Step by step guide to Java Collection Framework interview questions and FAQs..

8.What are the primary classes that implement the List interface in Java?
There are several important classes in Java that implement the List interface each designed to handle different scenarios for storing and accessing elements:
ArrayList: This is a dynamic array that grows only when needed. It is effective for applications involving rapid non deterministic access to elements. Adding or deleting items from a list is a slower operation for the beginning and middle of lists because it moves all the rest of the items.
LinkedList: Doubly Linked List implementation whose access time in case of indexing is slower as compared to the ArrayList. Adding or deleting any element at any end is also fast.
Vector: Essentially an ArrayList but is synchronized so it's safe to be accessed by threads. Though much less used now than in years gone by as most modern replacements exist, still sometimes used where backward compatibility has to be supported.
Stack: It is an extension of the class Vector which has implemented last in first out. It will be used for operations where a LIFO element gets added or popped.

9.Which of classes implement the Set interface in Java?

There are many classes in Java that implement the Set interface and each is designed with a particular purpose in mind. The most common implementations are HashSet, LinkedHashSet and TreeSet and each has some unique strengths. A HashSet is like a simple bag-it can quickly check whether an item is already inside, but it doesn't care about the order of the items. This class is very handy for quick add, remove or contains element operations. The LinkedHashSet has a similar function but maintains an order in which elements were inserted. That way, if you iterate over this class, items are returned in the same order in which they have been inserted. A TreeSet has it a notch higher because the elements in the tree will be sorted by default alphabetically or numerically. All of these classes adhere to the fundamental principle of a Set no duplicates allowed. So, if you try to add something that already exists, it won't add it again. In short, depending on your needs you would use the appropriate Set implementation, HashSet when you need speed, LinkedHashSet when the order matters and TreeSet for the case if sorting is important.

10.What is the Iterator interface in Java and how is it used to iterate over collections?
The Iterator interface is in Java what is it used for to step through collections The Iterator interface in Java is designed to traverse through the elements of a collection, whether it is List, Set, or Queue. It provides three key methods that make iterating over collections straightforward and safe:
hasNext(): Checks if there are more elements left in the collection to iterate over.
next(): Retrieves the next element in the collection.
remove(): Removes the last element returned by the iterator. This operation is optional and not all collections may support it.
The Iterator lets you access collections without concerning yourself with underlying structures, like indices in a List. In this way, it is quite efficient and failsafe, notably when working on collections that will be accessed from multiple threads of execution where safe and unbroken iteration may be important.

11.What is the difference between ArrayList and LinkedList in Java?
In Java, the main difference between ArrayList and LinkedList lies in their internal data structures and the resulting performance for different operations.
ArrayList is based on a resizable array to hold its elements. This design makes it fast to access elements at random positions in O(1) time. This makes it useful when accessing items is essential, such as when doing lookup in a collection. On the other hand, it takes O(n) time for insertion or deletion operations at arbitrary positions because elements have to be shifted.
LinkedList is a doubly linked list, and every node contains pointers to its preceding node and following node. This data structure permits fast insertion and deletion from either end of the list, which has an O(1) time complexity. Access by index, on the other hand, takes linear time, or O(n).
In essence, ArrayList would then shine with fast access when needed to elements, while LinkedList would be the star if you need high frequency of insertions and deletions.

12.What is the Vector class in Java? How is it different from ArrayList?
The Vector class is similar to ArrayList but with a few main differences:
Thread Safety: Vector is synchronized. This ensures that only one thread can access the list at a time, so it's safe to use in a multi-threaded environment. But this comes at a performance trade-off because operations may be slower than those for ArrayList, which is not synchronized.
Resizing Behavior: The two classes differ in how they handle resizing. Vector doubles its size when it runs out of space, whereas ArrayList only increases its size by 50%. This means Vector can potentially use more memory than ArrayList, particularly when the list size is considerably large.
Legacy Status: Although Vector was a part of the original Java collection framework, it is now legacy. In contrast, ArrayList is a part of the modern java.util package and is used much more often due to its better performance in most use cases.

