Java Programming Interview Guide : 200+ Questions & Answers

158. A PriorityQueue is a concrete implementation of the Queue interface that orders its elements according to their natural ordering or by a Comparator provided at queue construction time. It supports adding and removing elements, as well as retrieval of the least element (according to the priority), with constant-time semantics for these operations.

159. Example implementations of the BlockingQueue interface include:

• LinkedBlockingQueue: A queue based on linked nodes where each insert or removal operation is performed atomically as a single operation upon an entire node, ensuring thread safety without external synchronization.
• ArrayBlockingQueue: A queue with an internal array that bounds the number of elements held in the queue.
• PriorityBlockingQueue: A priority queue where the head of the queue at all times contains the least element.
• SynchronousQueue: A queue that locks on both submit and poll operations, ensuring that they happen one after another, effectively allowing only one thread to operate on the queue at a given time.

160. The Map interface in Java is a collection that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value. The Map interface extends from the Collection interface, which means a map is also a collection because it can be used in any collection operation (like containsKey, putAll, etc.).

161.The main difference between Map and SortedMap is the order in which the mappings are kept. A Map does not specify an order for its entries. An implementation may place no restrictions on the order of the set's iterators. In contrast, a SortedMap maintains its mappings in key order. The SortedMap interface extends Map, and it requires that the keys be sorted according to their natural ordering or by a Comparator provided at map creation time.

162. As of my knowledge cutoff in 2023, there is no direct SynchronousMap in Java's standard library. However, you can implement a thread-safe Map using concurrent collections like ConcurrentHashMap and synchronization mechanisms like Collections.synchronizedMap.

163. A WeakHashMap associates keys and values but, in the absence of manual removal, will remove entries whose keys are no longer strongly reachable (i.e., no part of the running program is holding onto them). This can be useful in memory-conscious applications like caching or keeping track of soft references.

164. A IdentityHashMap differs from a standard Map by treating all null keys as equivalent, and by making all non-null references act as their own identity (i.e., two distinct objects that happen to have the same internal representation will not be treated as equal in an IdentityHashMap).

165. A Hashtable is an legacy implementation of a dictionary or associative array, functioning as a map. It is synchronized and thus safe for concurrent use by multiple threads without external synchronization. However, it is generally recommended to use HashMap, ConcurrentHashMap, or other more recent and efficient implementations over Hashtable for most applications.

166. A LinkedHashMap maintains doubly linked nodes of its entries (and, if values are references, of their corresponding values). This allows it to traverse the map in the order in which the entries were inserted (insertion order), or in the order of access if the access order is maintained (access-order mapping).

167. A TreeMap automatically sorts keys upon iteration and maintains a Red-Black tree to keep its keys sorted at all times, ensuring that the keys are sorted according to their natural ordering or by a Comparator provided at map creation time.

168. A ConcurrentHashMap provides concurrency lock-free, synchronization-free operation for most methods, and thus significantly reduces contention and improves performance for large data sets or when mapping objects to both true and false (which would require locking within a traditional synchronized map).

169. A HashMap is a hash table implementation that associates keys with values. It does not guarantee the order of its entries, which can be useful for applications that are sensitive to iteration order but do not require key-sorted behavior.

170. A HashMapInstance typically has an initial capacity (which can be set to a specific size) and a load factor that determines the resizing condition. When the number of elements in the map exceeds the capacity times the load factor, the map is rehashed with more buckets.

171. The HashMap class provides a way to iterate over its entries via its entrySet() method, which returns a Set view of the mapping between keys and values. This set's iterator returns Map.Entry objects, allowing access to both keys and their corresponding values.

172. A LinkedHashMap is useful when you want to iterate over elements in the order they were inserted or accessed, which can be important for implementing least-recently-used (LRU) caches, among other things.

173. The ConcurrentHashMap class is designed to replace Hashtable in performance-critical applications where threads access a data source simultaneously, as it provides the same guarantees but significantly better performance.

174. A SoftMap or WeakHashMap can be created using Collections.synchronizedSortedMap(new TreeMap<>()) or Collections.synchronizedSortedMap(new WeakHashMap<>()), respectively, while ensuring thread safety with synchronization. However, these are not standard Java collections and require additional import statements (java.util.Collections).

175. The EnumMap is a specialized map class designed to hold entries where keys are of an enum type. It has the same functionality as other Map implementations but uses the enum constants for its keys, which can offer a more natural and type-safe way of organizing data.

176. The AbstractMap class serves as a base class for implementing custom map classes. It is an abstract class that provides the basic structure to create your own mapping from keys to values.

177. A Stream<T> is a sequence of elements representing a lazily evaluated, ordered collection of primitive elements or objects. The behavior of streams and their operations is defined by the java.util.stream.Stream interface and its specialized variants for primitive types (e.g., IntStream, DoubleStream, etc.).

