Java Builder with compile time checking of full initialization (Strict Builder pattern)

example.java

public class Person {
    private final String username;
    private final int age;

    Person(PersonBuilder personBuilder) {
        this.username = personBuilder.username;
        this.age = personBuilder.age;
    }


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


    public static class PersonUsernameBuilder {
        public PersonAgeBuilder username(String username) {
            return new PersonAgeBuilder(username);
        }
    }

    public static class PersonAgeBuilder {
        public final String username;

        PersonAgeBuilder(String username) {
            this.username = username;
        }

        public PersonBuilder age(int age) {
            return new PersonBuilder(username, age);
        }
    }

    public static class PersonBuilder {
        public final String username;
        public final int age;

        PersonBuilder(String username, int age) {
            this.username = username;
            this.age = age;
        }

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


/*
1. Any method has only access for reading for fields (except of changing inner collections).
2. Only full initialized Person can be build
3. Any step of initialization has only one initialization method.

Person.builder()
            .username("Bob")
            .age(10)
            .build()          

*/

I think it need for project lombok. Because this pattern variant has lot of sub-builders. Exactly for code generation.

Well, of course you can have some other variants of controlled partial initialization with this pattern. By creating another chains of sub-builders. Same as multiple constructors. But I can't recommend it. Better solution only one chain with nulls.

Once I made something similar for retrofit
let me show you... (but it is bit dotnettish...)

https://gist.github.com/norritt/add48ae376c4d79d2d253b197bff1ebd that's it

Yes, something similar. C#, but yes :) Pattern it is description for solution of typical problem. Main idea of this pattern it is compiler checked initialization without any additional checks.

Created issue in lombok project
projectlombok/lombok#3367