Vector is a synchronized class, making it thread-safe. This means that it is safe to use Vector in a multithreaded environment without additional synchronization. ArrayList and LinkedList are not synchronized, while HashMap is also not thread-safe without additional synchronization.

The Collections.unmodifiableList(list) method returns an unmodifiable view of the specified list. Any attempt to modify the returned list will result in an UnsupportedOperationException.

The remove(int index) method in an ArrayList removes the element at the specified index, shifting any subsequent elements to the left

A PriorityQueue in Java is implemented using a heap data structure, typically a binary heap, which allows efficient retrieval of the highest (or lowest) priority element.

To create a custom collection class that allows iteration over its elements, you should implement the Iterable interface. This interface requires implementing the iterator() method, which returns an Iterator.

The SortedSet interface provides a total ordering on its elements. Elements are ordered using their natural ordering or a comparator provided at set creation time. TreeSet is a common implementation of SortedSet.

The List interface allows random access to elements, meaning elements can be accessed by their index. Classes like ArrayList and LinkedList implement this interface, with ArrayList providing constant-time positional access.

The main difference is that a TreeSet maintains elements in their natural order or according to a provided comparator, while a HashSet does not maintain any order of elements.

EnumSet is faster than HashSet for storing enums because it is implemented as a bit vector. It is a specialized Set implementation specifically for use with enum types.

The Iterator interface allows traversal of a collection in the forward direction only. The ListIterator interface, on the other hand, allows bidirectional traversal.

The Collections.singletonList(T o) method returns an immutable list containing only the specified element.

The Collections.unmodifiableMap(map) method returns an unmodifiable view of the specified map, making it read-only.

LinkedList can be used to implement a stack as it provides methods like addFirst() and removeFirst() for pushing and popping elements, respectively. Stack is also available as a class but is considered legacy.

The List interface allows duplicate elements. Set does not allow duplicates, and while Map allows duplicate values, it does not allow duplicate keys.

The average time complexity of retrieving an element from a HashMap given the key is O(1). This is because a HashMap uses a hash table for storage, which allows for constant-time complexity for basic operations such as get() and put(), assuming a good hash function and low collision rates.

LinkedHashSet maintains the order of elements based on their insertion order. It combines the unique element property of HashSet with the ordering of LinkedList.

None of the mentioned classes (HashMap, TreeMap, LinkedHashMap) allow duplicate keys. All these classes implement the Map interface, which does not permit duplicate keys.

The LinkedList class implements the List interface. It provides a doubly-linked list data structure, allowing for constant-time insertions and deletions. ArrayList is another common class that implements the List interface.

StringBuilder is preferred for representing and manipulating sequences of characters when thread safety is not required. It is more efficient than StringBuffer as it is not synchronized.

HashSet does not implement the List interface, so you cannot assign a HashSet to a variable of type List. ArrayList, LinkedList, and Vector all implement the List interface.

The synchronizedCollection() method of the Collections class can be used to make a collection thread-safe by wrapping the specified collection.

The PriorityQueue class implements a queue that processes elements based on their priority, defined either by their natural ordering or by a Comparator provided at the queue’s construction time.

In a HashMap, if you add a key that already exists, the old value associated with that key is replaced with the new value. The key itself is not duplicated.

The default initial capacity of an ArrayList when created using the default constructor is 10. This means that it initially allocates space for 10 elements, and will resize automatically when more elements are added.

The subList(2, 4).clear() method call removes the elements from index 2 (inclusive) to index 4 (exclusive). So, elements at index 2 and 3 (i.e., 3 and 4) are removed from the list.

The get() method is not in the Collection interface; it is specific to the List interface, which provides positional access to elements. The Collection interface includes methods such as add(), remove(), and size().

The Collections.unmodifiableCollection method can be used to make an existing collection immutable. Any attempt to modify the returned collection will result in an UnsupportedOperationException.

TreeSet is a class that implements the NavigableSet interface and is based on a TreeMap. It allows elements to be accessed in their natural order or by a comparator provided at the time of the set’s creation.

The CopyOnWriteArrayList class provides a thread-safe variant of ArrayList where all mutative operations (add, set, etc.) are implemented by making a fresh copy of the underlying array. Vector is also thread-safe but uses synchronized methods.

To define a natural order for a custom class, the class should implement the Comparable interface. This requires implementing the compareTo(T o) method, which compares the current object with the specified object.

The Set interface does not allow duplicate elements. This is a key characteristic of sets in the Java Collections Framework. Both HashSet and TreeSet implement the Set interface, ensuring that no duplicates are stored.

The clear() method is used to remove all elements from a collection. This method is available in various collection classes such as ArrayList, HashSet, HashMap, etc.

LinkedList can be used to implement a queue (FIFO) because it provides methods like addLast() and removeFirst(), which help maintain the FIFO order. Stack is LIFO (last-in-first-out), and HashSet and TreeSet do not guarantee any specific ordering.

Arrays is not part of the Java Collections Framework. It is a utility class that provides methods for manipulating arrays (such as sorting and searching). ArrayList, HashMap, and TreeSet are part of the Java Collections Framework.

The remove(Object o) method is used to remove the first occurrence of a specified element from a List. If the element is not found, the list remains unchanged.

CopyOnWriteArraySet is a thread-safe variant of HashSet. It is backed by a CopyOnWriteArrayList and is designed for use cases where set size is typically small and iteration is more common than modification.

The add(int index, E element) method is used to insert an element at a specific position in a List. The other add() method adds elements to the end of the list, while put() and set() are used in different contexts (Map and modifying an element in a List, respectively).

The Properties class is not considered a part of the Java Collections Framework. It extends Hashtable and is used to maintain lists of values in which the key is a string and the value is also a string. ArrayList, Hashtable, and TreeMap are part of the Collections Framework.

A Binary Heap is the most suitable data structure for implementing a priority queue. It allows efficient retrieval and removal of the highest (or lowest) priority element.

LinkedHashMap maintains the insertion order of its elements. This means that when iterating over the map, elements are returned in the order in which they were inserted.

LinkedList can be used to implement a FIFO queue as it provides methods such as addLast() and removeFirst(), which maintain FIFO order.

Fail-fast iterators throw a ConcurrentModificationException if the collection is modified while iterating. Fail-safe iterators, on the other hand, operate on a copy of the collection and do not throw exceptions when the collection is modified.

An unmodifiable collection does not support operations that modify the collection, such as adding or removing elements. It only allows operations that do not alter the collection, like iteration and retrieval.

HashSet is the best collection type for storing a collection of unique elements with no particular order. It does not allow duplicate elements and has a constant time complexity for basic operations.

The Collections.shuffle(List<?> list) method randomly permutes the elements in the specified list, effectively shuffling them into a random order.

The Map interface does not extend the Collection interface. It is a separate interface that deals with key-value pairs. List, Set, and Queue all extend the Collection interface.

The sort() method in the Collections class is used to sort a List. It sorts the list according to the natural ordering of its elements or by a specified Comparator.

LinkedHashMap is typically used for implementing a LRU cache. By overriding the removeEldestEntry method, you can specify the conditions for removing the oldest entries when new entries are added.

The Map interface in Java provides the ability to store elements in key-value pairs. It is a collection that maps keys to values, with no duplicate keys allowed, but duplicate values are permitted.

The List.of(T… elements) method, introduced in Java 9, returns an immutable list containing the specified elements. Arrays.asList() returns a fixed-size list, which is not the same as an immutable list.