If you want to implement a class that stores the first and last name of a person, you would normally implement something like this:

    public class PersonMutable { 
      private String firstName; 
      private String lastName; 
      private Date birthDate; 
 
      public String getFirstName() { 
        return firstName; 
      } 
 
      public void setFirstName(String firstName) { 
        this.firstName = firstName; 
      } 
 
      public String getLastName() { 
        return lastName; 
      } 
 
      public void setLastName(String lastName) { 
        this.lastName = lastName; 
      } 
      public Date getBirthDate() { 
        return birthDate; 
      } 
 
      public void setBirthDate(Date birthDate) { 
        this.birthDate = birthDate; 
      } 
 
    }

You can convert this class into an immutable class by following the rules explained earlier. The following is the result:

    public final class PersonImmutable { 
 
      final private String firstName; 
      final private String lastName; 
      final private Date birthDate; 
 
      public PersonImmutable (String firstName, String lastName,
                              String address, Date birthDate) { 
        this.firstName=firstName; 
        this.lastName=lastName; 
        this.birthDate=birthDate; 
      } 
 
      public String getFirstName() { 
        return firstName; 
      } 
 
      public String getLastName() { 
        return lastName; 
      }

      public Date getBirthDate() { 
        return new Date(birthDate.getTime()); 
      } 
 
    }

Essentially, you followed the basic principles of an immutable class, which are as follows:

• The class is marked as final.
• The attributes are marked as final and private.
• The value of the attributes can only be established in the constructor of the class.
  Its methods return the value of an attribute, but they don't modify them.
• For mutable attributes (the birthDate attribute in our case), we return a defensive copy of the get() method by creating a new object.