Set set you don't know

This issue is wonderful ：

Set Interface

Basic introduction

Set Implementation class of interface

1、HashSet

characteristic  

HashSet The implementation of the

Code knowledge  

2、TreeSet

characteristic

Code knowledge

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Set Interface

Basic introduction

characteristic ：

• Set： disorder ( The sequence of deposit and withdrawal is different, so the subscript cannot be used to traverse ), No repetition
• List： Orderly , Element is not repeatable

Traverse ：foreach, iterator

Capacity expansion ： Initial capacity 16, Load factor 0.75, Expansion increment 1 times

Set、Queue、List implement Collection, and Map Will not be achieved Collection Interface

 ThreeSet And Collection etc. Relational UML chart

Those who can't understand the relationship arrow in this figure can go to this blog , It's introduced in detail UML chart
List aggregate &UML chart _ Little Alfie _ The blog of -CSDN Blog UML The secret that you can understand at a glance and List How much do you know about the set in the interface ？https://blog.csdn.net/yifei_345678/article/details/125483171?spm=1001.2014.3001.5501

Set Implementation class of interface

Set and List Collections also belong to interfaces , Cannot directly create an instantiated object , You need to implement classes to create ,HashSet、ThreeSet Are commonly used implementation classes

1、HashSet

characteristic  

• Non-thread safety
• from HashMap Support
• There is no guarantee of insertion order
• Exist in java.util package Class in , It is also called a set , In this container Only non repeating objects can be stored
• It stores The only element And allow null values （ According to the hashcode To determine whether the element exists ）
• performance parameter ： Initial capacity , Load factor （ The default value is ： Initial capacity 16, Load factor 0.75）

HashSet The implementation of the

about HashSet for , It is based on HashMap Realized ,HashSet Bottom use HashMap To save all elements , therefore HashSet The implementation of is relatively simple , relevant HashSet The operation of , Basically, they call the underlying layer directly HashMap To complete , We should be Save to for HashSet Object overlay in hashCode() and equals()

Known implementation interfaces are ：

Serializable, Cloneable, Iterable<E>, Collection<E>, Set<E>

Known subclasses ：

JobStateReasons, LinkedHashSet

Code knowledge  

Once there was a problem , Ask how to ArrayList Remove the repeated elements in ( duplicate removal ), There are many ways , Little apes can try to write a few by themselves , Here is a simple and fast method ： Use HashSet duplicate removal

public class SetDemo {
	
	private List<Integer> list = new ArrayList<>();
	
    @BeforeAll
	public void setup() {
		set.add(1);
		set.add(1);
		set.add(2);
		set.add(2);
		set.add(3);
		set.add(3);
	}
	
	@Test
	public void test01() {
		List<Integer> tmp = new ArrayList<>(new HashSet<Integer>(list));
		System.out.println(tmp);
	}
}

The output is 123（ArrayList It can store repeated elements , But here we use HashSet duplicate removal ） 

1、 Create a new one Set Set and put elements

public class SetDemo {
	
    private Set<Integer> set = new HashSet<>();
	
	@BeforeAll
	public void setup() {
		set.add(1);
		set.add(1);
		set.add(2);
		set.add(4);
		set.add(5);
		set.add(3);
	}
}

2、 You can use the following two methods to traverse the elements , because HashSet Only non duplicate objects can be stored in , Therefore, the repeated elements will be automatically de duplicated when outputting  

@Test
	public void test02() {
		for(Integer e: set) {
			System.out.println(e);
		}
	}
	
    // Using Iterators 
	@Test
	public void test03() {
		Iterator<Integer> it = set.iterator();
		while(it.hasNext()) {
			System.out.println(it.next());
		}
	}    

3、set.remove(i) What is passed in is the element , This is because HashSet There is no guarantee of insertion order , No subscript concept , So you can only pass in elements

    @Test
	public void test05() {
		set.remove(3);
		System.out.println(set);
	}

Delete the first step Set The elements in the collection 3, The output is 1245 

2、TreeSet

characteristic

• Is a containing Orderly And There are no repeating elements Set
• Role is Provide orderly Set aggregate , Natural ordering ( such as 123456...) Or according to the information provided Comparator（ The comparator ） Sort
• TreeSet Is based on TreeMap Realized , and ThreeMap The bottom is Map( Key value pair )

Code knowledge

Create a student class and implement the comparator interface

public class Student implements Comparable<Student>{
	
	private Integer sid;	
	private String sname;
	private int age;

	public Student(Integer sid, String sname, int age) {
		super();
		this.sid = sid;
		this.sname = sname;
		this.age = age;
	}

	public Integer getSid() {
		return sid;
	}

	public void setSid(Integer sid) {
		this.sid = sid;
	}

	public String getSname() {
		return sname;
	}

	public void setSname(String sname) {
		this.sname = sname;
	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

}

It also needs to be implemented in this student class hashCode and equals Method ：

    /*
    *hashCode You can try to turn the student name into a unique number , Compared with ID card 
    */
    @Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((sid == null) ? 0 : sid.hashCode());
		result = prime * result + ((sname == null) ? 0 : sname.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (age != other.age)
			return false;
		if (sid == null) {
			if (other.sid != null)
				return false;
		} else if (!sid.equals(other.sid))
			return false;
		if (sname == null) {
			if (other.sname != null)
				return false;
		} else if (!sname.equals(other.sname))
			return false;
		return true;// Students with the same student number, name and age are the same students 
	}

	@Override
	public String toString() {
		return "Student [sid=" + sid + ", sname=" + sname + ", age=" + age + "]";
	}
    
    /**
	 *  Realization Comparable<Student> Methods that need to be rewritten after the interface 
     *  Sort by age , The same age is compared with the student number , The default is ascending 
	 */
	@Override
	public int compareTo(Student o) {
		if(this.getAge()-o.getAge()==0) {
			return this.getSid()-o.getSid();
		}
		return this.getAge() - o.getAge();
	}

And then use ThreeSet

    @Test
	public void test04() {
		TreeSet<Student> stu = new TreeSet<>();
		
		stu.add(new Student(1,"zs", 18));
		stu.add(new Student(1,"zs", 18));
		stu.add(new Student(2,"ls", 19));
		stu.add(new Student(4,"lihao", 10));
		stu.add(new Student(7,"lihao", 18));
		stu.add(new Student(5,"zengfanyan", 20));
		stu.add(new Student(3,"we", 30));
				
		for(Student s: stu) {
			System.out.println(s);
		}
	}	

Run output ：