Java API

Question 1

API offered by Queue Interface

Answer 1

add(E element) - insert element into queue if possible (else exception)

E element() - retrive but not remove the head (exception wheen empty)

boolean offer(E) - add if possible (else return false)

E peek() - same as element(), but returns null when empty

E poll() - similar to peek, but removes the element

E remove() - similar to poll, but throws exception when empty

Question 2

API offered by the Deque interface

Answer 2

addFirst(E)/addLast(E) - add element if possible, otherwise exception

offerFirst(E)/offerLast(E) - add element if possible, otherwise false

E removeFirst()/removeLast() - remove element, otherwise exception if empty

E pollFirst()/pollLast() - retrive and remove element or null

E peekFirst()/peekLast() - retrive element or null

E getFirst()/getLast() - retrieve element or exception

Question 3

What are the Stack equivalent methods of a Deque?

Answer 3

push(E) -> addFirst(E)

E pop() -> E removeFirst()

E peek() -> E peekFirst()

Question 4

Which class should be used instead of a Stack and why?

Answer 4

A Deque (a LinnkedList for example) as it is more “complete” and “consistent”, and a Stack is an extension of the Vector class, which is not recommended when used in a non-concurrent environment.

Question 5

Can you sub-class a final class in java?

Answer 5

No, it is not allowed.

Question 6

How can you prevent a class from being subclassed without declaring it final?

Answer 6

Make its constructor(s) private, thus subclasses won’t be able to inherit from it.

Question 7

When is the finalize method called in java?

Answer 7

Before the class gets garbage collected.

Question 8

PriorityQueue class API

Answer 8

add(E) - same as offer

clear()

offer(E)

E peek() - retrive but does not revome or null

E poll() - retrive and remove or null

T[] toArray(T[]) - convert to an array

Question 9

Can you iterate over a PriorityQueue using an Iterator to retrive elements in ascending or descending order?

Answer 9

No. It supports iterator, but its ordering is not guaranteed or sorted. If you need to access the elements in a specific order, then you have to convert it to an array usiing toArray(T[]) and then sort the array using Arrays.sort()

Question 10

Describe some of the Scanner API methods.

Answer 10

boolean hasNext() - are there more tokens?

boolean hasNextInt()/hasNextBoolean()/… - next token can be interpreted as int/boolean/…

next()/nextLine()/nextInt()/nextBoolean()/… - read next token as String/int/boolean/etc

Question 11

Lock interface API

Answer 11

void lock() - acquires lock

Condition newCondition() - return a new condition bound to this lock

tryLock() -

tryLock(long time, TimeUnit unit) - trys to lock

unlock() - releases this lock

Question 12

ReadWriteLock interface API

Answer 12

Lock readLock() - obtains a read lock

Lock writeLock() - obtains a write lock

Question 13

Which classes implement the Lock interface?

Answer 13

ReentrantLock

ReentrantReadWriteLock

Question 14

Semaphore class API

Answer 14

Semaphore(int permits) / Semaphore() - constructor

acquire() - acquire a permit or wait until one is available

acquire(int permits) - acquire more than one permit

release()/release(int permits) - releases a permit

boolean tryAcquire()/tryAcquire(int permits) - tries to acquire pemits (all or nothing)

Question 15

CountdownLatch API

Answer 15

CountdownLatch(int count) - constructor

await()/await(int time, TimeUnit units) - waits for counter to reach zero

countDown() - decrements the count of the latch

Question 16

What is the ordering of elements of the PriorityQueue class? Which one it returns first?

Answer 16

It returns the least element of its contents.