What's wrong with my builder pattern?

Question

I have a problem in realisation of Builder pattern. I 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 is when I'm trying to call a Builder from my client code:

Student studentOne = new Student.Builder("Andrue", "Booble", /*Date variable here*/);

I'm getting a compiler problem :

Error:(24, 30) java: incompatible types: course_2.Student.Builder cannot be converted to course_2.Student

Can somebody help me with understanding, why does this happen and how I can solve it? Thanks!

Answer 1

You need to add the following to your Builder:

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

And call it like this:

    Student studentOne = new Student.Builder("Andrue", "Booble", null).build();