4. Java 8: Collection operations with Lambda

Question 1

How could you create a stream of Employee objects given the following class:
class Employee {
// Field and getters/setters omitted
Employee(String name);
}

Answer 1

Stream.of(new Employee(“Jan”), new Employee(“Henk”));

Question 2

Given a Stream of Employee objects create a list of Integer objects.
class Employee {
// Field and getters/setters omitted
Employee(String name);
}

Answer 2

Stream employees = Stream.of(new Employee("Jan"), new Employee("Henk"));
Stream names = employees.map(Employee::getName);
Stream lenghts = names.map(String::lenght);

Question 3

What are the primitive specialized Stream interfaces?

Answer 3

• IntStream
• DoubleStream
• LongStream

Question 4

What are the Stream methods for mapping a Stream to a primitive specialized stream?

Answer 4

• mapToInt
• mapToLong
• mapToDouble

Question 5

Primitive specialized streams come with some convenience methods, what are these methods?

Answer 5

min()
max()
sum()
average()

Question 6

What is the difference between the findFirst and findAny method?

Answer 6

The findAny methods behavior is explicitly nondeterministic; it is free to select any element in the stream. This is to allow for maximal performance in parallel operations;

While findFirst while always return the first element (or an empty optional)

Question 7

Using a Stream of String objects, use the anyMatch method to see if the name “Henk” is present.

Answer 7

Stream names = Stream.of(“Jan”, “Henk”, “Benny”);

boolean test = names.anyMatch(e -> “Henk”.equalsIgnoreCase(e)); // True

Question 8

Using a Stream of String objects, use the allMatch method to determine if all names start with a “B”.
And use the noneMatch to determine that none of the names start with a “A”

Answer 8

Stream names = Stream.of(“Bob”, “Benny”);

names. allMatch(e -> e.startsWith(“B”)); // True
names. nonMatch(e -> e.startsWith(“A”)); // True

Question 9

How would one create a empty Optional?

Answer 9

Optional.empty();

Question 10

How would one create a Optional from a non-null object?

Answer 10

Optional.of(T);

Question 11

How would one create a Optional from a object that might be NULL?

Answer 11

Optional.ofNullable(T);

Question 12

What is the result of the following?
String s = null;
Optional optional = Optional.from(s);
System.out.println(optional.isPresent());

Answer 12

NullPointerException

Optional.of can only be used for non-null objects. If you want to wrap a (potential) null object use the Optional.ofNullable(T) factory method.

Question 13

How can you retrieve the value of a Optional?

Answer 13

optional.get();

Question 14

What is the difference between
Optional.orElse(T other)
or
Optional.orElseGet(Supplier extends T> other)

Answer 14

orElseGet is lazy

The supplier code is only executed when the Optional is really empty (it’s lazy). Whit orElse you really need to provide a object that is already created.

Question 15

Give a Optional wrapping a String how would one write the code to get the value but if it doesn’t exist it will throw a IllegalArgumentException?

Answer 15

String s = null;
Optional opt = Optional.ofNullable(s);
String value = opt.orElseThrow(() -> new IllegalArgumentException());

Question 16

Given a Optional wrapping a String how would I print the actual value only if it’s not null?

Answer 16

String s = “Hello World”;
Optional opt = Optional.of(s);
s.ifPresent(v -> System.out.println(v)));

Question 17

What is the Optional method to check if a value is not null?

Answer 17

Optional.isPresent();

Question 18

Given the following Stream:
Stream names = Stream.of(“Bob”, “Benny”);
How would you write the code to retrieve the shortest and the longest name?

Answer 18

Optional minName = names.min(Comparator.comparing(item -> item.length()));
Optional maxName =
names.max(Comparator.comparing(item -> item.length()));

Question 19

Given the following Stream:
Stream names = Stream.of(“Bob”, “Benny”);
How would you write the code that calculates the number of items?

Answer 19

long count = names.count();

Question 20

What specialized primitive stream can be used to store short primitive?

Answer 20

IntStream

short, char, byte and boolean can all be stored in a IntStream because they will fit.

Question 21

What spcialized primitive stream can be used to store a float prmitive?

Answer 21

DoubleStream

A float primitive can be stored in a DoubleStream because it can fit in a double.

Question 22

How would one create a IntStream with the following numbers (1, 5, 10) and calculate the min and max?

Answer 22

IntStream numbers = IntStream.of(1,5,10);
IntOptional min = numbers.min();
IntOptional max = numbers.max();

Question 23

Given a Stream of strings how can it be sorted using the default sorting?

Answer 23

strings.sorted().collect(Collectors.toList());

Question 24

The stream.sorted() method sorts objects in a Stream. What does the sorted method require from a Object to be sorted? And what will happen if that Object does’nt meet that requirement?

Answer 24

The object should implement the Comparable interface.

If the object doesn’t implement this interface it will throw a ClassCastException.

Question 25

Using the sorted(Comparator c) method and comparators how can we sort the following stream of Strings first on length and then on the string itself?
Stream names = Stream.of(“Bob”, “Benny”, “Annie”);

Answer 25

Comparator c1 = Comparator.comparing(e -> e.length());
Comparator c2 = (e1, e2) -> e1.compareTo(e2);

names.sorted(c1.thenComparing(c2))
.collect(Collectors.toList());

Question 26

What are the Collectors methods to collect the results of a stream in a List or Set?

Answer 26

toList()

toSet()

Question 27

What are the Collectors methods available besides toList and toSet?

Answer 27

Collectors.averagingDouble(ToDoubleFunction super T> mapper);
Will collect the average for a stream, for example: stream().collect(Collectors.averagingDouble(s -> s.length());

Collectors.groupingBy(Function super T, ? extends K> classifier);
A group by operation grouping the elements according to the classification function and returning the result in a map.
For example: Map groupedBy = stream.collect(Collectors.groupingBy(Employee::getDepartement));

Collectors.joining()
Joins all Objects in a Stream using the toString() method into a single String.
For example:
String joinedString = str.map(Employee::getName).collect(Collectors.joining);

Collectors.partitioningBy(Predicate super T> predicate)
Will group all elements into a map where the predicate either returns true or false.
For Example:
Map> = str.collect(Collectors.partitioningBy(e -> “Management”.equals(“e.getDepartment()));