Chapter 15: Functional Programming Flashcards
1
Q
Which functional interface from java.util.function takes 0 parameters and returns a generic object (T)?
A
Supplier
2
Q
Which functional interface from java.util.function takes 1 parameter and returns void?
A
Consumer
3
Q
Which functional interface from java.util.function takes 2 parameters and returns void?
A
BiConsumer
4
Q
Which functional interface from java.util.function takes 1 parameter and returns a boolean?
A
Predicate
5
Q
Which functional interface from java.util.function takes 2 parameters and returns a boolean?
A
BiPredicate
6
Q
Which functional interface from java.util.function takes 1 parameter and returns a generic object (R)?
A
Function
7
Q
Which functional interface from java.util.function takes 2 parameters (T, U) and returns a generic object (R)?
A
Function
8
Q
Which functional interface from java.util.function takes 1 parameter (T) and returns a generic object (T)?
A
UnaryOperator
9
Q
Which functional interface from java.util.function takes 2 parameters (T, T) and returns a generic object (T)?
A
BinaryOperator
10
Q
When should the Supplier interface be used?
A
When you want to generate or supply values without taking any input.
11
Q
What method does the Supplier functional interface have?
A
get()
12
Q
When should the Consumer interface be used?
A
When you want to do something with a parameter but not return anything.
13
Q
What method does the Consumer and BiConsumer functional interfaces have?
A
accept(T)
accept(T, U)
14
Q
What method does the Predicate and BiPredicate functional interfaces have?
A
test(T)
test(T, U)
15
Q
What method does the Function and BiFunction functional interfaces have?
A
apply(T)
apply(T, U)
16
Q
When should the Function interface be used?
A
When you want to turn one parameter into a value of potentially different type and return it.
17
Q
Which functional interface should be used when you want to combine two strings and return a new one?
A
BiFunction or BinaryOperator (better)
18
Q
UnaryOperator and BinaryOperator are similar to Function and BiFunction. How do they differ?
A
UnaryOperator and BinaryOperator require the input and output to be of the same type.
19
Q
What interface does UnaryOperator extend?
A
Function
20
Q
What interface does BinaryOperator extend?
A
BiFunction
21
Q
Is the following code valid?
UnaryOperator u1 = String::length;
A
No. The statement returns an Integer but UnaryOperator requires the return type be the same as the input type (String).
To make this code valid, Function needs to be used.
22
Q
What functional interface would you use in the following situation:
Returns a String without taking any parameters.
A
Supplier
23
Q
What functional interface would you use in the following situation:
Returns a Boolean (not a primitive) and takes a String
A
Function
Predicate returns a primitive boolean.
24
Q
What functional interface would you use in the following situation:
Returns an Integer and takes two Integers.
A
BinaryOperator or BiFunction (both are equivalent, but BinaryOperator is more specific).
25
What functional interface would you use in the following situation:
Returns a boolean primitive and takes a String
Predicate
Predicate returns a primitive boolean. Function could be used to return a Boolean (Wrapper)
26
What functional interface(s) belong on the blank lines?
____ ex = x -> "".equals(x.get(0));
It pases one List and returns a boolean. Therefore this could be a Predicate or a Function.
27
What functional interface(s) belong on the blank lines?
____ ex = (Long l) -> System.out.println(l);
It passes a long and doesn't return anything. Therefore this should be a Consumer.
28
What functional interface(s) belong on the blank lines?
____ ex = (s1, s2) -> false;
BiPredicate
Takes two parameters and returns a boolean. Since the generics don't specify a Boolean, it can't be a Function.
29
What is wrong with the following statement?
Function< List> ex = x -> x.get(0);
A Function always needs to specify two generics, one input and one output type.
30
What is wrong with the following statement?
UnaryOperator ex = (long l) -> 3.14;
To use a UnaryOperator, the same type must be returned (in this case a Long). The statement returns a double instead.
31
What is wrong with the following statement?
Predicate ex = String::isEmpty;
Predicate expects a generic but since it wasn't supplied, the parameter that was passed in is treated as an Object, instead of a String.
32
What convenience methods does Predicate contain?
- and()
- negate()
- or()
33
How can the following code be rewritten using conveniance methods?
