Introduction to Maps

A Map stores pairs. Each pair has a key you look things up by and a value you get back. Where a list answers what is at position 2, a map answers what is the price of apex-guide.

Declaring a Map

A map declaration names two types: the key type first, then the value type.

Map<String, Integer> prices = new Map<String, Integer>();

You can also fill a map as you create it, using => between each key and value:

Map<String, Integer> stock = new Map<String, Integer>{ 'pens' => 12, 'paper' => 4 };

Adding and Reading Pairs

put() stores a pair and get() reads the value back:

prices.put('guide', 25);
Integer guidePrice = prices.get('guide'); // 25

Putting the same key twice does not create a second pair. The new value replaces the old one, so size() stays the same.

Missing Keys

get() returns null when the key was never stored. Ask containsKey() first when a missing key needs different handling:

if (prices.containsKey('poster')) {
	Integer posterPrice = prices.get('poster');
}

Common Mistakes

Do not assume a map keeps the order you added pairs in, and remember that size() counts pairs, not keys and values separately. Also watch out for assigning get() of a missing key straight into an Integer, because the null you get back will throw as soon as you do math with it.

Why This Matters

Maps turn a slow search into an instant lookup. Instead of looping through a list of accounts to find the one with a given id, you build a map once and read any record straight out of it.