How to implement a tree data-structure in Java?

Question

Is there any standard Java library class to represent a tree in Java?

Specifically I need to represent the following:

• The sub-tree at any node can have

Answer 1

If you're doing whiteboard coding, an interview, or even just planning to use a tree, the verbosity of these is all a little much.

It should further be said that the reason a tree is not in there like, say, a Pair (about which the same could be said), is because you should be encapsulating your data in the class using it, and the simplest implementation looks like:

/***
/* Within the class that's using a binary tree for any reason. You could 
/* generalize with generics IFF the parent class needs different value types.
 */
private class Node {
  public String value;
  public Node[] nodes; // Or an Iterable nodes;
}

That's really it for an arbitrary width tree.

If you wanted a binary tree it's often easier to use with named fields:

private class Node { // Using package visibility is an option
  String value;
  Node left;
  Node right;
}

Or if you wanted a trie:

private class Node {
  String value;
  Map nodes;
}

Now you said you want

to be able to get all the children (some sort of list or array of Strings) given an input string representing a given node

That sounds like your homework.
But since I'm reasonably sure any deadline has now passed…

import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;

public class kidsOfMatchTheseDays {
 static private class Node {
   String value;
   Node[] nodes;
 }

 // Pre-order; you didn't specify.
 static public List list(Node node, String find) {
   return list(node, find, new ArrayList(), false);
 }

 static private ArrayList list(
     Node node,
     String find,
     ArrayList list,
     boolean add) {
   if (node == null) {
     return list;
   }
   if (node.value.equals(find)) {
     add = true;
   }
   if (add) {
     list.add(node.value);
   }
   if (node.nodes != null) {
     for (Node child: node.nodes) {
       list(child, find, list, add);
     }
   }
   return list;
 }

 public static final void main(String... args) {
   // Usually never have to do setup like this, so excuse the style
   // And it could be cleaner by adding a constructor like:
   //     Node(String val, Node... children) {
   //         value = val;
   //         nodes = children;
   //     }
   Node tree = new Node();
   tree.value = "root";
   Node[] n = {new Node(), new Node()};
   tree.nodes = n;
   tree.nodes[0].value = "leftish";
   tree.nodes[1].value = "rightish-leafy";
   Node[] nn = {new Node()};
   tree.nodes[0].nodes = nn;
   tree.nodes[0].nodes[0].value = "off-leftish-leaf";
   // Enough setup
   System.out.println(Arrays.toString(list(tree, args[0]).toArray()));
 }
}

This gets you use like:

$ java kidsOfMatchTheseDays leftish
[leftish, off-leftish-leaf]
$ java kidsOfMatchTheseDays root
[root, leftish, off-leftish-leaf, rightish-leafy]
$ java kidsOfMatchTheseDays rightish-leafy
[rightish-leafy]
$ java kidsOfMatchTheseDays a
[]