Predicate brownEggs = s -> s.contains("egg") && s.contains("brown");
Predicate egg = x -> x.contains("egg");
Predicate brown = x -> x.contains("brown");
Predicate brownEgg = egg.and(brown);
34
How can the following code be rewritten using conveniance methods?
Predicate brownEggs = s -> s.contains("egg") && !s.contains("brown");
Predicate egg = x -> x.contains("egg");
Predicate brown = x -> x.contains("brown");
Predicate brownEgg = egg.and(brown.negate());
35
What convenience methods does Consumer contain?
- andThen()
36
What convenience methods does Function contain?
- andThen()
| - compose()
37
What conveniance method can be used to combine the following consumers?
Consumer c1 = x -> System.out.println(x);
Consumer c2 = x -> System.out.println(x);
andThen()
Consumer combined = c1.andThen(c2);
38
What conveniance method can be used to pass the output of f1 to the input of f2?
Function f1 = x -> x + 1;
Function f2 = x -> x * 2;
compose()
Function combined = f2.compose(f1);
39
What does the following code do?
Optional opt = Optional.empty();
opt.get();
Throws a NoSuchElementException
40
How do we prevent the code from throwing an exception (without using a try-catch)?
Optional opt = Optional.empty();
opt.get();
replace opti.get() with:
if (opt.isPresent())
opt.get();
41
What does the Optional static method ofNullable() do?
Creates an empty optional if the provided value is null or creates an optional with the value if it is not null.
42
What does the Optional instance method ifPresent(Consumer c) do if the Optional is empty?
Does nothing.
43
What does the Optional instance method ifPresent(Consumer c) do if the Optional contains a value?
Calls Consumer with value.
44
What does the Optional instance method orElse(T other) do if the Optional contains a value?
Returns value
45
What does the Optional instance method orElse(T other) do if the Optional is empty?
Returns other parameter
46
How can we use Optional to get the value, or else throw an exception if it is empty?
Using the method orElseThrow().
We can leave the parameters empty to throw a NoSuchElementException, or provide a Supplier to throw another exception.
47
Why does the following code not compile?
```
Optional{Double} opt = Optional.empty();
System.out.println(opt.orElseGet(() -> new IllegalStateException()));
```
The opt variable is an Optional with generic Double which means the Supplier must return a Double. This Supplier returns an exception instead.
48
What is a stream in Java?
A stream in Java is a sequence of data.
49
What is a stream pipeline in Java?
A stream pipeline consists of the operations that run on a stream to produce a result.
50
What are the parts of the stream pipeline?
- source: where the stream comes from
- Intermediate operations: Transforms the stream into another one.
- Terminal operation: actually produces a result. The stream is no longer valid after a terminal operation completes.
51
The Stream interface is located in what package?
java.util.stream
52
What are ways to create finite Streams?
- Stream.empty()
- Stream.of(varargs)
- someCollection.stream()
- someCollection.parallelStream()
- Streams.iterate(seed, predicate, unaryOperator)
53
Given the following list (or any other Collection), how do we create a stream out of this?
var list = List.of("a", "b", "c");
Stream fromList = list.stream();
54
Given the following list (or any other Collection), how do we create a parallel stream out of this?
var list = List.of("a", "b", "c");
Stream fromList = list.parallelStream();
55
What are ways to create infinite Streams?
- Stream.generate(supplier)
- Stream.iterate(seed, unaryOperator)
- Streams.iterate(seed, predicate, unaryOperator)
56
Make a statement for creating an infinite stream that returns random numbers.
Stream randoms = Stream.generate(Math::random);
57
Make a statement for creating an infinite stream that returns every odd number.
Stream oddNums = Stream.iterate(1, n -> n + 2);
58
Make a statement for creating an infinite stream that returns every odd number, less than 100.
Stream oddNums = Stream.iterate(1, n -> n < 100, n -> n + 2);
(page 690)
59
What does the Stream method count() do?
Looks at each element in the stream and returns a single value.
Stream s = Stream.of("monkey", "gorilla", "bonobo");
System.out.println(s.count());
60
Does count() terminate infinite streams?
no
61
What does the Stream method min() & max() do?
