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This article mainly introduces Java intern interview questions, which have certain reference value, interested friends can refer to, I hope you have a lot of gains after reading this article, let Xiaobian take you to understand.

Java data structures have you used? What is the difference between map and set?

A data structure is a collection of data elements that have one or more specific relationships with each other.

According to logical relationships, data structures are divided into linear and nonlinear data structures.

What is the essential difference between Map and Set?

Set cannot contain duplicate elements, zui has a null value, inherited from the Collection interface, the underlying is the Map implementation mechanism. Map cannot contain duplicate keys. Each key zui corresponds to more than one mapped value. No keys with empty values are allowed. The methods provided by the two interfaces are not exactly the same.

What are the common Java data structures?

Common Java data structures include Collection and Map, where Collection interface includes List and Set interfaces, and there are multiple implementation classes under it, such as ArrayList, LinkedList, Vector and other implementation classes under List; HashSet, LinkedSet and other implementation classes and SortedSet interface under Set; LinkedHashSet subclass under HashSet; and TreeSet implementation class under SortedSet interface. Under the Map interface there are HashMap(with LinkedHashMap subclass), HashTable(with Properties subclass) implementation class and SortedMap interface (with TreeMap implementation class).

Java data structures include List, Set, Map, ArrayList, HashSet, HashMap, Properties, etc.

How does Annotation work in Java?

An annotation type is only visible at runtime if it is defined as runtime retention, and annotations saved in class files when they are loaded are read by the virtual machine.

Annotations provide a mechanism for associating elements of a program such as classes, methods, attributes, parameters, local variables, packages, and metadata. This allows the compiler to store metadata in Class files. Virtual machines and other objects can then use this metadata to decide how to use these program elements or change their behavior.

5. What is the role of Dao layer in layered architecture? If Dao layer is not needed, can the advantages of Dao layer be realized?

Dao layer is also called data access object layer, encapsulating the underlying operation details of the database, such as database connections and CRUD operations on data, which not only shields the difference in the final medium of data storage, but also shields the difference in specific implementation technologies. There is usually a Service layer on top of it to encapsulate business operations for further decoupling. Without DAO layer can also achieve the advantages of the Dao layer, such as their own encapsulation of a similar function of the Dao layer database operation template class.

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