Exercise: Objects And Collections

Questions for: Objects And Collections

Which statement is true for the class java.util.ArrayList?
A:
The elements in the collection are ordered.
B:
The collection is guaranteed to be immutable.
C:
The elements in the collection are guaranteed to be unique.
D:
The elements in the collection are accessed using a unique key.
Answer: A

Yes, always the elements in the collection are ordered.

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Which of the following are true statements?
  1. The Iterator interface declares only three methods: hasNext, next and remove.
  2. The ListIterator interface extends both the List and Iterator interfaces.
  3. The ListIterator interface provides forward and backward iteration capabilities.
  4. The ListIterator interface provides the ability to modify the List during iteration.
  5. The ListIterator interface provides the ability to determine its position in the List.
A:
2, 3, 4 and 5
B:
1, 3, 4 and 5
C:
3, 4 and 5
D:
1, 2 and 3
Answer: B

The ListIterator interface extends the Iterator interface and declares additional methods to provide forward and backward iteration capabilities, List modification capabilities, and the ability to determine the position of the iterator in the List.

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x = 0;
if (x1.hashCode() != x2.hashCode() )  x = x + 1;
if (x3.equals(x4) )  x = x + 10;
if (!x5.equals(x6) ) x = x + 100;
if (x7.hashCode() == x8.hashCode() )  x = x + 1000;
System.out.println("x = " + x);
and assuming that the equals() and hashCode() methods are properly implemented, if the output is "x = 1111", which of the following statements will always be true?
A:
x2.equals(x1)
B:
x3.hashCode() == x4.hashCode()
C:
x5.hashCode() != x6.hashCode()
D:
x8.equals(x7)
Answer: B

By contract, if two objects are equivalent according to the equals() method, then the hashCode() method must evaluate them to be ==.

Option A is incorrect because if the hashCode() values are not equal, the two objects must not be equal.

Option C is incorrect because if equals() is not true there is no guarantee of any result from hashCode().

Option D is incorrect because hashCode() will often return == even if the two objects do not evaluate to equals() being true.

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Which two statements are true about comparing two instances of the same class, given that the equals() and hashCode() methods have been properly overridden?
  1. If the equals() method returns true, the hashCode() comparison == must return true.
  2. If the equals() method returns false, the hashCode() comparison != must return true.
  3. If the hashCode() comparison == returns true, the equals() method must return true.
  4. If the hashCode() comparison == returns true, the equals() method might return true.
A:
1 and 4
B:
2 and 3
C:
3 and 4
D:
1 and 3
Answer: A

(1) is a restatement of the equals() and hashCode() contract. (4) is true because if the hashCode() comparison returns ==, the two objects might or might not be equal.

(2) and (3) are incorrect because the hashCode() method is very flexible in its return values, and often two dissimilar objects can return the same hash code value.

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What two statements are true about properly overridden hashCode() and equals() methods?
  1. hashCode() doesn't have to be overridden if equals() is.
  2. equals() doesn't have to be overridden if hashCode() is.
  3. hashCode() can always return the same value, regardless of the object that invoked it.
  4. equals() can be true even if it's comparing different objects.
A:
1 and 2
B:
2 and 3
C:
3 and 4
D:
1 and 3
Answer: C

(3) and (4) are correct.

(1) and (2) are incorrect because by contract hashCode() and equals() can't be overridden unless both are overridden.

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