567472/GUI/java/Optimal-solution-creating-multiple-dialog
I'm trying to develop a simple swing desktop application where I imagine alot of different dialog's jumping around to fetch user input. Would need to present labels, textfields, passwordfields, combobxes, checkboxes etc in various dialog windows.
For example: creating the database firsthand, creating the first admin account, adding users, changing user accounts etc.
I have an understanding that JOptionPane is used to create simple quick & easy modal dialog's. I would really like to know why one would choose one over another in this case. Which one is more preferable to use: JOptionPane vs. JDialog
Also I could use some pointers how one should appropriately design and implement this.
Thank you.

Here's a statement I found on the Java website that says one key point about the difference between the two.
How to make Dialogs
A Dialog can be modal. When a modal Dialog is visible, it blocks user input to all other windows in the program. JOptionPane creates JDialogs that are modal. To create a non-modal Dialog, you must use the JDialog class directly.
So it sounds like you would use JOptionPane if you want a user to have to make a choice and close the box before returning to the main screen. If you use a JDialog box, then they can just click around it and get back to the main screen without making a choice. For example, say you wanted to make a user choose the number of results before clicking submit, you wouldn't want them to be able to click around that window and click submit. You would use JOptionPane to force them to select a value first before going back to submit.

Check out http://docs.oracle.com/javase/tutorial/uiswing/components/dialog.html it pretty much has everything you would need.
As i understand it, JOptionPane is great for what it can do, but you can't really change the functionality beyond that (not easily). JDialog is better to inherit from if you want to create your own custom Dialogs.