Tree.java

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Objects;
import java.util.stream.Stream;

import static java.util.Arrays.asList;

final class Tree<T> {

    private final List<Tree<T>> children = new LinkedList<>();
    private final T value;

    Tree(final T value, final List<Tree<T>> children) {
        this.value = Objects.requireNonNull(value);
        this.children.addAll(children);
    }

    @SafeVarargs
    Tree(T value, Tree<T>... children) {
        this(value, asList(children));
    }

    T value() {
        return value;
    }

    List<Tree<T>> children() {
        return Collections.unmodifiableList(this.children);
    }

    Stream<Tree<T>> flattened() {
        return Stream.concat(
            Stream.of(this),
            this.children.stream().flatMap(Tree::flattened));
    }
}