13.How does ArrayList handle dynamic resizing in Java?
ArrayList in Java automatically handles resizing when more elements are added than it can accommodate. Once the internal array becomes full, the ArrayList grows to a larger, usually by a factor 1.5 of the existing size, creating a new array in memory, copy elements into a new array and continues the pattern. In that sense, the growth allows the addition of new elements by providing an allocation of further space while inducing some performance hit. Every time the list changes size, its elements have to be copied over to a new array; such operations slow down when data becomes large in number.

14.When to use LinkedList over ArrayList in Java?
You should use LinkedList over ArrayList in Java if your application performs insertions or deletion operations very often especially for a long list at the beginning or middle. As it is efficient for deleting and inserting elements at one of the ends with O(1) and faster than ArrayList for modifying elements in the middle. But LinkedList has its trade off it is slower on random access because it has to traverse the list, while an ArrayList is based upon an array and thus very fast with access (O(1)). So, if your use case is more towards frequent changes of the list and not so much for quick element access then LinkedList would be the better choice.

15.What is the Stack class in Java and how does it work?
The Stack class is a class in Java that extends Vector. It implements the LIFO data structure. The class java.util belongs to the package which offers several significant methods for stack operations.
push(E item): Adds an element on top of the stack.
pop(): Removes the top element and returns the top one on the stack if empty it throws an exception.
peek(): Returns top of the stack, does not remove.
empty(): Returns if this stack is empty.
Although Stack is part of the legacy collection classes in many places the stack is actually replaced by a more up to date collection class, such as Deque that has the addFirst() and removeFirst() methods and may be used as a stack.

16.What is the difference between HashSet, LinkedHashSet and TreeSet in Java? All of these three classes implement the Set interface but they differ from each other concerning the handling of element order:
HashSet: It is the most basic implementation of a set and doesn't offer a specific order of elements. It uses a hash table for storage that provides O(1) time complexity for common operations such as add, remove contains assuming a good hash function is in place.
LinkedHashSet: This implementation maintains the insertion order of elements combining a hash table with a linked list. Therefore, the order of elements is predictable based on their insertion order. It still offers constant time complexity (O(1)) for basic operations but ensures elements are iterated in the order they were added.
TreeSet: This is unique because it keeps the elements sorted. It is based on a Red-Black tree and provides O(log n) time complexity for add, remove and contains so it's great for any application where you want elements to be in a sorted sequence.

17.How does HashSet maintain uniqueness of elements in Java?
The uniqueness of the elements of the HashSet is ensured using a hash table. At the time of addition it computes the hash code for the added element. Then it searches for any existing element with the same hash code in the set. If any element already exists with the same hash code, it will not be added into the set because there would have been a duplication of an element. If no element is found with the same hash code, then that element is added to the set. This mechanism is efficient because it ensures that only unique elements are stored, and the hash code plays a crucial role in checking for duplicates.

18.What is the difference between a SortedSet and an ordinary Set in Java?
A SortedSet is a Set interface that further extends the element storage in such a way that its elements have a specific order. The order can be implemented either based on the natural ordering of the elements if they are instances of Comparable or by using a comparator provided explicitly or implicitly. The main distinction between a SortedSet and an ordinary Set is that a SortedSet provides other methods that allow it to better handle its sorting.
These methods include:
first(): Returns the first (lowermost) element in the set.
last(): Returns the last (highest) element in the set.
headSet(E from, toElement): Returns a subset of elements that are strictly less than the specified element.
A typical usage of SortedSet is TreeSet.

19.Why does TreeSet elements maintain sorted in Java?
In TreeSet, to maintain its element in sorted, it uses an implementation of red-black tree of self-balancing binary search. This permits elements to be ordered automatically either according to their natural ordering or according to a comparator provided by the user. The Red-Black tree guarantees that this set remains balanced in the sense that all operations are performed with a time complexity of O(log n) for insertion, removal, and searching. When elements are added, the tree automatically orders itself to maintain the sorting; therefore, the elements of a TreeSet are always ordered.

20. What is the fundamental distinction between HashMap and TreeMap in Java?
Following are the major differences between HashMap and TreeMap.
Order: HashMap doesn't maintain any order of its elements. The entries in TreeMap are kept in ascending order of their keys. A TreeMap can be ordered either using a natural ordering for its keys or by a comparator provided by users for ordering of keys.
Performance: HashMap provides an average time complexity of O(1) to most operations considering a good hashing function. As against this, the operations carried out in the TreeMap are found to be within O(log n) time, since it resorts to a Red-Black tree to maintain a state of order.
Null Keys/Values: It is possible for HashMap to provide one null key and multiple null values. Meanwhile, TreeMap would not accept any null key though it can retain null values.

