What is the difference between a collection and a map?

Collections and maps are commonly used data structures in Java, with their main difference being in storage and access methods.

1. 存储方式：

1. A collection is a group of objects that may contain duplicate elements. The Collection interface has two subinterfaces, List and Set, which store elements in the form of a linear list and a collection, respectively.
2. Map is a mapping table of key-value pairs, where each key corresponds to a unique value. Keys and values in Map can be any object, but keys are unique while values can be repeated.

1. Method of access:

1. A Collection accesses elements in a collection using an iterator, and can also be traversed using a foreach loop.
2. Maps allow for accessing corresponding values using keys. You can use the get() method to retrieve values based on keys, or iterate through the map using key sets, value sets, or key-value pairs.

Summary:

1. Collections are suitable for storing a group of objects, which can include duplicate elements, and accessing elements requires iterating through them using an iterator.
2. Maps are suitable for storing key-value pairs, where each key corresponds to a unique value and the value can be accessed by searching with the key.