Maps

package my.pkg;
import java.util.Objects;

public class EmailAddr {
    private String email;

    private EmailAddr(String email) {  // private - from outside only factory!
        this.email = email;  // no checks here - done in factory!
    }
    public static EmailAddr create(String email) {  // static factory method
        checkValid(email);
        return new EmailAddr(email);
    }
    public static void checkValid(String email) { // only effect is exception if needed
        if (email == null) {
            throw new java.lang.IllegalArgumentException("email is null");  // unchecked
        }
        if (email.isBlank()) {
            throw new java.lang.IllegalArgumentException("email has no content");
        }
        // more checks, see https://en.wikipedia.org/wiki/Email_address
    }
    @Override
    public int hashCode() {   // created by IDE
        return Objects.hash(email);
    }
    @Override
    public boolean equals(Object obj) {   // created by IDE
        if (this == obj)  return true;
        if (!(obj instanceof EmailAddr))  return false;
        EmailAddr other = (EmailAddr) obj;
        return Objects.equals(email, other.email);
    }
    @Override
    public String toString() {
        return email;
    }
    // There is NO SETTER for email, so it is IMMUTABLE. Otherwise the email address
    // could be changed while used as a key!
    // For such cases we would need a "property change event" implementation!
}

package my.pkg;
import java.time.LocalDate;

public class Person {
    private String name;
    private EmailAddr emailAddr;
    private LocalDate birthDate;
    private String info;

    public Person(String name, EmailAddr emailAddr) {
        setName(name);
        setEmailAddr(emailAddr);
    }
    public String getName() { return name; }
    public EmailAddr getEmailAddr() { return emailAddr; }
    public LocalDate getBirthDate() { return birthDate; }
    public String getInfo() { return info; }

    public void setName(String name) { this.name = name; }
    public void setEmailAddr(EmailAddr emailAddr) { this.emailAddr = emailAddr; }
    public void setBirthDate(LocalDate birthDate) { this.birthDate = birthDate; }
    public void setInfo(String info) { this.info = info; }
    @Override
    public String toString() {
        return "Person [name=%s, email=%s, birthDate=%s, info=%s]" //
                .formatted(name, emailAddr, birthDate, info);
    }

package my.pkg;
import java.time.LocalDate;
import java.util.HashMap;
import java.util.Map;

public class FriendsBook {
    private Map<EmailAddr, Person> friendsMap;

    public FriendsBook() {
        friendsMap = new HashMap<>(); // accessing Person by EmailAddr hashCode+equals
    }
    public static void main(String[] args) {
        FriendsBook myFriendsBook = new FriendsBook();
        myFriendsBook.populate();
        myFriendsBook.show("after populate(): Ede, Ida");
        myFriendsBook.change();
        myFriendsBook.show("after change(): name Ede -> Edi");
        myFriendsBook.remove();
        myFriendsBook.show("after remove(): ida");
        myFriendsBook.badChange();
        myFriendsBook.show("after badChange(): email ede@.. -> eduard@.. (email != key)");
        System.out.println("Now try to create invalid EmailAddr '   ' :");
        myFriendsBook.produceException();
    }
    public void populate() {
        EmailAddr ea;
        Person p;
        friendsMap.put(ea = EmailAddr.create("ede@nixnet.at"), //
                p = new Person("Ede", ea));
        p.setBirthDate(LocalDate.of(1999, 12, 31));
        p.setInfo("Some Details");
        friendsMap.put(ea = EmailAddr.create("ida@mailth.at"), //
                p = new Person("Ida", ea));
        p.setInfo("Important facts");
    }
    public void change() {
        Person p = friendsMap.get(EmailAddr.create("ede@nixnet.at"));
        p.setName("Edi");
    }
    public void remove() {
        EmailAddr ea;
        Person p = friendsMap.remove(ea = EmailAddr.create("ida@mailth.at"));
        System.out.format("removed key=%s -> %s%n", ea, p);
    }
    public void badChange() {
        // change emailAddr of person while in map -> emailAddr not equals key any more!
        Person p = friendsMap.get(EmailAddr.create("ede@nixnet.at"));
        p.setEmailAddr(EmailAddr.create("eduard@nixnet.at"));
    }
    public void show(String title) {
        System.out.format("FriendsMap (%d items) %s:%n", friendsMap.size(), title);
        for (EmailAddr ea : friendsMap.keySet()) {
            System.out.format("  key=%s -> %s%n", ea, friendsMap.get(ea));
        }
        System.out.println();
    }
    public void produceException() {
        EmailAddr ea = EmailAddr.create("   ");
    }
}

FriendsMap (2 items) After populate with Ede, Ida:
  key=ede@nixnet.at -> Person [name=Ede, email=ede@nixnet.at, birthDate=1999-12-31, info=Some Details]
  key=ida@mailth.at -> Person [name=Ida, email=ida@mailth.at, birthDate=null, info=Important facts]

FriendsMap (2 items) After change name Ede -> Edi:
  key=ede@nixnet.at -> Person [name=Edi, email=ede@nixnet.at, birthDate=1999-12-31, info=Some Details]
  key=ida@mailth.at -> Person [name=Ida, email=ida@mailth.at, birthDate=null, info=Important facts]

removed key=ida@mailth.at -> Person [name=Ida, email=ida@mailth.at, birthDate=null, info=Important facts]
FriendsMap (1 items) After remove ida:
  key=ede@nixnet.at -> Person [name=Edi, email=ede@nixnet.at, birthDate=1999-12-31, info=Some Details]

FriendsMap (1 items) After badChange email ede@.. -> eduard@.. (email != key):
  key=ede@nixnet.at -> Person [name=Edi, email=eduard@nixnet.at, birthDate=1999-12-31, info=Some Details]

Now try to create invalid EmailAddr '   ':
Exception in thread "main" java.lang.IllegalArgumentException: email has no content
	at my.scribble2021q4a.mapdemo.EmailAddr.checkValid(EmailAddr.java:21)
	at my.scribble2021q4a.mapdemo.EmailAddr.create(EmailAddr.java:13)
	at my.scribble2021q4a.mapdemo.FriendsBook.produceException(FriendsBook.java:67)
	at my.scribble2021q4a.mapdemo.FriendsBook.main(FriendsBook.java:26)