21.What is a Java LinkedHashMap and how is this class different than HashMap?
Java LinkedHashMap class implements HashMap. Elements are ordered, which is ordered based on an insertion order that has been achieved in this implementation. Although it's using the hash table and therefore can take direct access to store the key - value pairs inside the HashMap implementation, it orders them. LinkedHashMap keeps a linked list of its entries, so elements are iterated in the same order they were added, making its iteration predictable. HashMap does not guarantee any specific order.
Key differences include:
Order: HashMap does not maintain any order, while LinkedHashMap preserves the insertion order.
Performance: The performance of LinkedHashMap is slightly less efficient than HashMap because it involves additional overhead in maintaining the linked list, which slows down operations.

22. How does HashMap handle collisions and what is its internal structure?
HashMap handles collisions using a technique called chaining. In this technique, multiple entries are stored in the same bucket if their hash codes conflict. The internal structure of a HashMap is an array of buckets, with each bucket holding either a linked list or since Java 8 a balanced tree. The choice of using a linked list or tree depends on the number of elements in the bucket. When a new key-value pair is added HashMap computes the hash code of the key and identifies the appropriate bucket. If two or more entries map to the same bucket due to a hash code conflict, they are stored together in a linked list or tree within that bucket.
This collision management method allows HashMap to function efficiently.
However, if many entries with the same hash code end up in one bucket, performance may decline, especially if a linked list is used instead of a tree structure.

23. What are the differences between HashMap and ConcurrentHashMap in Java?
The core difference between HashMap and ConcurrentHashMap is their approach to thread safety:
HashMap is not thread safe. Access and modifications of a map by several threads might bring about unpredictable outcomes. For safe, thread-safe operations with a HashMap, some manual synchronization would have to be added.
ConcurrentHashMap on the other hand is designed to handle concurrent access without any external synchronization. It is thread-safe by default and enables multiple threads to read and write data simultaneously without corrupting the map’s contents. This is possible because the map is divided into several smaller segments, each of which can be locked independently. This segmentation strategy enhances concurrency compared to locking the entire map at once.
Whereas HashMap is fine for single-threaded applications or if the synchronization is being done from elsewhere, ConcurrentHashMap is best suited for multi-threaded applications where the thread safety is a concern.

24. How does the Hashtable class differ from HashMap in Java?
The Hashtable class is one of the older classes in Java that performs very much like HashMap but has differences, primarily due to its synchronization.
Here's how they differ:
Synchronization: Hash table is a synchronized Hash table is thread-safe but slow in performance. HashMap is not synchronized, so if thread safety is not an issue, it performs better.
Null Values and Keys: A Hash Table does not accept a null key or null value whereas HashMap accepts one null key but accepts multiple null values.
Legacy: Hashtable is part of the first versions of Java and is now considered legacy. Whereas HashMap was introduced as part of Java Collections Framework by Java 1.2 and is used in most real time Java applications.
Although Hashtable is thread safe its performance limits and unavailability for null keys or values make HashMap the better option in most cases. In case of necessity to have it thread-safe, an alternative option will be a ConcurrentHashMap which would be more efficient.

25. What are Java Collections generics and how do they ensure type safety?
Java provides the ability to write code that can handle a variety of types of data yet retain type safety through the use of generics. Using type parameters you define the types of elements that should exist in any collection in order to prevent runtime errors at compile time due to incompatibility of types.
Example
List<String> list = new ArrayList<>();
list.add("Hello");
// list.add(1); // Compile-time error
Here, List<String> means that the list is allowed to hold only String objects. If you try to add a number or any object other than a string, you'll get a compile time error. This way, you'll avoid runtime issues of ClassCastException because your code will be safer and easier to maintain.

26. How does the Comparable interface facilitate in Java Collections for ordering objects?
It can be used for establishing a natural order for the objects of the class in Java. You set how one object should be compared to another while implementing the compareTo() method. This comparison method allows objects to be sorted within collections like lists and sets based on the predefined order.
Here's an example:
public class Person implements Comparable<Person> {
private String name;
private int age;
@Override
public int compareTo(Person other) {
return this.age - other.age; /*Sorting by age */
}
}
By implementing the Comparable interface of the Person class, instances of Person will now automatically be sorted on age when being added to a sorted collection, such as a TreeSet or an ArrayList with Collections.sort().

