Collections - Beginning Java 8 Language Features - page 581

// Reverse the list
Collections.reverse(list);
System.out.println("After Reversing: " + list);

// Swap elements at indexes 1 and 3
Collections.swap(list, 1, 3);
System.out.println("After Swapping (1 and 3): " + list);

// Rotate elements by 2
Collections.rotate(list, 2);
System.out.println("After Rotating by 2: " + list);

List: [John, Richard, Donna, Ken]
After Shuffling: [Ken, Donna, Richard, John]
After Reversing: [John, Richard, Donna, Ken]
After Swapping (1 and 3): [John, Ken, Donna, Richard]
After Rotating by 2: [Donna, Richard, John, Ken]

Creating Different Views of a Collection
You can get a LIFO Queue view of a Deque using the asLifoQueue() static method of the Collections class:

•

<T> Queue<T> asLifoQueue(Deque<T> deque)

Some Map implementations have corresponding Set implementations too. For example, for HashMap , you have a
HashSet ; for TreeMap , you have a TreeSet . If you want to use a Map's implementation as a Set implementation, you
can use the newSetFromMap() static method of the Collections class:

<E> Set<E> newSetFromMap(Map<E, Boolean> map)
Note that the idea is to use the implementation of the Map as a Set , not to share elements between a Map and a
Set . This is the reason that the Map must be empty when you use it in this method and you are not supposed to use
the Map directly at all. There is a WeakHashMap implementation class for the Map . However, there is no corresponding
WeakHashSet implementation class for the Set . Here is how you can get a weak hash set implementation:

•

Map map = new WeakHashMap(); // Do not populate and use the map
Set wSet = Collections.newSetFromMap(map); // You can use wSet

Use the weak hash set wSet as a Set and it acts as the WeakHashMap implementation. Since you are not
supposed to use the Map object, it is better to use the following statement to create the set using the WeakHashMap
implementation class:

// Do not keep the reference of the Map
Set wSet = Collections.newSetFromMap(new WeakHashMap());

When the JVM needs memory, the garbage collector can remove elements from wSet as it does from any
WeakHashMap . By using one line of code, you get a Set that has features of a WeakHashMap .