Introduce ListItemIterator

src/aud/exam/prep/list/DoubleListItem.java

@@ -1,7 +1,17 @@
 package aud.exam.prep.list;
 
-public class DoubleListItem<T> {
+public class DoubleListItem<T> implements ListItem<T> {
     public T key;
-    public DoubleListItem<T> next;
     public DoubleListItem<T> prev;
+    public DoubleListItem<T> next;
+
+    @Override
+    public T head() {
+        return key;
+    }
+
+    @Override
+    public ListItem<T> tail() {
+        return next;
+    }
 }

src/aud/exam/prep/list/ListItem.java

@@ -0,0 +1,8 @@
+package aud.exam.prep.list;
+
+public interface ListItem<T> {
+
+    T head();
+
+    ListItem<T> tail();
+}

src/aud/exam/prep/list/ListItemIterator.java

@@ -0,0 +1,24 @@
+package aud.exam.prep.list;
+
+import java.util.Iterator;
+
+public class ListItemIterator<T> implements Iterator<T> {
+
+    private ListItem<T> current;
+
+    public ListItemIterator(ListItem<T> current) {
+        this.current = current;
+    }
+
+    @Override
+    public boolean hasNext() {
+        return current != null;
+    }
+
+    @Override
+    public T next() {
+        T t = current.head();
+        current = current.tail();
+        return t;
+    }
+}

src/aud/exam/prep/list/SingleListItem.java

@@ -1,6 +1,17 @@
 package aud.exam.prep.list;
 
-public class SingleListItem<T> {
+public class SingleListItem<T> implements ListItem<T> {
+
     public T key;
     public SingleListItem<T> next;
+
+    @Override
+    public T head() {
+        return key;
+    }
+
+    @Override
+    public ListItem<T> tail() {
+        return next;
+    }
 }