178. A Function<T, R> represents a mathematical function that maps an input of type T to an output of type R. In functional programming terms, it's a unary operation. Functions are first-class citizens in Java 8 and above, which means they can be passed as arguments, returned from methods, and composed or chained together.

179. The Collectors class provides methods to collect the elements of a stream into various data structures such as lists, sets, maps, etc., or to reduce the elements to a single value, which is useful for the terminal operations of the streams API (e.g., collect()).

180. The Optional<T> class provides a way to represent an optional value—a box that may or may not contain a non-null value. It is used to avoid NullPointerException and to express intent that a certain operation might not find a result, thereby making the intention of your code clearer to other developers.

181. A Predicate<T> represents a predicate (a mathematical function that produces a boolean value). It's a functional interface with a single method test(T t) that checks if the given input t is an element on which the predicate operates.

182. A Consumer<T> is a functional interface that accepts a single input argument and returns no result (i.e., it is a void-returning operation). It's useful for performing actions on the elements of a stream or any other input, such as logging information, without having to wrap these operations into a class.

183. A Supplier<T> is a functional interface that supplies a result (a value of type T). Its single method get() does not take any arguments and returns the computed or retrieved value. It's useful for providing a lazy-evaluated value, such as generating a random number, fetching a value from the database, or reading a file.

184. A BiConsumer<T, U> is a functional interface that accepts two input arguments and returns no result. It can be used to operate on pairs of elements without needing to define an explicit class for this purpose.

185. A BiFunction<T, U, R> is a functional interface that takes two arguments of types T and U and produces a result of type R. It's useful when you need to apply a function to pairs of values.

186. A BiPredicate<T, U> represents a predicate that takes two arguments of types T and U and produces a boolean value (true or false).

187. A ToIntBiFunction<T, U> is a functional interface that maps a pair of input elements (of types T and U) to an int value. It's one of the specialized variants provided in Java'9 for handling primitive types in the streams API for better performance.

188. A BiConsumer<T, U> is used when you need to operate on pairs of elements within a stream and perform some action that doesn't return a result (like updating a database with two related values).

189. The stream() method can be called on any object that implements the Streamable interface, including collections like lists, sets, and maps, arrays, or even generating streams with methods like Stream.generate(Supplier<T> s).

190. Parallel streams are a feature introduced in Java 8 that allows you to take advantage of multiple threads to process streams more efficiently when dealing with large datasets or CPU-bound operations. This is achieved by dividing the stream into partitions and processing each partition in parallel.

191. The parallelStream() method on collection classes creates a parallel stream, which can then be processed using the operations defined by the Stream API. Parallel streams are particularly useful when performing operations that can be easily split into smaller tasks, such as filtering or mapping, and when the overhead of parallelism is outweighed by the performance gains from concurrent processing.

192. A Collector<T, A, R> is a functional operation that takes elements of type T, accumulates them into an intermediate accumulator of type A, and then produces a result of type R. It's used to combine stream operations into a single associative and identity-valued operation.

193. A java.util.concurrent package provides a set of concurrency utilities that facilitate the development of multithreaded programs through the use of concurrent collections, locking mechanisms, atomic variables, and executors.

194. An ExecutorService is an abstraction for executing tasks concurrently. It can execute each submitted task in a new thread or hand it off to a pool of worker threads. It's used to manage a pool of threads and schedule tasks to be executed within those threads.

195. A ScheduledExecutorService extends the functionality of an ExecutorService by providing methods for scheduling commands to run after a delay, at fixed intervals, or at a specified fixed rate.

196. The CompletableFuture<T> class is introduced in Java 8 as part of the java.util.concurrent package, providing a way to write asynchronous and non-blocking code for Java applications. It represents a future that may eventually hold a result or be cancelled.

197. A CompletableFuture can be completed with a value (thenAccept(Action afterAccept)) or an error (thenApply(Function<Throwable,T> fn)), and it can transform values using methods like thenApply(Function<? super T,? extends U> fn) or combine results with other futures using methods like thenCombine(CompletableFuture<?> other, BiFunction<? super T, ? super R, V> resultCombiner).

198. The ForkJoinPool class is a concurrent task executor for recursively splitting tasks until they are small enough to execute directly. It's designed for high-level parallelism and is particularly well suited for the "fine-grained" division of tasks characteristic of recursive algorithms.

199. A ForkJoinTask<V> is a functional interface representing a task that can be divided into smaller subtasks and executed in a pool of threads in a ForkJoinPool, or executed directly if it's small enough (i.e., its compute size is less than a threshold).

200. The java.util.concurrent.atomic package contains classes for thread-safe immutable objects that support high-performance lock-free programming on single variables, which can be used to implement more efficient concurrent data structures and algorithms.