PhantomType.java

/**
 * Type-safe builder pattern in Java with Phantom Types
 */

public class PhantomType {
    private static interface Filled {}
    private static interface Empty {}
    public static class Builder<T1, T2> {
        private String name;
        private String address;
        private String email;
        private Builder(String name, String address, String email) {
            this.name = name;
            this.address = address;
            this.email = email;
        }
        public static Builder<Empty, Empty> getBuilder() {
            return new Builder<Empty, Empty>(null, null, null);
        }
        // Mandatory field name
        public Builder<Filled, T2> setName(String name) {
            return new Builder<Filled, T2>(name, address, email);
        }
        // Mandatory field address
        public Builder<T1, Filled> setAddress(String address) {
            return new Builder<T1, Filled>(name, address, email);
        }
        // Optional field email
        public Builder<T1, T2> setEmail(String email) {
            return new Builder<T1, T2>(name, address, email);
        }
    }
    private String name;
    private String address;
    private String email;
    public PhantomType(Builder<Filled, Filled> builder) {
        name = builder.name;
        address = builder.address;
        email = builder.email;
    }
    public static void main(String[] args) {
        new PhantomType(Builder.getBuilder().setEmail("useless").setName("foo").setAddress("bar"));
        // email is optional, i.e., ignoring it won't affect the compilation
        new PhantomType(Builder.getBuilder().setAddress("bar").setName("foo"));
        // The following two won't compile
        //new PhantomType(Builder.getBuilder().setName("foo"));
        //new PhantomType(Builder.getBuilder().setAddress("bar"));
    }
}