Cannot add value to methods return value in java

I have a method in each of my employee subclasses that calculates their earnings and returns that value.
In my main method I want to add 200 to this value if the employee has been in the workplace for over 5 years.
My approach was to compare the date that the employees joined to the current date by looping through my array of different employees and checking if it has been more than 5 years. If so, I want to get the employees earnings and add 200 to it.

I get an error at employeeArray.get(i).earnings() += 200; -> "Variable expected"

Relevant Code:

Test Class

for(int i = 0; i < employeeArray.size(); i ++){
            if(employeeArray.get(i).getDateJoined().compareTo(LocalDate.of(2015, 10, 16)) <= 0)
                {
                    employeeArray.get(i).earnings() += 200;
                }
            }

Employee Class

    public abstract double earnings();

Example employee (boss) earnings method (Subclass to Employee)

 public double earnings() {
        try{
            if(weeklySalary < 100){
                throw new Exception();
            }
        }
        catch (Exception lowWageException){
            System.out.println();
        }
        return weeklySalary;
    }

employeeArray.get(i).earnings() += 200; 
-> "Variable expected"

Yes, some expressions can occur on the left side of an assignment, some cannot.

For example,

a = 5;

is ok, but

a + 1 = 5;

is not. A method call (like earnings()) is one of those things that cannot appear on the left side of an assignment.

For this, you need a setter (or something similar) that takes a value so you can set the earnings.

public void setEarnings( double earn ) {
   weeklySalary = earn;
}

Then you can call that to set the value of weeklySalary.

if(employeeArray.get(i).getDateJoined()
      .compareTo(LocalDate.of(2015, 10, 16)) <= 0)
{
   double temp = employeeArray.get(i).earnings() + 200;
   setEarnings( temp );
}