Restructures demos.

demos/java/src/app/App.java

@@ -0,0 +1,58 @@
+package app;
+
+import app.GraphData.EdgeInfo;
+
+public class App {
+
+    public static void main(String[] args) {
+        // watch `list.visualize()`
+        LinkedList<String> list = new LinkedList<String>();
+        list.append("1");
+        list.append("2");
+        list.append("3");
+        list.append("4");
+    }
+}
+
+class LinkedList<T> {
+    private Node<T> head;
+
+    public void append(T value) {
+        if (head == null) {
+            head = new Node<T>(value);
+        } else {
+            Node<T> m = head;
+            while (m.next != null) {
+                m = m.next;
+            }
+            m.next = new Node<T>(value);
+        }
+    }
+
+    public String visualize() {
+        Node<T> list = new Node<T>(null);
+        list.next = this.head;
+
+        return GraphData.from(list, (item, info) -> {
+            if (item != list) {
+                info.id = item.value.toString();
+            } else {
+                info.label = "list";
+            }
+            if (item.next != null) {
+                EdgeInfo<LinkedList.Node<T>> ei = info.addEdge(item.next);
+                ei.label = "next";
+            }
+            return info;
+        }).toString();
+    }
+
+    static class Node<T> {
+        public T value;
+        public Node<T> next;
+
+        public Node(T value) {
+            this.value = value;
+        }
+    }
+}