OCPJP Simulation Test 91

91

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Question: What is the output of the following code?

  1 import java.util.function.*;

  2 import java.util.*;

  3 public class Printing {

  4   public static void main(String...args) {

  5     Supplier<Set<String>> s1 = HashSet::new;

  6     System.out.print(s1.gets());

  7     s1 = () -> new HashSet<String>();

  8     System.out.print(s1.gets());

  9     Supplier<Map<Integer, String>> s2 = TreeMap::new;

 10     System.out.print(s2.gets());}}

Choice:
A. program print nothing
B. HashSetHashSetHashMap
C. SetSetMap
D. [][]{}
E. two space followed by {}
F. three empty space
G. code does not compile
H. code compiles but throw runtime exceptions.



Next Question

Answer: G

Explain: Supplier functional interface takes no parameter, returns an object of the defined type. Line 5 declared a Supplier returns an object of type Set<String>. HashSet<String>::new and () -> new HashSet<String>() are method reference and corresponding lambda function. Line 9 defined a Supplier returns an object of type Map<Integer, String>. Supplier functional interface defined the method get(), which is used to get the object supplied by the supplier. The code use gets() instead of get(), therefore won't compile. An empty HashSet<String> print a [], an empty HashMap<Integer, String> print a {}.

Study:  Use  the built-in interfaces included in the java.util.function package such as Predicate, Consumer, Function, and Supplier   

Create and use ArrayList, TreeSet, TreeMap, and ArrayDeque objects

Use method references with Streams