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HashSet<E> (Java Questions)

Class HashSet<E>
java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.HashSet<E>
Type Parameters:
E - the type of elements maintained by this set
All Implemented Interfaces:
Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>
Direct Known Subclasses:
JobStateReasons, LinkedHashSet
  1. What is a HashSet?

    Answer:
    A Java HashSet is an unordered and unsorted set.
    Here's a more detailed explanation about HashSet in Java:
    HashSet Characteristics in Java:
    • Unordered: Elements in a HashSet do not maintain any particular order. The order of insertion is not guaranteed to be preserved when iterating over the set.
    • Unsorted: Unlike TreeSet, HashSet does not keep its elements sorted. The elements are stored based on their hash codes, which leads to a more or less random order from an external perspective.

    Key Points About HashSet:
    1. Implementation:
      • HashSet internally uses a HashMap to store its elements, with each element being a key in the map (and the value is typically a dummy object).
    2. Performance:
      • Add, Remove, Contains: These operations have an average time complexity of O(1), making HashSet very efficient for these operations.
      • Iteration: Since it's unordered, iteration might not be in the order elements were added.
    3. No Duplicates:
      • HashSet does not allow duplicate elements. If you try to add a duplicate element, the set will simply ignore the addition.
    4. Null Elements:
      • HashSet allows one null element, unlike HashMap which allows one null key.
    5. Methods:
    • add(E e): Adds the specified element to this set if it's not already present.
    • remove(Object o): Removes the specified element from this set if it is present.
    • contains(Object o): Returns true if this set contains the specified element.
    • size(): Returns the number of elements in this set (its cardinality).
    • isEmpty(): Returns true if this set contains no elements.

    Code Example:
    import java.util.HashSet;
    import java.util.Set;
    
    public class HashSetExample {
        public static void main(String[] args) {
            Set<String> set = new HashSet<>();
    
            // Adding elements
            set.add("apple");
            set.add("banana");
            set.add("cherry");
            set.add("apple"); // Duplicate, will not be added
    
            // Check if an element is in the set
            System.out.println("Contains apple: " + set.contains("apple")); // true
            System.out.println("Contains date: " + set.contains("date")); // false
    
            // Print the size of the set
            System.out.println("Size of the set: " + set.size()); // 3
    
            // Iterate over the set
            for (String element : set) {
                System.out.println(element);
            }
            // Note: The order might not match the insertion order
            
            // Remove an element
            set.remove("banana");
            System.out.println("After removing banana: " + set);
        }
    }
    
    In this example, even though we add "apple" twice, it will only appear once in the HashSet. Also, note that the iteration order might not reflect the order of insertion, showcasing the unordered nature of HashSet.


  2. What is a LinkedHashSet?

    Answer:
    A LinkedHashSet is an ordered but not sorted set that maintains the order in which the objects were inserted.

  3. In what order are the elements of a hashtable placed?

    Answer:
    In a hashtable the elements are placed in a chaotic order based on the hashcode of the key.

  4. What does it mean when a collection is ordered?

    Answer:
    When a collection is ordered, it means you can iterate through the collection in a specific (not-random) order.


  5. How does an ArrayList keep order?

    Answer:
    An ArrayList keeps the order established by the element's index position (just like an array). An ArrayList maintains order by means of an index position.
    In Java SE 17, an ArrayList maintains order through its underlying data structure, which is a dynamically resizing array. Here's how it keeps order:
    1. Insertion Order:
      • Elements in an ArrayList are stored in the order they are added. When you use methods like add(E e), the element e is appended to the end of the list. If you use add(int index, E element), the element is inserted at the specified position, shifting all elements after that position one place to the right.
    2. Index-Based Access:
      • Each element in an ArrayList has an index starting from 0. The order is preserved by maintaining the association of each element with its index. When you retrieve an element using get(int index), you get the element at that exact position.
    3. Dynamic Resizing:
      • When the number of elements exceeds the capacity of the internal array, ArrayList automatically increases its size. This resizing operation copies all elements to a new, larger array, preserving their order.
    4. Modification:
      • Methods like set(int index, E element) replace the element at the given index without changing the order of other elements. Similarly, remove(int index) removes the element at the specified index, shifting all subsequent elements one position to the left to maintain order.

    Here's a simple example to illustrate:
    import java.util.ArrayList;
    
    public class ArrayListOrderExample {
        public static void main(String[] args) {
            ArrayList list = new ArrayList<>();
            
            // Adding elements in order
            list.add("First");
            list.add("Second");
            list.add("Third");
            
            // Inserting an element at a specific index
            list.add(1, "Inserted");
            
            // Printing the list to show order
            System.out.println(list);
            
            // Modifying an element
            list.set(2, "Modified");
            
            // Removing an element
            list.remove(0);
            
            System.out.println(list);
        }
    }
    

    This code would output:
    [First, Inserted, Second, Third]
    [Inserted, Modified, Third]
    
    In summary, ArrayList in Java SE 17 uses an array internally to keep elements in the exact order they were inserted or modified, providing methods to maintain this order through insertion, removal, and modification operations.



  6. How does the LinkedHashSet keep order?

    Answer:
    The LinkedHashSet keeps the order established by insertion.

  7. How do you insert an element at a specific index position in an ArrayList ?

    Answer:
    To insert an element in ArrayList at a specific position, use ArrayList.add(index, element) function where index (= i-1) specifies ith position and the element is the one that is inserted.
    When the element is inserted, the elements from i-th position are shifted right side by a position.

  8. How do you describe collections that maintain the natural order of elements?

    Answer:
    One says that these collections are ordered and sorted.

  9. What is a sorted collection?

    Answer:
    A sorted collection means that the order in the collection is determined according to some rule or rules, known as the sort order.
    Here's a more detailed explanation of what a sorted collection implies in the context of programming:
    Definition of a Sorted Collection A sorted collection is a data structure where elements are organized according to a predefined rule or set of rules, known as the sort order. This means:
    • Consistency: Every time you access the collection, the elements will be in a predictable, ordered sequence based on the sort criteria.
    • Comparison: Elements are compared against each other using a comparator or a comparable interface, where each element can be evaluated as less than, equal to, or greater than another.

    Characteristics of Sorted Collections
    • Implementation:
      • ArrayList: Resizable-array implementation. Best for scenarios where frequent access by index is required but less effective for insertions/deletions at arbitrary positions.
      • LinkedList: Doubly-linked list. Better for frequent insertions and deletions, especially at the beginning or end, but slower for random access.
    • Features:
      • Allow duplicate elements.
      • Maintain insertion order (unless specified otherwise by operations like sort()).
      • Common methods include add(), get(int index), remove(int index), set(int index, E element).
    • Use Case: When you need a sequence of elements where order matters, or when you frequently need to add or remove elements from specific positions.

    Example in Java Here's how you might see this in practice:
    import java.util.TreeSet;
    
    public class SortedCollectionExample {
        public static void main(String[] args) {
            // A TreeSet sorts its elements in natural order
            TreeSet sortedSet = new TreeSet<>();
            
            sortedSet.add("Banana");
            sortedSet.add("Apple");
            sortedSet.add("Cherry");
            
            // Iterating over the set will give elements in sorted order
            for (String s : sortedSet) {
                System.out.println(s);
            }
        }
    }
    
    This would output:
    Apple
    Banana
    Cherry
    

    In summary, a sorted collection ensures that the elements are always in a specific order according to some sorting criteria, providing a structured and predictable way to manage data.

  10. What is sorting based on ?

    Answer:
    Sorting is done based on properties of the object's themselves.

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