The min() and max() take a Comparator and find the smallest or largest value in a stream.
62
Given the following stream, how can we print the smallest value, if present?
Stream s = Stream.of("monkey", "ape", "bonobo");
Optional min = s.min((s1, s2) -> s1.length()-s2.length());
| min.ifPresent(System.out::println);
63
What do the Stream method min() and max() return?
An Optional
64
Does min() or max() terminate infinite streams?
No
65
What does the Stream method findAny() & findFirst() do?
Return an element of the stream unless the stream is empty. They always return an Optional.
66
Does findAny() & findFirst() terminate infinite streams?
Yes.
67
What do the Stream method anyMatch(), allMatch() & noneMatch() do?
Search a stream and return information about how the stream pertains to the predicate.
68
What interface to anyMatch, allMatch() and noneMatch() take to test the stream values?
A Predicate.
69
What do the Stream method anyMatch(), allMatch() & noneMatch() return?
a boolean.
70
What interface does forEach() take as a parameter (streams)?
a Consumer
71
What does forEach() return (streams)?
void
72
Does forEach() terminate infinite streams?
No.
73
What does the reduce() method do? (streams)
The reduce() method combines a stream into a single object. It is a reduction, which means it processes all elements.
74
What are the method signatures for reduce()? (streams)
- T reduce (T identity, BinaryOperator accumulator)
- Optional reduce(BinaryOperator accumulator)
- U reduce(U identity, BiFunction accumulator, BinaryOperator combiner)
75
What is the identity of a reduce() method? (streams)
Identity – an element that is the initial value of the reduction operation and the default result if the stream is empty
76
What is the accumulator of a reduce() method? (streams)
Accumulator – a function that takes two parameters: a partial result of the reduction operation and the next element of the stream
77
What is the combiner of a reduce() method? (streams)
Combiner – a function used to combine the partial result of the reduction operation when the reduction is parallelized or when there's a mismatch between the types of the accumulator arguments and the types of the accumulator implementation
78
What does the following code print?
Stream numbers = Stream.of(1,2,3,4,5,6);
int result = numbers.reduce(0, (n1, n2) -> n1 + n2);
System.out.println(result);
21
79
What does the following code print?
Stream numbers = Stream.of("1","2","3","4","5","6");
String result = numbers.reduce("", (n1, n2) -> n1 + n2);
System.out.println(result);
123456
80
What does reduce() return?
```
Stream numbers = Stream.of("1","2","3","4","5","6");
var result = numbers.reduce((n1, n2) -> n1 + n2);
```
Optional
If no identity is specified, Optional is used (as there is no initial/default value)
81
What does the following code print?
Stream numbers = Stream.of("1");
Optional result = numbers.reduce((n1, n2) -> n1 + n2);
System.out.println(result.get());
1
If there is one value, it will be returned by reduce(). the accumulator won't be applied.
82
What does the following code print?
```
Stream stream = Stream.of("w", "o", "l", "f!");
var result = stream.reduce(0, (i, s) -> i+s.length(), (a, b) -> a+b);
System.out.prinln(result);
```
5
| page 696
83
What does the collect() method do? (streams)
The collect() method is a reduction method that uses a mutable object to transform data from a stream.
84
What does the following code print?
```
Stream stream = Stream.of("w", "o", "l", "f!");
var result = stream.collect(
StringBuilder::new,
StringBuilder::append,
StringBuilder::append);
```
System.out.println(result);
wolf!
| page 697
85
What is the difference between a terminal and an intermediate stream operation?
An intermediate operation returns a stream as its result, while a terminal operation does not.
86
What does the filter() method do? (streams)
The filter() method returns a Stream with elements that match a given expression.
87
What does the distinct() method do? (streams)
The distinct() method returns a Stream with duplicate values removed. Java does this by calling equals() on the elements.
88
What does the filter() method take as a parameter? (streams)
a Predicate
89
What does the limit() and skip() method do? (streams)
The limit() and skip() methods can make a Stream smaller, or they could make a finite stream out of an infinite stream.
```
Stream limit(long maxSize)
Stream skip(long n)
```
90
What does the following code print?
