Question

HashSet set1 = new HashSet(); set1.add (1) ; set1.add (2) ; set1. add (1) ; Hashset \(\operatorname{set} 2=\) new HashSet ()\(;\) \(\operatorname{set} 2 \cdot \operatorname{add}(4) ;\) set \(2 \cdot \operatorname{add}(3) ;\) set1.addAll \((\operatorname{set} 2) ;\) tem.out.println(set1): a. \([1,2,3,4]\) b. \([1,2,1,4,3]\) c. \([1,2,4,3]\) d. \([2,1,4,3]\)

Short answer

c. [1,2,4,3]

Step-by-step solution

Initialize the Sets

Start by creating two instances of HashSet: set1 and set2.

Add Elements to set1

Add the elements 1 and 2 to set1, then try to add element 1 again. Since HashSet doesn't allow duplicate values, set1 will still contain {1, 2}.

Add Elements to set2

Add the elements 4 and 3 to set2. At this point, set2 contains {4, 3}.

Merge set2 into set1

Use the addAll method to combine set2 into set1. The resulting set1 will be {1, 2, 4, 3}, because HashSet is unordered and merges without duplicates.

Print the Result

Use System.out.println to print the elements of set1. The output will be of the form {1, 2, 4, 3}. Since sets are unordered, any order of these four elements is correct.

Key concepts

HashSet operations

In Java, a HashSet is a collection that does not allow any duplicate elements. It is part of the java.util package.
A HashSet is typically used for fast data retrieval and ensures that each element is unique.
Common operations on a HashSet include:

• add(): Adds a specified element to the set if it is not already present. Example: set1.add(1);
• addAll(): Combines another collection into the HashSet. Example: set1.addAll(set2);
• remove(): Removes a specified element from the set if it is present. Example: set1.remove(1);
• contains(): Checks if a specified element exists in the set. Example: set1.contains(1);

Understanding these operations helps you manage sets effectively in Java programming.

Duplicate elements in HashSet

One of the key features of a HashSet is that it does not allow duplicate elements. When you attempt to add a duplicate element:

• The duplicate is ignored, and the set remains unchanged.

For example, in the exercise:
set1.add(1);
set1.add(2);
set1.add(1);
The second add(1) operation is ignored because 1 is already present in set1.
Thus, set1 ends up containing only {1, 2} after these operations.
This property is particularly useful when you need to ensure data uniqueness without additional checks.

Merging HashSets

To merge two HashSets, you can use the addAll() method. This combiner function includes all elements from the second set into the first set.
Let's look at the exercise example:
set1.addAll(set2);
Here, set1 gets updated with all elements of set2:

• Initial set1: {1, 2}
• set2: {3, 4}
• Resulting set1 after merge: {1, 2, 3, 4}

Note that if any elements in set2 were already present in set1, they would not be duplicated.
This operation is efficient for consolidating unique data from multiple collections.

Unordered collection in Java

Unlike arrays or lists, a HashSet does not maintain the order of elements.
The internal structure of a HashSet ensures uniqueness and quick access but in an unordered fashion.
For example, after adding elements {1, 2, 3, 4} to set1, the printed order can be different each time:
System.out.println(set1);
The output could be any permutation of the set like [1, 2, 3, 4] or [4, 3, 2, 1].
This unordered nature is beneficial for use cases where the order of data does not matter.
It also improves performance for search, insert, and delete operations because of the underlying hash table implementation.