compiler-design-eth/docs/hw1/SimpleVisitor.java

import java.lang.*;

abstract class TreeNode {
	public int key;
	public TreeNode leftchild;
	public TreeNode rightchild;

	public TreeNode(int k, TreeNode left, TreeNode right) {
		key = k;
		leftchild = left;
		rightchild = right;
	}
}

class BlueNode extends TreeNode {
	public BlueNode(int k, TreeNode left, TreeNode right) { super(k, left, right); }
}

class GreenNode extends TreeNode {
	public GreenNode(int k, TreeNode left, TreeNode right) { super(k, left, right); }
}

class Visitor {
	public void visit(TreeNode node) {
		System.out.print(node.key + ": ");

		if (node instanceof BlueNode)
			System.out.println("Blue");
		else if (node instanceof GreenNode)
			System.out.println("Green");

		if (node.leftchild != null) visit(node.leftchild);
		if (node.rightchild != null) visit(node.rightchild);
	}
}

public class SimpleVisitor {
	public static void main(String [] args) {
		TreeNode root = new BlueNode(1,
				new GreenNode(21,
						new BlueNode(31, null, null),
						new GreenNode(32, null, null)),
				new GreenNode(22,
						new BlueNode(33, null, null),
						new GreenNode(34, null, null)));
		Visitor v = new Visitor();
		v.visit(root);
	}
}