java hashmap hashset

Set / Map in Java

HashSet

HashSet is a part of the Java Collections Framework and implements the Set interface. It is a collection that stores unique elements in a hash table structure. This means that it uses a hash function to map elements to indices in an array, and performs operations such as add, remove, and search in constant time on average, regardless of the number of elements in the set.

HashSet does not maintain the order of elements, which means that the elements are not stored in the same order as they were inserted. If you need to maintain the order of elements, you should use a LinkedHashSet instead.

One important aspect of HashSet is that it allows null elements. However, there can only be one null element in a HashSet at any given time.

HashSet provides a number of methods for adding, removing, and searching for elements, as well as methods for performing set operations such as union, intersection, and difference.

In summary, HashSet is a fast and efficient implementation of the Set interface that is used to store unique elements and provides constant time access to elements.

HashMap

A HashMap is a class in Java that implements the Map interface. It stores key-value pairs in a hash table data structure, where each unique key is mapped to a single value. The key is used to efficiently retrieve the associated value. The hash function is used to map the keys to indices in the internal array, allowing for constant-time average complexity for operations such as adding, removing, and searching for elements.

HashMap does not maintain the order of elements, and allows one null key and multiple null values.

HashMap provides a number of methods for adding, removing, and searching for key-value pairs, as well as methods for performing operations such as checking if a key exists in the map, getting the size of the map, and clearing all elements from the map.

In summary, HashMap is a useful data structure in Java for storing and retrieving key-value pairs in an efficient manner.

TreeSet / TreeMap

A TreeSet is a class in Java that implements the Set interface and uses a tree data structure to store its elements. It stores elements in sorted order, which can be either their natural order or a custom order defined by a Comparator. The elements are stored in a balanced tree structure, which allows for logarithmic time complexity for operations such as add, remove, and search.

A TreeSet does not allow null elements.

A TreeMap is a class in Java that implements the Map interface and uses a tree data structure to store its key-value pairs. Like a TreeSet, it stores elements in sorted order, either based on their natural order or a custom order defined by a Comparator. The keys are stored in a balanced tree structure, which allows for logarithmic time complexity for operations such as add, remove, and search.

A TreeMap does not allow a null key, but allows multiple null values.

In summary, both TreeSet and TreeMap are classes in Java that provide a sorted data structure for storing elements and are useful when you need to maintain the order of elements or efficiently retrieve elements in a sorted manner.