HashMap in Java
HashMap in Java is a collection that stores key-value pairs. It uses hashing internally to provide average O(1) time complexity for put, get, and remove operations. It's part of the Java Collections Framework.
Basic operations:
Map map = new HashMap<>();
// Insert
map.put("apple", 10);
map.put("banana", 20);
// Get
int count = map.get("apple"); // 10
// Check existence
boolean hasApple = map.containsKey("apple"); // true
// Remove
map.remove("banana");
// Iterate over keys
for (String key : map.keySet()) {
System.out.println(key + ": " + map.get(key));
}
// Iterate over entries
for (Map.Entry entry : map.entrySet()) {
System.out.println(entry.getKey() + " -> " + entry.getValue());
}
Important points:
- Keys must implement hashCode() and equals() correctly.
- HashMap is not thread-safe; use ConcurrentHashMap for concurrent access.
- Initial capacity and load factor affect performance.
- Allows one null key and many null values.
Two Minute Drill
- HashMap stores key-value pairs with O(1) average time.
- Methods: put, get, containsKey, remove, keySet, entrySet.
- Keys must override hashCode() and equals().
- Not thread-safe; use ConcurrentHashMap for concurrency.
- Resizes when load factor exceeded.
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