Updating Map Entries

A map is not frozen once you build it. You can replace the value behind a key, grow a value that is already there, or drop a pair completely.

put() Replaces Instead of Duplicating

Calling put() with a key the map already holds does not add a second pair. The new value takes the place of the old one and size() stays the same.

Map<String, Integer> stock = new Map<String, Integer>{ 'pens' => 12 };
stock.put('pens', 20);
System.debug(stock.get('pens')); // 20
System.debug(stock.size()); // 1

Growing a Value That Is Already There

To change a value based on what is already stored, read it with get(), work out the new value, and put it back under the same key.

stock.put('pens', stock.get('pens') + 5);
System.debug(stock.get('pens')); // 25

Counting with containsKey()

The most common update pattern is a tally. Because get() returns null for a key the map has never seen, check with containsKey() first and start that key at 1.

List<String> visits = new List<String>{ 'apex', 'soql', 'apex' };
Map<String, Integer> tally = new Map<String, Integer>();
for (String pageName : visits) {
	if (tally.containsKey(pageName)) {
		tally.put(pageName, tally.get(pageName) + 1);
	} else {
		tally.put(pageName, 1);
	}
}
System.debug(tally.get('apex')); // 2

remove() Drops a Pair

remove() deletes the pair and hands back the value it was holding.

Integer removedCount = tally.remove('soql'); // 1
System.debug(tally.size()); // 1

Common Mistakes

Do not skip the containsKey() check when you are counting. Adding 1 to the null that a missing key returns throws a null pointer exception on the very first word.

Why This Matters

Tallying is everywhere: how many opportunities each owner has, how many errors of each type a job hit, how many contacts sit under each account. One map plus one loop answers all of them.