Type-safe Builder in Java

TypeSafeBuilder.java

public static class Example {
    private final String name;
    private final Integer age;
    private final String address;

    private Example(Builder<Filled, Filled> builder) {
        this.name = builder.name;
        this.age = builder.age;
        this.address = builder.address;
    }

    public static Builder<Required, Required> builder() {
        return new Builder<>();
    }

    public static class Builder<Name, Age> {
        private String name; // 必須
        private Integer age; // 必須
        private String address; // オプショナル

        public Builder<Filled, Age> setName(String name) {
            this.name = name;
            return this.unsafeCast();
        }

        public Builder<Name, Filled> setAge(Integer age) {
            this.age = age;
            return this.unsafeCast();
        }

        public Builder<Name, Age> setAddress(String address) {
            this.address = address;
            return this.unsafeCast();
        }

        private Builder() {
        }

        @SuppressWarnings("unchecked")
        private <Name_, Age_> Builder<Name_, Age_> unsafeCast() {
            return (Builder<Name_, Age_>) this;
        }
    }

    @Override
    public String toString() {
        return "Example{name='" + name + "', age=" + age + ", address='" + address + "'}";
    }

    // メインメソッド
    public static void main(String[] args) {
        Example example = new Example(Example.builder()
                .setAge(30)
                .setName("John")
                .setAddress("123 Street"));

        System.out.println(example);
    }

    public static class Required {}
    public static class Filled {}
}