Question 1

What will be the output of the program? public static void main(String[] args) 
{
    Object obj = new Object() 
    {
        public int hashCode() 
        {
            return 42;
        }
    }; 
    System.out.println(obj.hashCode()); 
}

Accepted Answer

This code is an example of an anonymous inner class. They can be declared to extend another class or implement a single interface. Since they have no name you can not use the "new" keyword on them. In this case the annoynous class is extending the Object class. Within the {} you place the methods you want for that class. After this class has been declared its methods can be used by that object in the usual way e.g. objectname.annoymousClassMethod()

Question 2

Which two statements are true about comparing two instances of the same class, given that the equals() and hashCode() methods have been properly overridden? If the equals() method returns true, the hashCode() comparison == must return true. If the e…

Accepted Answer

(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.

Question 3

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? x = 0;
if (x1.hashCode() != x2.hashCode() )  x = x + 1;
if (x3.equals(x4) )  x = x + 10…

Accepted Answer

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.

Question 4

What two statements are true about properly overridden hashCode() and equals() methods? hashCode() doesn't have to be overridden if equals() is. equals() doesn't have to be overridden if hashCode() is. hashCode() can always return the same value, re…

Accepted Answer

(3) and (4) are correct. (1) and (2) are incorrect because by contract hashCode() and equals() can't be overridden unless both are overridden.

Question 5

Which statement is true for the class java.util.HashSet?

Accepted Answer

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.

Question 6

Which of the following are true statements? The Iterator interface declares only three methods: hasNext, next and remove. The ListIterator interface extends both the List and Iterator interfaces. The ListIterator interface provides forward and backw…

Accepted Answer

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.

Question 7

Which statement is true for the class java.util.ArrayList?

Accepted Answer

Yes, always the elements in the collection are ordered.

Question 8

Which of the following statements about the hashcode() method are incorrect? The value returned by hashcode() is used in some collection classes to help locate objects. The hashcode() method is required to return a positive int value. The hashcode()…

Accepted Answer

(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.

Question 9

Which statement is true? class Test1 
{
    public int value;
    public int hashCode() { return 42; }
}
class Test2 
{
    public int value;
    public int hashcode() { return (int)(value^5); }
}

Accepted Answer

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.

Objects and Collections Questions