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Set集合

2022-06-30 11:53:00 【時宜】

目录

一、特点

二、遍历

三、扩容

四、实现

HashSet

TreeSet

自定义比较器

通过构造函数传入比较器

 实现排序接口

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

  1. 默认值： 初始容量16，负载因子0.75
  2. 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： 

 

原网站

版权声明
本文为[時宜]所创，转载请带上原文链接，感谢
https://blog.csdn.net/weixin_62270300/article/details/125495287

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