java - What's wrong with my builder pattern? -

i have problem in realisation of builder pattern. have 2 classes:

package course_2;  import java.util.date;  public class student {     private static int idstart = 0;      private final int id = idstart++;     private string name;     private string surname;     private string secondname;     private date birthdate;     private string address;     private string phone;     private int course;     private int group;      public static class builder {         // Обязательные параметры         private final string name;         private final string surname;         private final date birthdate;         // Необязательные параметры, инициализация по умолчанию         private string secondname = "";         private string address = "";         private string phone = "";         private int course = 1;         private int group = 1;          public builder(string name, string surname, date birthdate) {             this.name = name;             this.surname = surname;             this.birthdate = (date) birthdate.clone();         }          public builder secondname(string secondname) {             this.secondname = secondname;             return this;         }          public builder address(string address) {             this.address = address;             return this;         }          public builder phone(string phone) {             this.phone = phone;             return this;         }          public builder course(int course) {             this.course = course;             return this;         }          public builder group(int group) {             this.group = group;             return this;         }     }      private student(builder builder) {         this.name = builder.name;         this.surname = builder.surname;         this.secondname = builder.secondname;         this.birthdate = builder.birthdate;         this.address = builder.address;         this.phone = builder.phone;         this.course = builder.course;         this.group = builder.group;     } } 

the problem when i'm trying call builder client code:

student studentone = new student.builder("andrue", "booble", /*date variable here*/); 

i'm getting compiler problem :

error:(24, 30) java: incompatible types: course_2.student.builder cannot converted course_2.student

can me understanding, why happen , how can solve it? thanks!

you need add following builder:

        public student build(){             return new student(this);         } 

and call this:

    student studentone = new student.builder("andrue", "booble", null).build();