Crappy Java File

package my.classes.foo;

/**
 * Created by mikeholmes on 17/06/15.
 */
public class MyClassImpl implements IManager {
    public String firstname;
    public String lastname;
    protected int age;
    private int f;
    private boolean why;

    public MyClassImpl(String firstname) {
        this.firstname = firstname;
    }

    public MyClassImpl(String lastname, String firstname) {
        this.firstname = firstname;
        this.lastname = lastname;
    }

    public MyClassImpl(String firstname, int age, String lastname) {
        this.firstname = firstname;
        this.lastname = lastname;
        this.age = age;
    }

    /**
     *
     * @return
     */
    public String getFirstname() {
        return this.firstname;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

    public void changeAge(int age) {
        this.age = age;
    }

    /**
     *
     * @param years
     * @return
     */
    public boolean addYearToAge(int years) {
        this.age += years;
        return true;
    }

    public void setF(int g) {
        this.f = g;
        why = true;
    }

    /**
     *
     * @param newMarriedSurname
     * @param dateOfBirth
     */
    public void getMarriedOnBirthday(String newMarriedSurname) {
        this.lastname = newMarriedSurname;
    }

    public void doThingAndAddAgeOrAbbreviateFirstname(int age) {
        ManagerClass m = new ManagerClass();
        if (age < 10) {
            firstname = firstname.substring(0, 3);
        }
        else {
            this.age = m.otherThing(age);
        }
    }
}