Builder

DocumentItem documentItem = DocumentItem
        .builder()
        .title("Contract between X and Y")
        .build();

DocumentItem documentItem = DocumentItem
        .builder()
        .title("Contract between X and Y")
        .category(DocumentCategory.of("Contract"))
        .build();

DocumentItem documentItem = DocumentItem
        .builder()
        .title("Contract between X and Y")
        .category(DocumentCategory.of("Contract"))
        .signatures(Arrays.asList("Kowalski", "Nowak"))
        .build();

DocumentItem.DocumentItemBuilder documentItemBuilder = DocumentItem
                                                            .builder()
                                                            .title("Contract between X and Y");

// inny kod

documentItemBuilder.category(DocumentCategory.of("Contract")).build();

import java.util.List;
import java.util.stream.Collectors;

class DocumentItem {

    private final String title;
    private final DocumentCategory category;
    private final List<String> signatures;

    private DocumentItem(String title, DocumentCategory category, List<String> signatures) {
        this.title = title;
        this.category = category;
        this.signatures = signatures;
    }

    static DocumentItemBuilder builder() {
        return new DocumentItemBuilder();
    }

    public String toString() {
        return "DocumentItem(title=" + this.title + ", \ncategory=" + this.category
                + ", \nsignatures=" + this.signatures.stream()
                                           .map(String::toString)
                                           .collect(Collectors.joining(",")) + ")";
    }

    static class DocumentItemBuilder {

        private String title;
        private DocumentCategory category;
        private List<String> signatures;

        DocumentItemBuilder title(String title) {
            this.title = title;
            return this;
        }

        DocumentItemBuilder category(DocumentCategory category) {
            this.category = category;
            return this;
        }

        DocumentItemBuilder signatures(List<String> signatures) {
            this.signatures = signatures;
            return this;
        }

        DocumentItem build() {
            return new DocumentItem(title, category, signatures);
        }
    }
}

class DocumentCategory {

    private final String name;

    private DocumentCategory(String name) {
        this.name = name;
    }

    static DocumentCategory of(String name) {
        return new DocumentCategory(name);
    }

    public String toString() {
        return "DocumentCategory(name=" + this.name + ")";
    }
}

import java.util.Arrays;

public class Start {

    public static void main(String[] args) {

        DocumentItem documentItem = DocumentItem
                .builder()
                .title("Contract between X and Y")
                .category(DocumentCategory.of("Contract"))
                .signatures(Arrays.asList("Kowalski", "Nowak"))
                .build();

        System.out.println(documentItem);
    }
}