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This article introduces the relevant knowledge of "what are the characteristics of Set interface". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Set interface

The biggest feature of the Set collection is that repeating elements are not allowed to be saved, which is also the Collection subinterface.

Before JDK1.9, the definition of the Set collection is no different from that of the Collection collection. Set continues to operate using the methods provided in the Collection interface, but since JDK1.9, the Set collection also extends some static methods like the List collection. The definition of the Set collection is as follows:

Public interface Set extends Collection

It should be noted that the Set collection does not extend many new methods like the List collection, so the get () method provided in the List collection cannot be used, that is, the acquisition of the specified index data cannot be implemented. The inheritance relationship of the Set interface is as follows.

Set interface inheritance relationship

Since JDK1.9, the Set collection also provides static methods like of () in the List collection. Let's use this method to verify the characteristics of the Set collection.

Example: verify the characteristics of the Set collection

Import java.util.Set;public class JavaAPIDemo {public static void main (String [] args) throws Exception {/ / saves Set collection data and sets duplicate content Set all=Set.of ("Hello", "World", "MLDN", "Hello", "World"); all.forEach (System.out::println); / / Direct output / / Exception in thread "main" >

When using the new method of (), an exception is thrown directly if a duplicate element is found in the collection. This is consistent with the fact that traditional Set collections do not hold repeating elements, except that they throw exceptions themselves.

The general use of Set collections must rely on subclasses for instantiation, so there are two commonly used subclasses in the Set interface: HashSet and TreeSet.

HashSet subclass

HashSet is the most frequently used subclass in the Set API. Its most important feature is that the saved data is unordered, while the inheritance relationship of the HashSet subclass is as follows:

Public class HashSet extends AbstractSet implements Set, Cloneable, Serializable

This form of inheritance is very similar to the previous ArrayList, so now take a look at the inheritance structure of the class:

HashSet subclass

Example: observe the HashSet class

Import java.util.HashSet;import java.util.Set;public class JavaAPIDemo {public static void main (String [] args) throws Exception {Set all = new HashSet (); all.add ("MLDN"); all.add ("NiHao"); all.add ("Hello"); all.add ("Hello"); / / repeating element all.add ("World"); all.forEach (System.out::println) }} / * * NiHao* Hello* World* MLDN*/

Through the execution results, we can find the operational characteristics of HashSet: duplicate elements (defined by the Set interface) are not allowed to be saved, and another feature is that the data stored in HashSet is unordered.

TreeSet subclass

Another subinterface of the Set interface is TreeSet. The biggest difference from HashSet is that the data stored in the TreeSet collection is ordered. First, let's take a look at the definition of the TreeSet class:

Public class TreeSet extends AbstractSet implements NavigableSet, Cloneable, Serializable

In this subclass, the AbstractSet parent abstract class is still inherited, while a NavigableSet parent interface is implemented.

Example: using the TreeSet subclass

Import java.util.TreeSet;import java.util.Set;public class JavaAPIDemo {public static void main (String [] args) throws Exception {Set all = new TreeSet (); all.add ("MLDN"); all.add ("NiHao"); all.add ("Hello"); all.add ("Hello"); / / repeating element all.add ("World"); all.forEach (System.out::println) }} / * * Hello* MLDN* NiHao* World*/

When saving data with TreeSet, all data will be sorted automatically according to the ascending order of the data.

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