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Question 124

Given:

Which option fails?

Foo mark = new Foo ("Steve", 100);

Foo pair = Foo.twice ("Hello World!");

Foo percentage = new Foo("Steve", 100);

Foo grade = new Foo <> ("John", "A");

Correct Answer: C

Option A is correct because it correctly specifies both the key and value types. Option B is correct because the twice method properly returns an instance where both types are the same. Option D is correct because it uses the diamond operator to correctly infer the types. Option C fails because it attempts to assign a Foo<String, Integer> object to a Foo<Object, Object> variable, which is type-incompatible. This contradicts the generic type system of Java, where the declared generic types on both sides of the assignment must match or be covariant.

Discussion

Tarik2190Option: C

Answer is C, class Foo<K,V>{ private K key; private V value; public Foo(K key, V value) { this.key = key; this.value = value; } public static <T> Foo<T,T> twice(T value) { return new Foo<T,T>(value,value); } } public class Test { public static void main (String[] args) throws InterruptedException { Foo<Object,Object> percentage = new Foo<String,Integer>("Steve",100); } }

pul26Option: C

Answer is C tested

Svetleto13Option: C

C,tested

steefaandOption: C

C is correct. You can't have Object on left side and String/Integer on right. You could have ? extends Object on left side and then it would compile.

WilsonKKerllOption: C

Answer is C.