Lambda

Question 1

What is actually lambda?

Answer 1

It is quick implementation of an functional interface, a shortcut for compiler.
Compiler is implementing a anonymous class instead of lambda later.

Question 2

What is functional interface?

Answer 2

It is any interface that has only one abstract method.
It can be marked with @FunctionalInterface, but it is not mandatory.

Question 3

Is empty lambda valid?

Answer 3

Yes.
Eg. s -> {}

Question 4

Is “var” allowed as type reference for lambda?

Answer 4

No, there is no context in using “var”. It can be used inside lambda, or as parameter type.

Question 5

What methods are ignored when defined in functional interface?

Answer 5

equals()
hashCode()
toString()

Question 6

Can modifiers be used with lambda parameters?

Answer 6

Yes.
Eg. (final var x, @Deprecated var y) ->x.compareTo(…

Question 7

What are method references?

Answer 7

Quick implementation of functional interface and what will be ran in implementation of the interface method.

interface Test {
int test();
}
() -> Math.random()
Math::random
Test test = Math::random;
test.test();

What abstract method inside interface returns, so must method reference.

Question 8

What are some of predefined utility functional interfaces?

Answer 8

Supplier
Consumer
BiConsumer
Predicate
BiPredicate
Function
BiFunction
Unary Operator
Binary Operator

Question 9

Supplier?

Answer 9

• no parameters
• returns any type object
• get() method

Supplier< Integer > supplier = () -> 1;

Question 10

Consumer?

Answer 10

• has any type parameter, generic
• no returns
• accept() method
• can be chained with andThen()

Consumer< Integer > consumer = System.out::println;

Question 11

BiConsumer?

Answer 11

• has two any type parameters, generics
• no returns
• accept() method
• can be chained with andThen()

BiConsumer<Integer, String> biConsumer = (a, b) -> { System.out.println(a); };

Question 12

Predicate?

Answer 12

• has any type parameter, generic
• has boolean return
• test() method
• can be chained with and(), negate(), or()

Predicate< Integer > predicate = x -> x > 5;

Question 13

BiPredicate?

Answer 13

• has two any type parameters, generics
• has boolean return
• test() method
• can be chained with and(), negate(), or()

BiPredicate<Integer, Integer> biPredicate = (a, b) -> a < b

Question 14

Function?

Answer 14

• has any type parameter generic (first generic, first parameter)
• has return of the generic defined type
• apply() method
• can be chained with compose()

Function<Integer, Integer> function = x -> x * 2

Question 15

BiFunction?

Answer 15

• has two any type parameter generic
• has return of the generic defined type
• apply() method
• can be chained with compose()

BiFunction<Integer, Integer, Integer> biFunction = (a, b) -> a + b;

Question 16

UnaryOperator?

Answer 16

• has one any type parameter generic
• has return of the same type as parameter generic
• apply() method
• can be chained with compose()

UnaryOperator<Integer> unaryOperator = x -> x * 2;</Integer>

Question 17

BinaryOperator?

Answer 17

• has two any type parameter generics
• has return of the same type as parameter generic
• apply() method
• can be chained with compose()

BinaryOperator<Integer> binaryOperator = (a, b) -> a + b;</Integer>

Question 18

What are other combining predefined functional interfaces?

Answer 18

Each combination of main functional interfaces with Double, Int, Long, and combination of converting each of them into another, eg. DoubleToIntSupplier, IntToLongPredicate etc.