How to write the best possible Java code for a similar Ruby functionality?

First, a word of caution: that code is absolutely horrible. It provides almost no encapsulation, it leaks implementation details left and right, there's no way that a Tree object can maintain its own invariants or state. Secondly, it doesn't integrate at all with Ruby's collection framework.

As a consequence, my Java translation is also equally horrible, and it also doesn't integrate with Java's collection framework.

The two biggest drawbacks that your Java code has compared to your Ruby are

• in the Java version, the element type is hard-coded to String, whereas in the Ruby version, it can be any object, and even a mixture of objects within the same tree, and
• in the Java version, the iterators are hard-coded to printing the name(s), whereas in the Ruby version, the iterators take a block argument with the code to execute.

The first problem cannot be easily solved in Java. You can make the collection generic, so that it can hold elements of any type, but making it heterogeneous (i.e. being able to hold elements of different types in the same collection) is going to be a lot of work. So, I stuck with the partial solution: making the Tree generic.

The second problem can be solved by having the iterators take an object which contains the code. After all, a first-class subroutine is basically the same as an object with only one method. (Java 8 is going to take some of that pain away, I included examples in the code.)

import java.util.Collection;
import java.util.ArrayList;

interface Consumer<T> {
    void accept(T e);
}
// In Java 8, this interface is already part of the JRE.
// Just replace the 3 lines above with this import:
//import java.util.function.Consumer;

class Tree<T> {
    private String nodeName;
    private Collection<Tree<T>> children = new ArrayList<>();

    Tree(String name, Collection<Tree<T>> children) {
        nodeName = name;
        this.children = children;
    }

    Tree(String name) {
        nodeName = name;
    }

    public String getNodeName() { return nodeName; }
    public void setNodeName(String name) { nodeName = name; }

    public Collection<Tree<T>> getChildren() { return children; }
    public void setChildren(Collection<Tree<T>> children) { this.children = children; }

    void visitAll(Consumer<Tree<T>> block) {
        visit(block);
        for (Tree<T> tree : children) tree.visitAll(block);
    }

    void visit(Consumer<Tree<T>> block) {
        block.accept(this);
    }

    public static void main(String... args) {
        ArrayList<Tree<String>> children = new ArrayList<>();
        children.add(new Tree<String>("Reia"));
        children.add(new Tree<String>("MacRuby"));
        Tree<String> rubyTree = new Tree<>("Ruby", children);

        System.out.println("Visiting a node");
        rubyTree.visit(new Consumer<Tree<String>>() {
            public void accept(Tree<String> node) {
                System.out.println(node.getNodeName());
            }
        });
        // In Java 8, you can use a lambda.
        // Just replace the 5 lines above with this line:
        //rubyTree.visit(node -> System.out.println(node.getNodeName()));

        System.out.println();

        System.out.println("Visiting entire tree");
        rubyTree.visitAll(new Consumer<Tree<String>>() {
            public void accept(Tree<String> node) {
                System.out.println(node.getNodeName());
            }
        });
        // In Java 8, you can use a lambda.
        // Just replace the 5 lines above with this line:
        //rubyTree.visitAll(node -> System.out.println(node.getNodeName()));
    }
}