2. Java 8: Using Built-in Lambda Types

Question 1

Java 8 comes with some most-used functional interfaces, in which package are these interfaces hosted?

Answer 1

java.util.function

Question 2

Name the functional interface that represents a boolean valued function of one argument that takes the type of T.

Answer 2

Predicate

public boolean test(T t)

Question 3

Name the functional interface that represents a operation with no return value and accepts one argument of type T.

Answer 3

Consumer

public void accept(T t)

Question 4

Name the functional interface that represents a function that returns a value of R and accepts one argument of type T.

Answer 4

Function

public R apply(T t);

Question 5

Name the functional interface that is a supplier of values.

Answer 5

Supplier

public T get();

Question 6

Name the functional interface that represents a function that produces a double result.

Answer 6

ToDoubleFunction

public double applyAsDouble(T t);

Question 7

Name the functional interface that represents a boolean valued function of two arguments that takes the type of T and U.

Answer 7

BiPredicate

public boolean test(T t, U u);

Question 8

Name the functional interface that represents a operation on a single argument that returns the same type as the argument.

Answer 8

UnaryOperator extends Function

public T apply(T t); // inherited from Function

Question 9

Use the Function interface to create a Function that creates a new Boolean from a String. And then use that function to convert 3 strings to a Boolean:
true, false, dfa

Answer 9

Function function = s -> new Boolean(s);

s. apply(“true”); // true
s. apply(“false”); // false
s. apply(“dfa”); // false

Question 10

Use the Consumer interface to loop over a String list and System.out their values. The list is calles values.

Answer 10

Consumer consumer = s -> System.out.println(s);
for(String value : values) {
s.accept(value);
}

Question 11

Given the following class:
class Book {
   String title;
   String author;
// getters ommited
   public Book(title, author) {}
}

How would one use the Supplier interface to create and return a new instance and print the title.

Answer 11

Supplier supplier = () -> new Book("title", "author");
System.out.println(supplier.get().getTitle());

Question 12

How could you rewrite the following code to use a constructor reference?
Supplier supplier = () -> new User();

Answer 12

Supplier supplier = User::new;

Question 13

Use the UnaryOperator that takes a String and prints that with the addition of the string “ is Great!”.

Answer 13

UnaryOperator operator = s -> s + “ is Great!”;

Question 14

Use the Predicate to print true when a String is larger then 5 characters.

Answer 14

Predicate predicate = s -> s.length() > 5;

System.out.println(predicate.test(“verylargestring”));

Question 15

The usual FunctionalInterfaces only accept reference values as their parameters and return values. How can you use primitves in the FunctionalInterfaces so that no boxing and unboxing takes place?

Answer 15

Use the specialized interfaces for primitive types. For example given the Function interface there are additional interfaces for example:
IntFunction
IntToDoubleFunction

Question 16

What are the four types of method references available?

Answer 16

Static Method Reference
Constructor Reference
Instance method reference of particular class type
Instance method reference of a object

Question 17

Use a IntFunction to convert a int to a String (valueOf method) use a static method reference.

Answer 17

IntFunction f1 = String::valueOf;

f1.apply(100);

Question 18

Use a Function to convert a StringBuilder to a String using the String constructor as a method refernce.

Answer 18

Function f1 = String::new;
f1.apply(new StringBuffer().append("test"));

Question 19

Use a BiFunction to compare two strings (equalsIgnoreCase) using a instance method reference of a particular class type.

Answer 19

BiFunction f1 = String::equalsIgnoreCase;
fi1.apply(“hello”, “HELLO”);

When using a instance method reference of particular type the first argument is used as the target for the method so it would be written like:
“hello”.equalsIgnoreCase(“HELLO”);

Question 20

Use a Supplier and a instance method reference of a object to convert a Integer to a String (toString);

Answer 20

Integer i = new Integer(1);
Supplier f1 = i::toString;
f1.get();