Lecture 5

Question 1

What assumptions are made about collection elements?

Answer 1

they have a proper equals()

they have a proper hashCode() method for hashed collections

they have a proper compareTo() method for sorted collections

Question 2

how does the method removeIf work with collections?

Answer 2

takes a predicate and removes any element if true

myList.removeIf(x->x < 0);

Question 3

What exception is thrown if an illegal element is added to a collection?

Answer 3

ClassCastException

Question 4

In a List, elements are ordered

T or F

Answer 4

T

Question 5

In a List, elements may be duplicate

T or F

Answer 5

T

Question 6

add() inserts a new element at the front of a List

T or F

Answer 6

F - at the back

Question 7

How do you get List elements between certain indices?

Answer 7

sublist(fromIndex, toIndex)

inclusive, exclusive

Question 8

what does unmodifiableList() method do?

Answer 8

return a read only view of a collection

Question 9

what does synchronizedList() do?

Answer 9

return a syncrnoized, thread safe view of a collection

Question 10

what does checkedList() method do?

Answer 10

return a dynamically type safe view of a collection

Question 11

How can you give a method a variable number of arguments?

Answer 11

varargs:

public static int max(int … elements){

int result = Integer.MIN_VALUE;

for (int i : elements)

if (i > result) result = i;

return result

}

Question 12

How are vararg parameters passed?

Answer 12

In an array

Question 13

public static void someMethod(int … elements, int c)

^ this signature is a legal method signature in Java

T or F

Answer 13

False - varargs must be in last position

Question 14

What does @SafeVarargs annotation do?

Answer 14

suppresses compiler warnings about possibly unsafe use of varargs related to subtyping

Question 15

Object[] Collection.toArray() returns an array of objects that can be modified without changing the original collection

T or F

Answer 15

T

Question 16

what does Arrays.asList(1, 2, 3); do?

Answer 16

Returns a List<integer> with elements 1, 2 and 3</integer>

Question 17

Queues are typically LIFO (last in, first out)

T or F

Answer 17

F: Usually FIFO except for priority queues

Question 18

In a priority queue,

peek(), element() and size() take what time?

add(), remove(), offer(), poll() take what time?

remove() and contains() take what time?

Answer 18

constant time

O(log(n)) time

linear time in size of queue

Question 19

How do priorityqueues order elements?

Answer 19

Based on some priority for each element where the head has the lowest value

Question 20

What is an ArrayBlockingQueue<e>?</e>

Answer 20

bounded FIFO queue backed by an array

bounded = can’t change size after creation

new elements inserted at end

Question 21

What two blocking operations exist for an ArrayBlockingQueue?

Answer 21

attempts to add an element into a full queue block until the queue is no longer full

attempts to take an element from an empty queue block until the queue contains an element

Question 22

A circular buffer is often implemented using an ____________

Answer 22

ArrayBlockingQueue

Question 23

Java contains a blocking queue backed by a linked list

T or F

Answer 23

T

Question 24

BlockingQueue implementations are never thread safe

T or F

Answer 24

F - guaranteed to always be thread safe

Question 25

What is a Set?

Answer 25

collection with no duplicates

two sets equal if contain same elements

Question 26

what is a sortedset?

Answer 26

A set, where elements are ordered based on their comparator

Question 27

BitSet in Java implements Collection interface

T or F

Answer 27

F

Question 28

What is the difference between LinkedHashMap and HashMap?

Answer 28

Linked one stores elements in the order they are added. Not so with HashMap

Question 29

HashMap, LinkedHashMap and TreeMap are not cloneable

T or F

HashMap, LinkedHashMap and TreeMap are not synchronised

T or F

Answer 29

F

T

Question 30

How do you constrain type value in generic method?

Answer 30

public static <e>&gt; E min(E[] a) { ... }</e>

Question 31

When is an ArrayStoreException thrown?

Answer 31

When an attempt is made to store an element of the wrong type in an array

Question 32

What is covariant subtyping?

Answer 32

A T[] can be used in place of an S[] as long as T is a subtype of S

Question 33

Arrays do/do not have covariant subtyping in Java

Collections do/do not have covariant subtyping in Java

Answer 33

do

do not

Question 34

What is the Optional class?

Answer 34

Optional<t> which may or may not contain a value of type T</t>

method isPresent returns true if non-null value is present

method get() gets the value if it is not null

Question 35

Optional type is serialisable

T or F

Answer 35

F

Question 36

What is PECS?

Answer 36

Producer extends, consumer super

So if a method on a collection is a producer, it should be List extends T>

If the method is a consumer, it should be List super T>