Person.java

public final class Person {

    private final String firstName;
    private final String lastName;

    private Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public static Builder builder() {
        return new Builder();
    }

    public static final class Builder {
        private String firstName;
        private String lastName;

        private Builder() {
        }

        public Builder firstName(String firstName) {
            this.firstName = firstName;
            return this;
        }

        public Builder lastName(String lastName) {
            this.lastName = lastName;
            return this;
        }

        public Person build() {
            return new Person(firstName, lastName);
        }
    }
}

  public Person build();
    descriptor: ()LPerson;
    flags: ACC_PUBLIC
    Code:
      stack=5, locals=1, args_size=1
         0: new           #5                  // class Person
         3: dup
         4: aload_0
         5: getfield      #3                  // Field firstName:Ljava/lang/String;
         8: aload_0
         9: getfield      #4                  // Field lastName:Ljava/lang/String;
        12: aconst_null
        13: invokespecial #6                  // Method Person."<init>":(Ljava/lang/String;Ljava/lang/String;LPerson$1;)V
        16: areturn

  Person$Builder(Person$1);
    descriptor: (LPerson$1;)V
    flags: ACC_SYNTHETIC
    Code:
      stack=1, locals=2, args_size=2
         0: aload_0
         1: invokespecial #1                  // Method "<init>":()V
         4: return