What are the usage characteristics of HashSet in java

The difference between HashSet and TreeSet

1. The bottom layer of HashSet uses a Hash table. The principle of ensuring the uniqueness of elements: Determine whether the hashCode values ​​of the elements are the same. If they are the same, it will continue to determine whether the equals method of the element is true.

The bottom layer of TreeSet uses a red-black tree.

Ensuring the uniqueness of elements is achieved through the Comparable or Comparator interface.

The difference between HashSet and HashMap

2. In fact, the underlying implementation of HashSet is still HashMap, but it only uses Key, as follows:

(1) At the bottom of the add method of HashSet, use the put method of HashMap to construct key=e, value=PRESENT into a key-value pair. When e exists in the key of HashMap, the value will overwrite the original value. But the key remains unchanged, so if an already existing e element is added to the HashSet, the newly added element will not exist in the HashMap, so this satisfies the feature that the elements in the HashSet will not be repeated.

(2) The contains method of HashSet uses HashMap to get the containsKey method.

What are the advantages of Java

1. Simple, you only need to understand the basic concepts to write applications suitable for various situations;

2. Object-oriented ;

3. Distribution, Java is a network-oriented language;

4. Robustness, Java provides automatic garbage collection for memory management, preventing programmers from easily generating errors when managing memory. Error;

5. Security, Java used in network and distributed environments must prevent virus intrusion.

6. Architecture neutral, as long as the Java runtime system is installed, it can run on any processor.

7. Portability, Java can be easily transplanted to different machines on the network.

8. Interpretation and execution, the Java interpreter directly interprets and executes the Java bytecode.

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