Check if username is already registered in ArrayList

I have this JFrame program that just registers users by their full name, username, city, age. I have the checks for the text fields in a method. If you press the register JButton, then it checks if you have entered the information correctly and then creates a new User to an array list.

I need to add a check, that will tell me when I press the Register button if the username exists. I tried making an if statement in my method for checking my text fields right before a new user is created in the area list, but it is just ignored. I tried to make that if statement before the method for the Register button and still nothing. I will paste my Action Listener code below, with my methods.

Edit: posting the method that I use for more clarity.

public void fieldChecks()
{
if(fullnameField.getText().length() < 6)
{
Helpers.showError("Name must have at least 7 characters");
return;
}
if(usernameField.getText().length() < 4)
{
Helpers.showError("Username must have at least 4 characters!");
return;
}
if(cityField.getText().length() < 5)
{
Helpers.showError("City must have at least 5 characters");
return;
}
if(passwordField.getText().length() < 8)
{
Helpers.showError("Name must have at least 8 characters");
return;
}
if(ageField.getText().length() == 0)
{
Helpers.showError("Age is a required field!");
return;
}
//check if our boolean is true or false from Helpers class
if(Helpers.ageCheck(ageField.getText()) == false)
{
Helpers.showError("Age must be a whole number!");
return;
}
String fullname = fullnameField.getText();
String username = usernameField.getText();
String city = cityField.getText();
String password = passwordField.getText();
int age = Integer.parseInt(ageField.getText());
String sex = " ";
//check which radio button is selected
if(maleOption.isSelected())
{
sex = "Male";
}else
{
sex = "Female";
}
//we have to create a user now by having the above new vars in the user brackets in order of our Class constructor
User myUser = new User(fullname,username, city, password, sex, age);
//create the default add function for our arraylist that we added in the vars above
myUsers.add(myUser);
Helpers.showConfirmation("User added!");
}

@Override
public void actionPerformed(ActionEvent e) {
if(e.getSource() == registerButton)
{
//this method checks if my text fields have more 
//than X charactes typed before it creates a user.
// This method also adds the new user to my array list 
//after the checks are complete
fieldChecks();
}
else if(e.getSource() == printButton)
{
printUser();
}
else if(e.getSource() == searchButton)
{
searchUsers();
}

After:

if(usernameField.getText().length() < 4)
{
Helpers.showError("Username must have at least 4 characters!");
return;
}

I would verify if that username is taken with following code

if(myUsers.size() > 0)
{
for(User user : myUsers)
{
if(usernameField.getText() == user.username)
{
Helpers.showError("This username is taken");
return;
}
}
}

I assume that myUser is an ArrayList of type myUser. You want to override equals method in your myUser class and then call contains. Look up how to @Override equals but here is a short demonstration.

public static void main(String[] args) {
List<myUser> users = new ArrayList<>();
users.add(new myUser("Billy", "xBillx"));
List<myUser> usersToAdd = new ArrayList<>();
usersToAdd.add(new myUser("Jill", "jilly"));
usersToAdd.add(new myUser("Billy", "xBillx"));
for(myUser newUser : usersToAdd){
if(!users.contains(newUser)){
users.add(newUser);
System.out.println("New user added: " + newUser.fullname);
} else {
System.out.println("User already exists " + newUser.fullname);
}
}
}
static class myUser{
final private String fullname;
final private String username;
myUser(String fullName, String username){
this.fullname = fullName;
this.username = username;
}
@Override
public boolean equals(Object user){
if(user == this){
return true;
}
if(user instanceof myUser &&
this.username.equals(((myUser) user).username) &&
this.fullname.equals(((myUser) user).fullname)) {
return true;
}
return false;
}
}

it prints:

New user added: Jill

User already exists Billy

Edit: made a mistake, you can't compare strings with == in Java