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Lesson 38

HashMap and HashSet

HashMap stores key-value pairs. HashSet stores unique values only.

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Lesson content

HashMap: key-value pairs. HashSet: unique values only.

Key Terms

  • HashMap: Stores data as key-value pairs.
  • HashSet: Stores unique elements, no duplicates.
  • Key: Unique identifier in a HashMap.
  • Hashing: Converts data to hash code for storage.

Example: HashMap

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<Integer, String> students = new HashMap<>();
        students.put(101, "John");
        students.put(102, "Alice");
        students.put(103, "David");
        System.out.println(students);
        System.out.println("Student: " + students.get(102));
    }
}
{101=John, 102=Alice, 103=David}
Student: Alice

HashMap Methods

map.put(101, "John");       // add
map.get(101);               // retrieve
map.remove(101);            // delete
map.containsKey(101);       // check key
map.containsValue("John");  // check value
map.size();                 // count

Example: HashSet

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple");
        fruits.add("Mango");
        System.out.println(fruits);
    }
}
[Apple, Banana, Mango]

HashSet Methods

set.add("Java");       // add
set.remove("Java");    // remove
set.contains("Java");  // check
set.size();            // count
set.clear();           // clear all

Iterating HashMap

for (HashMap.Entry<Integer, String> entry : students.entrySet()) {
    System.out.println(entry.getKey() + " : " + entry.getValue());
}

Iterating HashSet

for (String fruit : fruits) {
    System.out.println(fruit);
}

FAQ

1. HashMap vs HashSet?

HashMap: key-value. HashSet: unique values only.

2. HashSet allow duplicates?

No. Auto-removed.

3. HashMap duplicate values?

Yes. Values can repeat. Keys cannot.

HashMap stores key-value pairs.
HashSet stores unique values only.
Keys in HashMap must be unique.
HashSet backed by HashMap internally.
Neither maintains insertion order.
HashMap allows one null key.
HashSet allows one null value.

Where Java is used

Java in real-world development

HashMap

Key-value pairs. Fast lookup by key. One null key allowed.

HashSet

Unique values only. Duplicates auto-removed. One null allowed.

HashMap Methods

put(), get(), remove(), containsKey(), containsValue(), size()

HashSet Methods

add(), remove(), contains(), size(), clear()

Use HashMap When

Data has key-value relationships. Fast key-based retrieval needed.

Use HashSet When

Unique values required. Duplicate prevention needed.

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