Set集合

一、特点

二、遍历

三、扩容

四、实现

一、特点

特点：无序，不重复

如何去重❓

先将包含重复元素的list构造出一个HashSet，再通过HashSet构造出一个ArrayList


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

效果图如下：

二、遍历

foreach，迭代器


 
	private Set<Integer>set=new HashSet<>();
	
	@Before
	public void setup() {
		set.add(1);
		set.add(2);
		set.add(2);
		set.add(3);
		set.add(4);
		set.add(5);
		set.add(6);
		set.add(7);
		
	}
	
 
    /**
	 * foreach遍历
	 */
	@Test
	public void test02() {
		for (Integer e : set) {
			System.out.println(e);
		}
	}
	
	/**
	 * 迭代器
	 */
	@Test
	public void test03() {
		Iterator<Integer> it = set.iterator();
		while(it.hasNext()) {
			System.out.println(it.next());
		}
	}

效果图如下：

三、扩容

扩容：初始容量16，负载因子0.75，扩容增量1倍

四、实现

HashSet

它存储唯一元素并允许空值，依据对象的hashcode来确定该元素是否存在

示例：

首先建立一个Student的实体类


package com.zking.set;
 
public class Student {
 
	private Integer sid;
	private String sname;
	private Integer sage;
	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 Integer getSage() {
		return sage;
	}
	public void setSage(Integer sage) {
		this.sage = sage;
	}
	public Student(Integer sid, String sname, Integer sage) {
		super();
		this.sid = sid;
		this.sname = sname;
		this.sage = sage;
	}
	public Student() {
		super();
	}
	@Override
	public String toString() {
		return "Student [sid=" + sid + ", sname=" + sname + ", sage=" + sage + "]";
	}
 
}

测试方法


public void test04() {
		Set<Student>stu=new HashSet<>();
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(2,"千帆",19));
		stu.add(new Student(3,"鼠标",17));
		stu.add(new Student(4,"耳机",22));
		stu.add(new Student(5,"安楠",30));
		
		for (Student s : stu) {
			System.out.println(s);
		}
	}

运行结果：发现并没有去重

在实体类里重写hascode以及equals方法后


@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((sage == null) ? 0 : sage.hashCode());
		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 (sage == null) {
			if (other.sage != null)
				return false;
		} else if (!sage.equals(other.sage))
			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;
	}

效果图如下：去重成功

注意：HashSet不保持插入顺序，非线程安全

性能参数：初始容量，负载因子

默认值：初始容量16，负载因子0.75
示例：new HashSet<>(20, 0.5f);

set删除（删除元素）


public void test05() {
		set.remove(7);
		System.out.println(set);
	}

效果图如下：

TreeSet

是一个包含有序的且没有重复元素的集合，作用是提供有序的Set集合，自然排序或者根据提供的Comparator进行排序。TreeSet是基于TreeMap实现的。

示例：如何根据年龄排序（从大到小）❓


	public void test04() {
     //自定义比较器
		TreeSet<Student>stu=new TreeSet<>(new Comparator<Student>() {
 
			@Override
			public int compare(Student o1, Student o2) {
				
				return o2.getSage()-o1.getSage();
			}
 
			
		});
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(2,"千帆",19));
		stu.add(new Student(3,"鼠标",17));
		stu.add(new Student(4,"耳机",22));
		stu.add(new Student(5,"安楠",30));
		
		for (Student s : stu) {
			System.out.println(s);
		}
	}

效果图如下：

删除对象

自定义比较器

通过构造函数传入比较器


public void test04() {
		TreeSet<Student>stu=new TreeSet<>(new Comparator<Student>() {
 
			@Override
			public int compare(Student o1, Student o2) {
				
				return o2.getSid()-o1.getSid();
			}
 
			
		});
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(2,"千帆",19));
		stu.add(new Student(3,"鼠标",17));
		stu.add(new Student(7,"鼠标",18));
		stu.add(new Student(4,"耳机",22));
		stu.add(new Student(5,"安楠",30));
		
		
		stu.remove(new Student(3,"鼠标",17));
		for (Student s : stu) {
			System.out.println(s);
		}
	}

效果图如下：

实现排序接口


package com.zking.set;
 
public class Student implements Comparable<Student>{
 
	private Integer sid;
	private String sname;
	private Integer sage;
	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 Integer getSage() {
		return sage;
	}
	public void setSage(Integer sage) {
		this.sage = sage;
	}
	public Student(Integer sid, String sname, Integer sage) {
		super();
		this.sid = sid;
		this.sname = sname;
		this.sage = sage;
	}
	public Student() {
		super();
	}
	@Override
	public String toString() {
		return "Student [sid=" + sid + ", sname=" + sname + ", sage=" + sage + "]";
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((sage == null) ? 0 : sage.hashCode());
		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 (sage == null) {
			if (other.sage != null)
				return false;
		} else if (!sage.equals(other.sage))
			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;
	}
	@Override
	public int compareTo(Student o) {
		// TODO Auto-generated method stub
		return this.getSid()-o.getSid();
	}
 
}

注意：在使用比较器进行删除时要注意比较器中的字段若删除的数据中有重复的元素而这个元素如果与比较器的字段一样那么可能会出现删除错误。因为比较器会根据你设定的字段排序，如果出现重复会自动进行去重。


public void test04() {
		TreeSet<Student>stu=new TreeSet<>(new Comparator<Student>() {
 
			@Override
			public int compare(Student o1, Student o2) {
				
				return o2.getSage()-o1.getSage();
			}
 
			
		});
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(2,"千帆",19));
		stu.add(new Student(3,"鼠标",17));
		stu.add(new Student(7,"鼠标",18));
		stu.add(new Student(4,"耳机",22));
		stu.add(new Student(5,"安楠",30));
		
		
		stu.remove(new Student(3,"鼠标",17));
		for (Student s : stu) {
			System.out.println(s);
		}
	}

效果图入下：会发现想删除的的是 Student(3,"鼠标",17)，却把Student(7,"鼠标",18)也删了

解决方案如下：


public void test04() { 
		TreeSet<Student>stu=new TreeSet<>(new Comparator<Student>() {
 
			@Override
			public int compare(Student o1, Student o2) {
				if(o1.getSage()-o2.getSage()==0) {
					return o1.getSid()-o2.getSid();
				}
				return o1.getSage()-o2.getSage();
			
			}
 
			
		});
		//stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(1,"苡桉",18));
		stu.add(new Student(2,"千帆",19));
		stu.add(new Student(3,"鼠标",17));
		stu.add(new Student(7,"鼠标",18));
		stu.add(new Student(4,"耳机",22));
		stu.add(new Student(5,"安楠",30));
		
		
		stu.remove(new Student(3,"鼠标",17));
		for (Student s : stu) {
			System.out.println(s);
		}
	}

效果图如下：

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原文地址：https://blog.csdn.net/weixin_62270300/article/details/125495287