HashMap

HashMap: keys to values

Half of programming is looking things up by name: the price for a product code, the user for a session token, the count for a word. Scanning a list for a match gets slower as data grows; a map answers by key in the same tiny time whether it holds ten entries or ten million, which is why some form of it sits inside almost every cache, index, and config system you will ever touch. A HashMap<K, V> is Java's dictionary: it stores key → value pairs and looks them up in constant time, like Python's dict.

import java.util.HashMap;

HashMap<String, Integer> stock = new HashMap<>();
stock.put("apple", 3);          // insert or overwrite
stock.get("apple")              // 3, or null if missing
stock.getOrDefault("kiwi", 0)   // 0 instead of null
stock.containsKey("apple")      // true
stock.remove("apple");
stock.size();

Two differences from Python worth flagging: a missing key returns null rather than raising an error (prefer getOrDefault to dodge null), and a HashMap has no promised order when you loop over it.

Looping over a map

A map is not a sequence, so looping takes one extra decision: do you want the keys, the values, or both? Each view has a method:

for (String key : stock.keySet()) { ... }        // keys only
for (int count : stock.values()) { ... }          // values only
for (Map.Entry<String, Integer> e : stock.entrySet()) {
  System.out.println(e.getKey() + ": " + e.getValue());  // both
}

A Map.Entry<K, V> is one key-value pair; entrySet() hands you all of them, and needs import java.util.Map;. Prefer entrySet when you use both halves — looping keySet and calling get(key) inside does a second lookup per pass for nothing.