functional interfaces & lambda expressions

Question 1

any interface that has exactly one abstract method is called …

Answer 1

a functional interface

Question 2

abstract method

Answer 2

methods from e.g. an interface that has a signature but not an implementation (the class that implements the interface will define the method with its own implementation - don’t forget annotation @Override)

Question 3

is List a functional interface?

Answer 3

no - it does not have only one method, it has many methods. A functional interface must have one method.

Question 4

can a developer create a new functional interface?

Answer 4

yes they can. Java libraries also provide many functional interfaces - they are called “the standard Java functional interfaces”. (we cover four in ATA)

Question 5

What are the four functional interfaces from Java that are covered in ATA?

Answer 5

Function, Supplier, Consumer, Predicate

Question 6

To implement an interface the standard way…

Answer 6

1. The class header declares which interface it’s implementing
2. The class implementation overrides each abstract method
3. Create and document entire class
   A great deal of effort just to use the single method inside!
   (hint: instead, use lambda expressions)

Question 7

lambda expressions provide…

Answer 7

a quick and easy way to implement a functional interface without expliciitly defining a class or even a formal method declaration.

Question 8

syntax of a lambda expression

Answer 8

argument -> method implementation
or 
(arg1, arg2) -> method implementation
or
() -> method implementation
or
(arg(s)) -> {
     mulit-line method
     implementation
}
Because each functional interface contains exactly one abstract method, lambda expressions allow us to implement only that one method (following the arrow) to satisfy the interface. We don't need to creat a class, and since the interface has only one method, we don't even need to name it!

Question 9

three parts to lambda expression

Answer 9

1. the method argument
2. the arrow separator ->
3. the method implementation

Question 10

Function interface

Answer 10

public interface Function {
     R apply(T t);
}
takes an input object, returns an output object

Question 11

why don’t we need to name the method we call in a lambda expression?

Answer 11

the lamda “operator” -> automatically calls the ONE method in the interface, that one method is defined on the right of the -> operator

Question 12

Supplier interface

Answer 12

public interface Supplier {
     T get();
}
no input, returns an output

Question 13

Supplier examples

Answer 13

List participants = {"ace", "queen", "diamond"}
Random random = new Random();
Supplier randomIndex = 
() -> random.nextInt(participants.size());
------
public void callOnParticipant(Supplier index) {
     String person = participants.get(indexChooser.get());
     System.out.println("Hello " + person);
}

Question 14

Supplier interface notes

Answer 14

1. Supplier does not take input directly, but may access any variable within its scope
2. called with empty parenthesis () -> the get method implementation
3. it doesn’t “appear” to have a return but does!

Question 15

note on keyword ‘return’ in lambda expressions

Answer 15

if you drop the curly braces from your one-line method implementation, and the functional interface that your lambda expression is implementing has a return value, you drop the keyword return. (most common usage of a lambda)

Question 16

method reference

Answer 16

if your lambda expression simply calls a method, this syntax is a way to “compactify” it:
long form: key -> method(key) or item -> item==null etc
I. Reference to an instance method of a particular object: containingObject::instanceMethodName
e.g.: recommendationsServiceClient::getBookRecs
(from .build(CacheLoader.from(^^)
II. Reference to an instance method of an arbitrary object of a particular type: ContainingType::methodName
e.g.: list.removeIf(String::isEmpty)
III. Reference to a static method:
ContainingClass::staticMethodName
e.g.: list.removeIf(StringUtils::isBlank)
e.g.: PublishingConverter::toPublishRecord

Question 17

Consumer interface

Answer 17

public interface Consumer {
void accept;
}

Question 18

when are consumers used?

Answer 18

Consumers are commonly used when:

1. performing an operation on each member of a data structure
2. updating another data structure by calling Consumer on each element

Question 19

example of Consumer

Answer 19

List names = fetchNames();
names.forEach(name -> System.out.println(“name: “ + name);

(note: forEach(Consumer action) is a List method)
fancier:
Consumer fancyLambda = myString -> System.out.println(“Hello there “ + myString “, how are you today?”);
names.forEach(fancyLambda);

Question 20

Predicate interface

Answer 20

public interface Predicate {
     boolean test(T t);
}

Question 21

what is Predicate used for?

Answer 21

Predicate is often used to filter (in or out) elements from a data structure, or to conditionally perform some operation on elements of a data structure.

A Predicate performs a test on an objects, and specifies whether it meets a certain condition or not.

Question 22

examples of Predicate

Answer 22

value -> value == null;

list. removeIf(value -> value == null);
(note: removeIf(Predicate filter) is a list method, it removes all elements of the collection that satisfy the given predicate. this method returns true if any element was removed.)

Question 23

remove all palindromes from a list

Answer 23

List noPalindrome = fetchWords;
noPalidrome.removeIf(text -> {
  String reverse = new StringBuffer(text).reverse.toString();
  return text.equals(reverse);
});

note:
1. with only one input, don’t need parentheis
2. the Predicate’s test(T t) method is inside the {} since multiple lines
3. the removeIf has regular parentheses () and end with ;
4. needs return since multiline (single line skips explicit return statement, it’s implied)
- ->( t -> { blabla; return boolean} );

If the input “satisfies the predicate” - means the test(input) returns true - the item is removed from the list.
**removeIf is a Collections method, which List inherits, so any collection can do this

Question 24

can a Predicate have two inputs

Answer 24

no - the signature test(T t) accounts for one input

Question 25

remove palindromes from list, defined as variable Predicate

Answer 25

Predicate noPalindrone = text -> {
String reverse = new StringBuffer.reverse().toString();
return text.equals(reverse);
};

question: is the ‘;’ correct at end?

Question 26

Function to open a file for reading data

Answer 26

Function fileOpener =
  filename -> {
      File file = new File(filename);
      try {
         return new FileInputStream(file); 
      } catch (FileNotFoundException e) {
         throw new IllegalArgumentException(e.getMessage..
      }
};

Question 27

Checked exceptions in lambda expressions

Answer 27

lamda cannot throw checked exceptions (not in the interface signature). What you need to do is use a try/catch block and throw a runtime exception.

e.g.: catch a FileNotFoundException (is a checked exception) and throw new IllegalArgumentException (is a runtime exception)
alternatively: just log it (dangerous): logger.warn(“Can’t open missing file: “ + filename);
return null; //maybe return a default file?

“make sure you plan for handling checked exceptions”