What does map do in java?

Map stores key-value pairs in Java and is used to store user preferences, cache data, implement lookup tables, associate objects, track status, etc. The specific usage method is: create a Map instance, add key-value pairs, and use the get() method to obtain the value.

The purpose of map in Java

Map is a data structure in Java that is used to store keys value pair. It is an unordered data collection, which means the order of the elements is unreliable. The keys of a Map are unique, which means that two values ​​cannot be stored with the same key.

Uses:

Map is widely used in Java for various purposes, including:

• Storing user preferences: Map can be used to store user preferences, such as language settings or theme selection. This makes it easy to access and modify these preferences.
• Caching data: Map can be used to cache frequently accessed data to improve application performance. By storing data in a Map, you avoid querying the database or file system multiple times.
• Implementing a lookup table: Map can be used to implement a lookup table, where the key is the input value and the value is the corresponding output. This makes it easy to find data items quickly.
• Associated objects: Map can be used to associate different objects. For example, a Map can be used to map customer IDs to customer objects in order to obtain customer information from the ID.
• Tracking state: Map can be used to track the current state of an object. For example, a Map can be used to store the board state of a game or the process state of an application.

How to use:

To use Map, you need to create an instance of the Map class. A variety of Map implementations are provided in Java, including HashMap, TreeMap and LinkedHashMap.

Here is an example of how to use HashMap:

Map<String, String> map = new HashMap<>();
map.put("name", "John Doe");
map.put("age", "30");
System.out.println(map.get("name")); // 输出: John Doe

In the above example, a HashMap is created where the keys are strings and the values ​​are also strings. The key "name" is then mapped to the value "John Doe" and the key "age" is mapped to the value "30". To get a value, you can use the get() method, passing the key of the value you want to retrieve.

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