Java

Front 1

What are the access specifiers?

Back 1

package

public

private

protected

Front 2

How about class types

Back 2

Anonymous

Inner

Top

Abstract

Front 3

Hashset vs. Treeset

Back 3

Hashset is not ordered, Treeset is ordered. Tresset add, remove and contains methods have O(logn) complexity. Hashset has O(1) complexity

Front 4

Difference between yield() and join() in threads

Back 4

* yield() - tell VM that willing to let other threads be scheduled in its place

* join() - Start executing when the other thread ends

Front 5

Vector vs. other collections? Why are collections better than vectors?

Back 5

Collections in the framework now use read locks as opposed to read/write locks

Front 6

What is the benefit of Generics at runtime (if any?)

Back 6

No typecasting required (typecasting requires JVM to look up class loaders)

Front 7

What are weak references and how do you use them?

Back 7

Use weak reference for a cache, for example. The JVM can free up the memory and you can get the data again if it goes away

Front 8

Stack vs. Heap?

Back 8

tbd

Front 9

Hashtable vs. Hashmap

Back 9

Hashtable is synchronized

HashMap can contain null values and keys, table cannot contain nulls at all

Front 10

Mechanisms for maintaining sessions?

Back 10

* URL rewriting (pose security risk)

* Cookies (preferred)

Front 11

Servlet lifecycle methods

Back 11

* init()

* service() --> doPost(), doGet()

* destroy() {Called once}

Front 12

Can an interface contain a variable?

Back 12

Yes they can (but probably shouldn't!). If so, they must be static because interfaces cannot be instantiated.

Can, but doesn't require a final specifier

Front 13

Can you define an enum inside an interface?

Back 13

Yes! Moreover, you can define methods inside that enum which is inside the interface ;)

Front 14

What are the major differences between doPost and doGet. When to use which?

Back 14

doPost() - parameters passed inline and are encrypted. No maximum size for data! Slower b/c does not write content length. Does not have to be idempotent

doGet() - Params appended to URL. Faster. SHOULD BE IDEMPOTENT

Front 15

What does: Executor.execute(Runnable) do?

Back 15

Executes asynchronously

Front 16

What does Executor.submit(Runnable) do?

Back 16

Executes, returns Future object. You can use this object to check when the thread ends

Front 17

What does Executor.submit(Callable) do?

Back 17

Executes, returns Future object. The Callable implementation can return a value!

Front 18

Using executors, how can you track the return value of a thread method call?

Back 18

Use Executor.submit(Callable) [[Not submit(Runnable) ]]!

Front 19

What does Executor.shutdown do?

Back 19

The Executor service will not shut down immediately, but will no longer accept new tasks. Once all threads have finished current tasks, the ExecutorService shuts down.

Front 20

How can you terminate an Executor immediately?

Back 20

Executor.shutDownNow()

Front 21

Explain the access specifiers:

* public

* protected

* private

* default

Back 21

public - accessible by anyone

protected - this class, or sub class

private - only this class

default - any class in this package

Front 22

What is a CountDownLatch?

Back 22

A kind of synchronizer which allows one Thread to wait for one or more Threads before it starts processing. (Not reusable once the count reaches zero!)

Example:

CountDownLatch latch = new CountDownLatch(3);

latch.await(); // Main thread waits:

// Each thread calls:

latch.countDown()

Front 23

What is a CyclicBarrier

Back 23

CyclicBarrier is initialized with the number of threads to wait for the barrier. Once the barrier is reached by all threads, they will all process.

Each thread calls await(), then blocks. When all threads called await(), they all cross the barrier and continue processing

Front 24

What is the advantage of new Lock interface over synchronized block in Java?

Back 24

Lock method tryLock allows method to back out if the lock is not available immediately or before a timeout expires. - good for solving deadlock problem.

Locks are not auto released like a sync block is. You need to explicitly release them (try-finally)

Front 25

What are the differences between sleep() and wait()

Back 25

sleep() is used for pausing. Locks are not released. wait() is used for inter-thread communication. Locks are released during a wait().

Front 26

what is volitile?

Back 26

guarantees visibility of changes to variables across threads (ensures each thread gets the value from memory and not thread cache).

Should also use synchronize with these variables.

Front 27

Why we call start() method which in turns calls run() method, why not we directly call run() method ?

Back 27

calling start() will instantiate a new thread and call run. Calling run directly calls it on the current thread!!