Iterator Pattern

A design pattern that uses an iterator to traverse the elements of some collection without exposing its underlying implementation.

Examples:

Iterators can be used to traverse linked lists, trees, maps, etc. Same interface, different underlying implementation.

Interface for Iterator/Main methods on Iterator:

hasNext()

next()

remove()

Sudo vs Su

su: a command line tool that allows you to switch users. It actually stands for "switch user".

sudo: a command line tool that allows you to run a command as the super user/root.

Gang of Four

Interviewer: Who are the gang of four?

Interviewee: A music group that plays really awesome songs!

Interviewer: FAIL

Interviewee:

Don't let the above happen to you : )

The "gang of four" are 4 authors that wrote a famous design pattern book called "Design Patterns: Elements of Reusable Object-Oriented Software"

Mutex vs Semaphore

Mutex: Mutual Exclusion Lock. Allows only 1 thread at a time to enter the protected area/controlled section. Once a thread has obtained a lock, other threads will wait until that thread exists the controlled section and releases the lock. The thread owns/obtains the lock, and only that thread can release it. Only one thread can own a lock a given point. Locking mechanism to control access to a resource.

Mutexs help prevent race conditions. The example usually given here is depositing money into a bank account.

A mutex is essentially a semaphore with value 1, except it differs in how threads "own" the lock.

Semaphore: Allows up to N threads to enter the protected area. Threads increment/decrement the semaphore when they enter the concurrent area and when they leave. So multiple threads can own the lock. More like a signaling mechanism with keeping track of count to access the shared resources.

Producer/Consumer example.

Object in Java

In Java, it's the root class. All objects implement the methods of this class.

clone(), creates and returns a copy of the object

equals(Object), tests whether the two objects are equal to each other.

finalize(), garbage collector calls this method when there are no more references to this object

getClass(), returns the runtime class of the Object

hashCode(), hash code value of the Object

toString(), string representation of the Object

Thread related functions:

notify(), notify/awaken a single thread waiting on this object

notifyAll(), notify/awaken all threads waiting on this object

wait(), causes the current thread to wait until another thread invokes notify

wait(timeout), causes the current thread to wait until another thread invokes notify or the timeout is up

Queue, Deque, Stack

Stack (LIFO, Last In First Out)

In Java, the Stack class extends Vector.

Deque (aka Double-Ended Queue), objects can only be added to or removed from the front or back. Doubly linked list is a good implementation structure for this.

In Java, the Deque class extends Queue.

Queue(FIFO, First In First Out)

In Java, the Queue class extends Collection.

LinkedList is a good data structure to implement all of these.

Methods on these classes

push(Object)

pop()

clear()

isEmtpy()

size()

List in Java

LinkedList, ArrayList, Vector, and Stack all implement this interface.

Important methods on this interface:

add(Object)

add(index, Object)

addAll(Collection)

addAll(index, Collection)

clear()

contains(Object)

containsAll(Collection)

equals(Object)

get(index) //return object

indexOf(Object)

isEmpty()

size()

iterator()

subList(index, index) //return List

toArray()

iterator()

set(index, element), replace specified postion with the passed in element

retainAll(Collection)

removeAll(Collection)

remove(Object)

remove(index)

lastIndexOf(Object) //return index of the last occurrence of this object

ArrayList:

clone()

toString()

removeRange(fromIndex, toIndex)

Vector:

clone()

toString()

elementAt(index)

setElementAt(Object, index)

firstElement()

ElementAtIndex(index)

removeAllElements()

removeRange(fromIndex, toIndex)

Stack, last-in-first-out (LIFO), extends vector

empty()

peek()

pop()

push()

search(object)

LinkedList //implements Deque Interface

descendingIterator - Returns an iterator over the elements in this deque in reverse sequential order.

removeFirst()

removeLast()

removeFirstOccurance(Object)

removeLastOccurance(Object)

peekFirst/Last() //retrieve and don't remove

offerFirst/Last() //insert

pollFirst/Last() //retrieves and removes