Map collection in apex

What is a map?

A map is a collection of key value pairs. The keys are always unique having a value associated.

Key and values can be of any of the following data types in apex:

• Primitive types, like string, integer etc..
• Collections, like list, set etc..
• SObjects
• User-Defined types

When to use a map

A map can be utilized in the following scenarios in salesforce:

• Relating records based on a unique identifier.

  To relate Contact records to their respective Account records, use a Map with AccountId as the key and a list of Contact records as the value.

• Aggregation of data based on a unique key.

  To calculate total Opportunity amounts per Account, use a Map with AccountId as the key and the total Opportunity Amount as the value.

• Building quick look ups

  To create a quick lookup for country capitals, use a Map with the country code as the key and the capital as the value.

How to declare a map?

You can declare a map using the Map<key, value> syntax. Following is an example of a map with string key and value.