The Java programming language has two distinct yet similar types of data containers, HashMap and HashSet . Both use a hash table to store data. A table is a table of values that uses a hash function to determine where to look and store data. This allowsquick access to data because a value does not have to be searched. Instead, the hash function can provide the exact location of the value. Despite so much use of hash tables, HashMap and HashSet are quite different from each other.
Maps
Maps store data in key-value relationships. A key is said to be mapped to a value. A dictionary is a good example of a map data container. The key is the word you are looking up and the value is the definition. All data in a map must be stored using a unique key, but there can be multiple, identical values. A HashMap is actually an implementation of Map.
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Sets
A set is a collection of data without duplicating elements. For example, the sequence of digits 0 through 9 is a set, but the sequence of 0, 1, 1, 2, 3 is not, since digit 1 is repeated. A set does not feature a key-value relationship like a map. It is simply a collection of non-duplicate values. a HashSet is actually an implementation of Set.
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Performance
HashMap provides constant time performance for the “Setting” and “Getting” methods, which are used to populate the data structure. Constant time performance means that no matter how big the data structure becomes, these operations will always take the same amount of time to perform. The HashSet provides constant time performance for the “add”, “remove”, “contain”, and “size” methods.
Optional features
The HashMap provides all the operations of the Java Map class and the HashSet provides all the operations of the defined Java class. This is because both HashMap and HashSet are implementations of the Java Map and Set classes, respectively.
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