Prompting the user for employment information
Wrote this on SO so excuse any compilation errors.

As a novice there is a lot to learn, I would start by understanding the SOLID Principles and also looking out for Top 10 errors Java programmers make.

The first thing you want to do is get away from the static main(String[] args) method that starts your program. You do not want to use static methods. Therefore you need instantiate your system (MySystem) so that you are in the world of objects.

You want to keep your responsibilities separate.

Always prefer immutability.

Here I have an immutable Employee object (it cannot be changed once instantiated). From the main method our DetailsRetriever will get the input from the user, this uses the DetailsValidator to ensure the input is correct. If all the details are correct a Factory creates our employee object. 

public class UseIt {

    public static void main(String[] args) {
        MySystem foo = new MySystem(new DetailsRetriever(new DetailsValidator()), new EmployeeFactory())
        foo.start();
    }
}




public class MySystem {

   private final DetailsRetreiver detailsRetriever;
   private final EmployeeFactory employeeFactory;

   public MySystem(DetailsRetreiver detailsRetriever, EmployeeFactory employeeFactory) {
         this.detailsRetriever = detailsRetriever;
         this.employeeFactory = employeeFactory;
   }

   public void start() {
         String name = detailsRetriever.retrieveName();
         String jobTitle = detailsRetriever.retrieveJobTitle();
         ..etc

         Employee employee = employeeFactory.create(name, jobTitle, ..etc);
   }

}




public class Employee {

    private final String name;
    private final String jobTitle;
    private final int age;
    private final int salary;

    public Employee(String name, String jobTitle, int age, int salary) {
        this.name = name;
        this.jobTitle = jobTitle;
        this.age = age;
        this.salary = salary;
    }

   public int calcTaxAmount() {
    int calculatedTax;
    if (salary &lt; 10_000)
        calculatedTax = 0;

    if ((salary &gt;= 10_000) &amp;&amp; (salary &lt; 30_000))
        calculatedTax = 28;

    if ((salary &gt;= 30_000) &amp;&amp; (salary &lt; 80_000))
        calculatedTax = 38;

    if (salary &gt;= 80_000)
        calculatedTax = 48;

    return taxAmount;
}

   public int calcSalary() {
    int calculatedSalaryPostTax = salary - calcTaxAmount();

    return calculatedSalaryPostTax;
   }

}




public class DetailsRetreiver {

    private final DetailsValidator detailsValidator;

    public DetailsRetreiver(DetailsValidator detailsValidator) {
      this.detailsValidator - detailsValidator;
    }

    public String getName() {
        String name = JOptionPane.showInputDialog("Enter your name:");
        if(detailsValidator.validateName(name)) {
          return name;
        } else {
           return getName(); // or throw an exception
        }
    }

    // etc ...

}




public class DetailsValidator {

     public boolean validateName(String input) {
          // add more validation as you need
          return !"".equals(input);
     }

     // etc
}




public class EmployeeFactory {

      public Employee create(String name, String jobTitle, int age, ... etc) {
           return new Employee(name, jobTitle, age .. etc); 
      }

}