What is java map?

Map provides a dual-column mapping storage collection, which can provide one-to-one data processing capabilities, the first column of the dual columns we It is called key, and the second column is value. A key can only appear at most once in a Map. Through a key, you can obtain the only corresponding value in the Map. It is this one-to-one mapping of data. Processing relationships, in practical applications, you can quickly locate the corresponding value through a key. (Recommended: java video tutorial)

Based on the above concepts, the following core points can be summarized:

1. Map storage is based on k-v key-value pairs The storage method is double column

2. The key in the Map is unique and cannot be repeated

3. The value corresponding to each key is unique

Map in Java is an interface. It does not inherit any other interface. It can be said that it is the top-level parent interface of all Maps in java. Its design concept completely follows the above rules, but there are many types of specific implementation classes, corresponding to the use of different application scenarios, so there may be differences in specific details and design.

Java's Map provides three Map views to display the contents of the Map:

1. Set collection containing only key

2. Set containing only value Collection

3. EntrySet that also contains key-value mapping

In addition, additional attention is required: you cannot use mutable objects as the keys of the Map, because once the object changes, it will cause the Map to The behavior is unpredictable (the changes here refer to changes that affect the comparison results of the equals method); at the same time, the Map itself cannot be used as the key of a Map, but the Map itself is allowed to be stored in the Map structure as a value.

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