Stream s = Stream.iterate(1, n -> n + 1);
s.skip(5).limit(2).forEach(System.out::println);
6
| 7
91
What does the map() method do? (streams)
The map() method creates one-to-one mapping from the elements in the stream to the elements of the next step in the stream.
92
What does the following code print?
Stream s = Stream.of("monkey", "gorilla");
s.map(String::length).forEach(System.out::println);
6
| 7
93
What does the flatMap() method do? (streams)
The flatMap() method takes each element in the stream and makes any elements it contains top-level elements in a single stream.
This is helpful when you want to remove empty elements from a stream or you want to combine a stream of lists.
94
What does the following code print?
```
List zero = List.of();
var one = List.of("Bonobo");
var two = List.of("Mama Gorilla", "Baby Gorilla");
Stream> animals = Stream.of(zero, one, two);
```
animals.flatMap(m -> m.stream()).forEach(System.out::println);
Bonobo
Mama Gorilla
Baby Gorilla
95
What does the sorted() method do? (streams)
The sorted() method returns a stream with the elements sorted. Java uses natural ordering unless we specify a comparator.
96
What is the result of the following code?
Stream s = Stream.of("monkey", "gorilla");
s.sorted(Comparator::reverseOrder);
The method reverseOrder() is not compatible with the abstract method that is defined in Comparator (different parameters and return type).
This however, works:
s.sorted(Comparator.reverseOrder());
97
What does the peek() method do? (streams)
The peek() method allows us to perform a stream operation without actually changing the stream.
Stream peek(Consumer super T> action)
98
What does the following code do?
Stream.generate(() -> "Elsa").filter(x -> x.length() == 4).sorted().limit(2).forEach(System.out::println);
It hangs until you kill the program or it throws an exception after running out of memory.
The sorted() method waits until everything to sort is present. That never happens because there is an infinite stream.
99
What does the following code do?
Stream.generate(() -> "Elsa").filter(x -> x.length() == 4).limit(2).sorted().forEach(System.out::println);
This prints "Elsa" twice.
The filter lets elements through, and limit() stops the ealier operations after two elements. Now sorted() can sort because we have a finite list.
100
What does the following code do?
Stream.generate(() -> "Olaf Lazisson").filter(x -> x.length() == 4).limit(2).sorted().forEach(System.out::println);
It hangs until you kill the program or it throws an exception after running out of memory.
The filter doesn't allow anything through, so limit() never sees two elements.
101
What are the types of Primitive Streams?
- IntStream
- LongStream
- DoubleStream
102
What does the common primitive stream method boxed() do?
Returns a Stream where T is the wrapper class associated with the primitive type value.
example: int will be boxed to Stream.
103
What is the difference between the primitive stream method range(a, b) and rangeClosed(a, b)?
range() returns a primitive stream from a (inclusive) to b (exclusive)
rangeClosed() returns a primitive stream from a (inclusive) to b (inclusive)
104
What method can we use to map from a Stream to a IntStream?
mapToInt()
105
What method can we use to map from a IntStream to a Stream?
mapToObj()
106
What is the difference with OptionalDouble and Optional?
OptionalDouble is for a primitive, while Optional is for a Wrapper class.
107
What is the return type of the method average() on any primitive stream?
OptionalDouble.
It is always double because the result could always become one.
108
What is the type of var?
```
LongStream longs = LongStream.of(5, 10);
var sum = longs.sum();
```
long.
Calling sum will always return the primitive associated with the stream.
109
What is the output?
```
var cats = new ArrayList();
cats.add("Rody");
var stream = cats.stream();
cats.add("Pooh");
System.out.println(stream.count());
```
2.
Streams are lazily evaluated, which means that the stream isn't actually created on the line where stream is assigned. Instead, an object is created that knows where to look for the data when it is needed.
The stream actually runs when count() is called.
110
Given the following stream, which collector should be used to create a single String where the words are seperated by commas? (lions, tigers, bears)
```
var ohMy = Stream.of("lions", "tigers", "bears");
String result = ohMy.collect(...);
```
Collectors.joining(", ");
111
Given the following stream, which collector should be used to get the average length of all elements?
```
var ohMy = Stream.of("lions", "tigers", "bears");
Double result = ohMy.collect(...);
```
Collectors.averagingInt(String::length);
112
Given the following stream, which collector should be used to Transform it into a TreeSet?
