The.

deque/src/deque/Main.java

@@ -17,26 +17,20 @@ class Examples {
 }
 
 // A doubly-linked list element.
-abstract class ANode<PointsTo> {
+abstract class ANode<T> {
 
-    PointsTo nxt; // Next element.
+    ANode<T> nxt; // Next element.
-    PointsTo pre; // Previous element.
+    ANode<T> pre; // Previous element.
 
-    ANode(PointsTo nxt, PointsTo pre) {
+    ANode(ANode<T> nxt, ANode<T> pre) {
         this.nxt = nxt;
         this.pre = pre;
     }
-}
 
-// Self-referential node.
+    void assignNode(ANode<T> nxt, ANode<T> pre) {
-abstract class ASRefNode {
+        throw new UnsupportedOperationException(
-
+            "Can't call assignNode() on non-Node."
-    ASRefNode nxt;
+        );
-    ASRefNode pre;
-
-    ASRefNode() {
-        this.nxt = this;
-        this.pre = this;
     }
 }
 
@@ -52,8 +46,10 @@ class Deque<A> {
 class Sentinel<T> extends ANode<ANode<T>> {
 
     Sentinel() {
+        // To create an empty list, this Sentinel points to itself.
         super(null, null);
         this.nxt = this;
+        this.pre = this;
     }
 }
 
@@ -65,4 +61,15 @@ class Node<T> extends ANode<Node<T>> {
         super(null, null);
         this.val = val;
     }
+
+    Node(T val, ANode<T> nxt, ANode<T> pre) {
+        this(val);
+        this.assignNode(nxt, pre);
+    }
+
+    void assignNode(ANode<T> nxt, ANode<T> pre) {
+        throw new UnsupportedOperationException(
+            "Can't call assignNode() on non-Node."
+        );
+    }
 }