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Java Programming - Objects and Collections

Exercise : Objects and Collections - Pointing out the correct statements
1.
class Test1 
{
    public int value;
    public int hashCode() { return 42; }
}
class Test2 
{
    public int value;
    public int hashcode() { return (int)(value^5); }
}
which statement is true?
class Test1 will not compile.
The Test1 hashCode() method is more efficient than the Test2 hashCode() method.
The Test1 hashCode() method is less efficient than the Test2 hashCode() method.
class Test2 will not compile.
Answer: Option
Explanation:

The so-called "hashing algorithm" implemented by class Test1 will always return the same value, 42, which is legal but which will place all of the hash table entries into a single bucket, the most inefficient setup possible.

Option A and D are incorrect because these classes are legal.

Option B is incorrect based on the logic described above.

2.
Which statement is true for the class java.util.HashSet?
The elements in the collection are ordered.
The collection is guaranteed to be immutable.
The elements in the collection are guaranteed to be unique.
The elements in the collection are accessed using a unique key.
Answer: Option
Explanation:

Option C is correct. HashSet implements the Set interface and the Set interface specifies collection that contains no duplicate elements.

Option A is wrong. HashSet makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.

Option B is wrong. The set can be modified.

Option D is wrong. This is a Set and not a Map.

3.
Which of the following statements about the hashcode() method are incorrect?
  1. The value returned by hashcode() is used in some collection classes to help locate objects.
  2. The hashcode() method is required to return a positive int value.
  3. The hashcode() method in the String class is the one inherited from Object.
  4. Two new empty String objects will produce identical hashcodes.
1 and 2
2 and 3
3 and 4
1 and 4
Answer: Option
Explanation:

(2) is an incorrect statement because there is no such requirement.

(3) is an incorrect statement and therefore a correct answer because the hashcode for a string is computed from the characters in the string.

4.
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.
1 and 2
2 and 3
3 and 4
1 and 3
Answer: Option
Explanation:

(3) and (4) are correct.

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

5.
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.
1 and 4
2 and 3
3 and 4
1 and 3
Answer: Option
Explanation:

(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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