Builder Pattern

FooBarZoo.java

public interface Foo {
    Collection<Bar> getBars();
    Zoo getZoo();
}

public interface Bar {}
public interface Zoo {}

FooDTO.java

@Entity(name = "Foo")
public final class FooDTO implements Foo {
    @Id
    @GeneratedValue
    private Long id;

    @NotEmpty
    @OneToMany(targetEntity = BarDTO.class)
    private Collection<Bar> bars = new HashSet<Bar>();

    @NotNull
    @ManyToOne(targetEntity = ZooDTO.class)
    private Zoo zoo;
 
    FooDTO() {}

    public Long getId() {
        return id;
    }

    @Override
    public Collection<Bar> getBars() {
        return Collections.unmodifiableCollection(bars);
    }

    @Override
    public Zoo getZoo() {
        return zoo;
    }

    void setId(final Long id) {
        this.id = id;
    }

    void setZoo(final Zoo zoo) {
        this.zoo = zoo;
    }

    void setBars(final Collection<? extends Bar> bars) {
        this.bars.clear();
        this.bars.addAll(bars);
    }
}

FooDTOBuilder.java

public final class FooDTO implements Foo {
    // ... described above

    public static final Builder newBuilder() {
        return Builder.create();
    }

    public static final Builder newBuilder(Foo foo) {
        return Builder.merge(foo);
    }

    public static final Builder updateBuilder(Foo foo) {
        return Builder.update(foo);
    }

    public static final class Builder implements com.example.Builder<Foo> {
        
        private FooDTO dto;
        
        private static final Builder create() {
            return new Builder();
        }
        
        private static final Builder update(Foo foo) {
            return new Builder(foo, false);
        }
        
        private static final Builder merge(Foo foo) {
            return new Builder(foo, true);
        }
        
        private Builder() {
            dto = new FooDTO();
        }
        
        private Builder(Foo foo, boolean newInstance) {
            this();
            dto.setId(newInstance ? null : foo.getId());
            dto.setZoo(foo.getZoo());
            dto.setBars(foo.getBars());
        }
        
        public Builder setZoo(Zoo zoo) {
            dto.setZoo(zoo);
            return this;
        }
        
        public Builder addBar(Bar bar) {
            dto.bars.add(bar);
            return this;
        }
        
        public Builder removeBar(Bar bar) {
            dto.bars.remove(bar);
            return this;
        }
        
        public Builder addAllBars(Collection<? extends Bar> bars) {
            dto.bars.addAll(bars);
            return this;
        }
        
        public Builder removeAllBars(Collection<? extends Bar> bars) {
            dto.bars.removeAll(bar);
            return this;
        }
        
        public Builder removeBars() {
            dto.bars.clear();
            return this;
        }
        
        @Override
        public Foo build() {
            return dto;
        }
    }
}