3. Java 8: Filtering Collections with Lambda's

Question 1

How could you loop over the following list and print their values using the forEach method?
List names = new ArrayList<>();
names.add(“jordi”);

Answer 1

names.forEach(name -> System.out.println(name));

Question 2

How could you loop over the following map and print the key and values using the forEach method?
Map settings = new HahsMap<>();
settings.put(“setting”, “option”);

Answer 2

settings.forEach((key, value) -> System.out.println(key + “:” + value));

Question 3

What are the four intermediate stream operations that you need to know for the exam?

Answer 3

Stream filter(Predicate super T> predicate);
Stream map(Function super T,? extends R> mapper);
Stream distinct();
Stream peek(Consumer super T> action);

Question 4

How can a stream be used to filter out all the items from a String list that do not start with a “a”. Given the lists name is names.

Answer 4

names.stream()
.filter(s-> s.startsWith(“a”)) // or s::startsWith;
.collect(Collectors.toList());

When the predicate returns true the item passes the filter and will move past it.

Question 5

How can a stream be used to transform all Strings in a list to Uppercase? Given the list name is names.

Answer 5

names.stream()
.map(s -> s.toUpperCase())
.collect(Collectors.toList());

Question 6

How can a stream be used to filter a list so that only the unique items remain? Given the list with String object is named names.

Answer 6

names.stream()
.distinct()
.collect(Collectors.toList());

Question 7

How can a stream be used to print every item that passes by. Given the list with String object is named names.

Answer 7

names.stream()
.peek(s -> System.out.println(s))
.collect(Collectors.toList());

Question 8

What are the six terminal stream operations that you need to know for the exam?

Answer 8

R collect(Collector super T, A, R> collector);
Optional min(Comparator super T> comparator);
Optional max(Comparator super T> comparator);
Optional findAny();
Optional findFirst();
long count();

Question 9

To collect the results of a stream operation several classes can be used. In what class you can find the built-in collectors?

Answer 9

Collectors

Question 10

How is a Stream pipeline built?

Answer 10

1. the first part is a source (an array, Collection, generator function, etc.)
2. zero or more intermediate operations wich transform a Stream into another Stream.
3. and finally one terminal operation which produces a result or side-effect. Such as count() or forEach(Consumer c);

Question 11

Given the following code what is the result:
names.stream
.peek(s -> System.out.print(s));

Answer 11

Nothing. As long as there is no terminal operation intermediate operations are not executed therefore streams are considered lazy.

Question 12

Given the following code what is the result:
List names =Arrays.asList(“Volha”, “Ivan”, “John”);
String name = names.stream();
.filter(s -> { System.out.println(“filtering: “ + s); return s.lenght == 4; })
.map(s -> { System.out.println(“uppercasing: “ + s); return s.toUpperCase(); })
.findFirst().get();
System.out.println(name);

Answer 12

filtering: Volha
filtering: Ivan
uppercasing: Ivan
IVAN