HashMap Operations & Iteration

Add, look up, remove, and loop through key/value pairs. Every method operates on the key — that's the whole point of a map.

Every operation uses the key. put(key, value), get(key), remove(key), containsKey(key). If you want to loop through all entries, use values(), keySet(), or entrySet() — three different views of the same map.

HashMap<String, String> statesAndCapitals

size(): 0

put(key, value) — add or update

get(key) — returns value or null

remove(key)

bulk

Iterating through a HashMap — three ways

Click a button to see what that iteration style produces.

Watch out: get("OK") on a map without an "OK" key returns null — not an error. If you try to use the returned value immediately, you'll hit a NullPointerException.

String value = map.get("OK"); if (value != null) { // safe to use value here System.out.println(value); } else { System.out.println("not in the map"); }

Call log

Tip: the list you see above won't always match insertion order — HashMaps use bucket order. If order matters, use a LinkedHashMap or a List instead.

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