class DefaultProjectDependencyGraph implements ProjectDependencyGraph {

    private final ProjectSorter sorter;

    private final List<MavenProject> allProjects;

    private final Map<MavenProject, Integer> order;

    private final Map<String, MavenProject> projects;

    /**
     * Creates a new project dependency graph based on the specified projects.
     *
     * @param projects The projects to create the dependency graph with
     * @throws DuplicateProjectException
     * @throws CycleDetectedException
     */
    public DefaultProjectDependencyGraph(Collection<MavenProject> projects)
            throws CycleDetectedException, DuplicateProjectException {
        this(projects, projects);
    }

    /**
     * Creates a new project dependency graph based on the specified projects.
     *
     * @param allProjects All collected projects.
     * @param projects    The projects to create the dependency graph with.
     * @throws DuplicateProjectException
     * @throws CycleDetectedException
     * @since 3.5.0
     * @deprecated Use {@link #DefaultProjectDependencyGraph(Collection, Collection)} instead.
     */
    @Deprecated
    public DefaultProjectDependencyGraph(List<MavenProject> allProjects, Collection<MavenProject> projects)
            throws CycleDetectedException, DuplicateProjectException {
        this((Collection<MavenProject>) allProjects, projects);
    }

    /**
     * Creates a new project dependency graph based on the specified projects.
     *
     * @param allProjects All collected projects.
     * @param projects    The projects to create the dependency graph with.
     * @throws DuplicateProjectException
     * @throws CycleDetectedException
     * @since 4.0.0
     */
    public DefaultProjectDependencyGraph(Collection<MavenProject> allProjects, Collection<MavenProject> projects)
            throws CycleDetectedException, DuplicateProjectException {
        this.allProjects = Collections.unmodifiableList(new ArrayList<>(allProjects));
        this.sorter = new ProjectSorter(projects);
        this.order = new HashMap<>();
        this.projects = new HashMap<>();
        List<MavenProject> sorted = this.sorter.getSortedProjects();
        for (int index = 0; index < sorted.size(); index++) {
            MavenProject project = sorted.get(index);
            String id = ProjectSorter.getId(project);
            this.projects.put(id, project);
            this.order.put(project, index);
        }
    }

    /**
     * @since 3.5.0
     */
    public List<MavenProject> getAllProjects() {
        return this.allProjects;
    }

    public List<MavenProject> getSortedProjects() {
        return new ArrayList<>(sorter.getSortedProjects());
    }

    public List<MavenProject> getDownstreamProjects(MavenProject project, boolean transitive) {
        Objects.requireNonNull(project, "project cannot be null");

        Set<String> projectIds = new HashSet<>();

        getDownstreamProjects(ProjectSorter.getId(project), projectIds, transitive);

        return getSortedProjects(projectIds);
    }

    private void getDownstreamProjects(String projectId, Set<String> projectIds, boolean transitive) {
        for (String id : sorter.getDependents(projectId)) {
            if (projectIds.add(id) && transitive) {
                getDownstreamProjects(id, projectIds, transitive);
            }
        }
    }

    public List<MavenProject> getUpstreamProjects(MavenProject project, boolean transitive) {
        Objects.requireNonNull(project, "project cannot be null");

        Set<String> projectIds = new HashSet<>();

        getUpstreamProjects(ProjectSorter.getId(project), projectIds, transitive);

        return getSortedProjects(projectIds);
    }

    private void getUpstreamProjects(String projectId, Collection<String> projectIds, boolean transitive) {<FILL_FUNCTION_BODY>}

    private List<MavenProject> getSortedProjects(Set<String> projectIds) {
        return projectIds.stream()
                .map(projects::get)
                .sorted(Comparator.comparingInt(order::get))
                .collect(Collectors.toList());
    }

    @Override
    public String toString() {
        return sorter.getSortedProjects().toString();
    }
}

        for (String id : sorter.getDependencies(projectId)) {
            if (projectIds.add(id) && transitive) {
                getUpstreamProjects(id, projectIds, transitive);
            }
        }
    
<no_super_class>