Question

What is the output? HashSet set \(1=\) new HashSet () ; set1.add(1) ; set \(1 \cdot\) add \((3)\); \(\operatorname{set} 1 \cdot \operatorname{add}(2) ;\) HashSet \(\operatorname{set} 2=\) new HashSet ()\(;\) \(\operatorname{set} 2 \cdot \operatorname{add}(2) ;\) \(\operatorname{set} 2 \cdot \operatorname{add}(3):\) System.out.println (set1. containsAll (set2)) ; a. True b. False c. Compilation error d. Runtime exception

Short answer

True

Step-by-step solution

- Initialize Set1

Create a new HashSet called 'set1' and add the elements 1, 3, and 2 to it.

- Initialize Set2

Create a new HashSet called 'set2' and add the elements 2 and 3 to it.

- Check Set Containment

Use the containsAll method on 'set1' to determine if it contains all the elements of 'set2'.

- Determine Output

If 'set1' contains all the elements of 'set2', the result will be true, otherwise false.

Key concepts

Java Collection Framework

The Java Collection Framework is a cornerstone of the Java programming language. It provides a comprehensive set of interfaces and classes to store and manipulate groups of objects. The key interfaces in the framework are List, Set, and Map. Each of these has numerous implementations that cater to different use cases. For instance, ArrayList and LinkedList are common implementations of the List interface, while HashSet and TreeSet implement the Set interface.

The framework simplifies many common tasks related to data structures. It offers a variety of operations such as adding, removing, or checking the presence of elements. Since it is part of Java's core libraries, you can use these data structures without needing to import third-party libraries.

One of the biggest advantages of the Java Collection Framework is its flexibility. You can switch between different implementations (like from ArrayList to LinkedList) with minimal code changes. This makes your programs more adaptable to changing requirements.

HashSet Operations

A HashSet is a part of Java's Collection Framework used to store unique elements. It implements the Set interface and extends the AbstractSet class. HashSets are backed by a HashMap and are particularly efficient for queries like checking whether an element is present.

Here's how you can use common HashSet operations in practice:

• **Adding Elements**: Use the add() method. For example: `set1.add(1);`
• **Removing Elements**: Use the remove() method: `set1.remove(2);`
• **Checking Presence**: Use the contains() method: `set1.contains(3);`
• **Checking Subset**: Use the containsAll() method to determine if all elements of one set are in another: `set1.containsAll(set2);`

HashSets do not maintain the order of their elements since they use a hash table for storage. This makes operations like insertion, deletion, and lookup pretty fast, usually with an average time complexity of O(1).

Java Set Methods

Sets in Java are used to store collections of unique elements. Here's a quick rundown of some of the essential Set methods:

• **add(E element)**: Adds the specified element to the set if it's not already present. Returns true if the set did not already contain the element.
• **remove(Object o)**: Removes the specified element from the set if it's present. Returns true if the set contained the specified element.
• **contains(Object o)**: Returns true if the set contains the specified element.
• **containsAll(Collection c)**: Returns true if the set contains all elements in the specified collection. This is particularly useful for checking if one set is a subset of another, as seen in the original exercise.
• **size()**: Returns the number of elements in the set.

Sets come in several flavors, such as HashSet, TreeSet, and LinkedHashSet. Each has its own use case and performance characteristics. For instance, TreeSet maintains sorted order, but this comes at the cost of O(log n) time complexity for insertion and lookup operations, whereas HashSet offers O(1) average time complexity for these operations but does not maintain order.