HashMap Example in Java
beginner· Collections · Map
Problem
A HashMap stores key-value pairs and gives near-instant lookup by key, without needing to scan through every entry the way a list search would.
Store a few people's ages in a HashMap keyed by name, then look one up and check whether another key exists.
Input
put("Alice", 30), put("Bob", 25), put("Charlie", 35)
Output
Bob's age: 25
Java Program
Java
import java.util.HashMap;
import java.util.Map;
public class HashMapExample {
public static void main(String[] args) {
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);
ages.put("Charlie", 35);
System.out.println("Bob's age: " + ages.get("Bob"));
System.out.println("Contains Alice: " + ages.containsKey("Alice"));
System.out.println("Contains Dave: " + ages.containsKey("Dave"));
}
}Output
Bob's age: 25
Contains Alice: true
Contains Dave: false
Core Logic
Storing each name-age pair with put() builds the map, and get()/containsKey() then query it directly by key instead of searching through it.
How It Works
- 1
put("Alice", 30)and the two calls after it each store one key-value pair in the map. - 2
get("Bob")looks up the value stored under the key"Bob"and returns it directly. - 3
containsKey("Alice")checks whether that exact key exists in the map, returning a boolean rather than the value itself. - 4
containsKey("Dave")returnsfalse, since no entry was ever stored under that key.
After the three
put() calls, get("Bob") returns 25, and containsKey("Dave") correctly reports false.💡
Key Point: get() on a missing key returns null rather than throwing — containsKey() is the safer check when a map might not have a given key at all.
Complexity
Time Complexity: O(1)Space Complexity: O(1)
Why: put(), get(), and containsKey() each hash the key directly to its bucket, giving average-case constant time; only a pathological hash collision pattern would push any single call toward O(n).
Key Concepts
HashMapput()get()containsKey()