```
var ohMy = Stream.of("lions", "tigers", "bears");
TreeSet result = ohMy.collect(...);
```
Collectors.toCollection(TreeSet::new);
113
Given the following stream, which Collector should be used to transform the stream into a map, where the key is the element and the value is the length?
```
var ohMy = Stream.of("lions", "tigers", "bears");
Map result = ohMy.collect(...);
```
Collectors.toMap(s -> s, String::length);
114
Why does this code generate an exception?
var ohMy = Stream.of("lions", "tigers", "bears");
Map result = ohMy.collect(Collectors.toMap(
String::length,
k -> k));
Two of the animal names are the same length. Java doesn't know how to solve this conflict if we don't specify how to deal with this.
115
What code must be added to fix the exception that will be thrown here?
var ohMy = Stream.of("lions", "tigers", "bears");
Map result = ohMy.collect(Collectors.toMap(
String::length,
k -> k));
var ohMy = Stream.of("lions", "tigers", "bears");
Map result = ohMy.collect(Collectors.toMap(
String::length,
k -> k,
(s1, s2) -> s1 + "," + s2));
116
How do we make the following code guarantee it returns a TreeMap?
var ohMy = Stream.of("lions", "tigers", "bears");
TreeMap result = ohMy.collect(Collectors.toMap(
String::length,
k -> k,
(s1, s2) -> s1 + "," + s2));
var ohMy = Stream.of("lions", "tigers", "bears");
TreeMap result = ohMy.collect(Collectors.toMap(
String::length,
k -> k,
(s1, s2) -> s1 + "," + s2,
TreeMap::new));
117
What Collectors method can we use to create a Map of List that is grouped by its length (key)?
Collectors.groupingBy(String::length);
groupingBy determines the key of the map.
118
# Fill in the blanks to comply with the type of result, where Integer is the length of the string and Set is a set of values that has that length.
```
var ohMy = Stream.of("lions", "tigers", "bears");
Map> result = ohMy.collect(...);
```
Collectors.groupingBy(String::length, Collectors.toSet()))
119
# Fill in the blanks to comply with the type of result, where Integer is the length of the string and Set is a set of values that has that length.
```
var ohMy = Stream.of("lions", "tigers", "bears");
TreeMap< Integer, Set> result = ohMy.collect(...);
```
Collectors.groupingBy(String::length, TreeMap::new, Collectors.toSet()))
120
# Fill in the blanks to comply with the type of result, where Integer is the length of the string and List is a set of values that has that length.
```
var ohMy = Stream.of("lions", "tigers", "bears");
TreeMap> result = ohMy.collect(...);
```
Collectors.groupingBy(String::length, TreeMap::new, Collectors.toList()))
121
Is the function in groupingBy() allowed to return null?
No. It does not allow null keys.
122
What is the difference between the Collectors methods groupingBy() and partitionBy()?
with partitionBy() there are only two possible groups: true and false. Partitioning is like splitting a list into two parts.
123
How can we partition the list with animals of length > 5 and animals with length <= 5?
```
var ohMy = Stream.of("lions", "tigers", "bears");
Map> result = ohMy.collect(...);
```
Collectors.partitionBy( s -> s.length() < = 5));
124
# Fill in the blanks to group all animals by name length (key) and the amount that have that length (value).
```
var ohMy = Stream.of("lions", "tigers", "bears");
Map result = ohMy.collect(...);
```
Collectors.groupingBy(String::length, Collectors.counting())
125
What are the reduction methods?
| bonus: which one is a mutable reduction?
- collect() // mutable
- count()
- max()
- min()
- reduce()
126
What happens when no terminal operations are encountered in the stream pipeline?
a Stream will be returned but not executed.
127
What are the terminal operations?
- collect()
- forEach()
- min()
- reduce()
- count()
- max()
- findFirst()
- findAny()
- anyMatch()
- allMatch()
- noneMatch()
128
What happens when anyMatch(), allMatch() or noneMatch() are used in an infinite stream?
They hang.
129
What do findFirst() and findAny() return?
An Optional
130
What do anyMatch(), allMatch() and noneMatch() return?
a boolean.