27. What does the Comparator interface do while sorting Java Collections?
Whereas the Comparable interface defines a natural order for objects the Comparator interface lets you define your own sort order. That comes handy when you want to sort objects in a way that is different from the default comparison or when objects are not even implementable using Comparable.
Here's how it works:
public class Person
{
private String name;
private int age;
// Constructor and getters
public static Comparator<Person> compareByName = new Comparator() {
@Override
public int compare(Person p1, Person p2) {
return p1.name.compareTo(p2.name); /* Sorting by name */
}
};
}
For instance, in the following example the Comparator class is used to sort Person objects based on the name property: Unlike Comparable which can define only one order Comparator offers you the ability to sort objects by different attributes or in a variety of ways when the class doesn't implement Comparable.

28. What are the differences between synchronized collections and concurrent collections in Java?
In Java, Synchronized Collections ensure that a collection is thread safe but work differently than concurrent collections and affect performance negatively.
A synchronized collection will lock the entire collection whenever any thread accesses or modifies it so only one thread can access or modify it at a time. Though this ensures that the collection is thread-safe, it then creates performance problems if many threads try to access the collection concurrently. Such collections are mostly parts of older Java classes, like Hashtable or Vector or may be explicitly synchronized using methods such as Collections.synchronizedList().
The collections are designed to be high concurrency. It supports multiple threads concurrently accessing the collection, hence provides better performance without sacrificing thread safety. Such advanced techniques as fine grained locking and lock-free algorithms are used by concurrent collections to ensure safe concurrent access.
ConcurrentHashMap, CopyOnWriteArrayList and BlockingQueue are some of the examples of concurrent collections optimized for high concurrency operation without sacrificing some typical performance lost inherent in synchronized collections.

29. How can the thread safe operations be done in Java on collections?
Thread safety becomes a significant feature while working on collections in a multi-threaded application using Java.
Here's how your operations on the collections remain thread safe:
Using synchronized collections: Java provides intrinsic methods such as Collections.synchronizedList() or Collections.synchronizedMap() that can wrap a collection so that only one thread will be able to access or modify the collection at any one time. This is useful when you want to control access to a collection between different threads.
List<Integer> synchronizedList = Collections.synchronizedList(new ArrayList<>());
Leverage concurrent collections: Java gives collections from the java.util.concurrent package for high performance, in particular in situations of high concurrency. Such collections include ConcurrentHashMap and CopyOnWriteArrayList that are designed for allowing multiple threads to access and modify the collection simultaneously without the overhead of synchronization.
Manual synchronization: Sometimes, you will need to explicitly synchronize parts of your code to ensure thread safety. By synchronizing specific blocks of code you can control when a thread is allowed to modify a collection thereby avoiding conflicts between threads.

30. What are common collection algorithms in Java (such as sorting and searching)?
Java offers several algorithms to interact with collections effectively. Some of the most commonly used algorithms are as follows:
Sorting: You use the Collections.sort() method for lists, for example ArrayList or LinkedList. Naturally ordered collections such as TreeSet or TreeMap, automatically perform sorting. You may define how the list will be sorted by passing a Comparator or use natural ordering if its elements are implementing the Comparable interface.
Finding: Java makes search in collections efficient. For sorted lists you can use Collections.binarySearch() to find the index of an element. For hash-based collections like HashSet and HashMap, lookup operations are performed in constant time, which assures very fast searching based on the element's hash code.
Filtering Streams had greatly made filtering and transforming collections much easier through the introduction of the Stream API in Java 8. You may use methods like filter(), map() and reduce() to process, query collections functional in the ways in which elements can be selected, transformed or combined based upon certain conditions.
Shuffling: You can use Collections.shuffle() to shuffle elements in a collection randomly. This comes in pretty handy especially if you need to shuffle the elements in a List within a card game or any application that uses cards or when generating a random dataset.
These are algorithms for working with Java collections for manipulation and interaction. This will enable you to manage, query, and update data efficiently.