ننسخ المرجع، ونصل لنفس الكائن

public class TestEmployee {
    public static void main(String[] args) {
        Employee e1 = new Employee("Mohammed", 20, 5000);
        e1.writeOutput();
        doubleSalary(e1);
        e1.writeOutput();
    }
    public static void doubleSalary(Employee e) {
        double s = e.getSalary();
        e.setSalary(s * 2);
    }
}

public class Employee {
    private String name;
    private int age;
    private double salary;

    public Employee(String n, int a, double s) {
        name = n;
        age = a;
        salary = s;
    }

    public String getName() { return name; }
    public void setName(String n) { name = n; }

    public double getSalary() { return salary; }
    public void setSalary(double s) { salary = s; }

    public void writeOutput() {
        System.out.println("Name: " + name + " Age: " + age + " Salary: " + salary);
    }
}