Set集合

Set

Set存储的元素是无序的,不可重复

1. 无序性 无序!=随机性,真正的无序性,指的是元素在底层存储的位置是无序的
2. 不可重新:当向Set中添加相同元素的时候,后面的不能添加进去

**注意:**要求添加进Set中的元素所在的类,一定要重写equals()和hashcode()方法

Set中元素的存储方式 :使用了哈希算法

当添加对象时,首先调用了对象所在类的hashCode()方法,计算对象的哈希值,此hash值决定了对象在Set中的存储位置,当此位置没有对象存储,就将对象存储在该位置上,有对象存储,通过equals方法比较对象是否相同,相同就无法添加,

要求:hashcode()和euqals方法统一一致

LinkedHashSet:

使用链表维护了一个天剑进集合中的顺序,导致当遍历LinkedHashSet集合元素时,按照添加进去的顺序遍历

注意:存储依然是无序的,通过存储的时候,前向索引和后向索引的指向,保证了顺序,

优点:遍历的时候块,插入性低

TreeSet

1. 只能添加同一个类的

2. 不可重复,有序,String和包装类,默认是按照从小到大的顺序

3. 自定义的类,没有实现Comparable接口是,不能向TreeSet中添加

   自然排序 实现lang包下的Comparable接口 ,重写自定义中的compareTo方法,定义按照哪个属性的顺序排列,在

   需要compareTo方法和HashCode和equals方法三者一致

**自然排序:**自定义一个Person类,name和age属性
实现Comparable接口 重写CopareTo方法,重写equals方法和hashcode方法

			package?com.zyd;
	
	import?java.util.Objects;
	
	public?class?Person?implements?Comparable{
	????private?String?name;
	????private?Integer?age;
	
	????public?Person()?{
	????}
	
	????public?Integer?getAge()?{
	????????return?age;
	????}
	
	????public?void?setAge(Integer?age)?{
	????????this.age?=?age;
	????}
	
	????public?String?getName()?{
	
	????????return?name;
	????}
	
	????public?void?setName(String?name)?{
	????????this.name?=?name;
	????}
	
	????public?Person(String?name,?Integer?age)?{
	????????this.name?=?name;
	????????this.age?=?age;
	????}
	
	????@Override
	????public?String?toString()?{
	????????return?super.toString();
	????}
	
	????@Override
	????public?int?hashCode()?{//return?age.hashCode()?+?name.hashCode();没下述的健壮性好。
	????????final?int?prime?=?31;
	????????int?result?=?1;
	????????result?=?prime?*?result?+?((age?==?null)???0?:?age.hashCode());
	????????result?=?prime?*?result?+?((name?==?null)???0?:?name.hashCode());
	????????return?result;
	????????//return?init++;//不能这样用
	????}
	
	????public?boolean?equals(Object?o)?{
	????????if?(this?==?o)?return?true;
	????????if?(o?==?null?||?getClass()?!=?o.getClass())?return?false;
	????????Person?person?=?(Person)?o;
	????????return?Objects.equals(name,?person.name)?&&
	????????????????Objects.equals(age,?person.age);
	????}
	
	????@Override
	????//当向TreeSet中添加Person类的对象时，依据此方法，确定按照哪个属性排列。
	????public?int?compareTo(java.lang.Object?o)?{
	????????if?(o?instanceof?Person){
	????????????Person?p?=?(Person)?o;
	????????????int?i?=?this.age.compareTo(p.age);
	????????????if?(i?==?0){
	????????????????return?this.name.compareTo(p.name);
	????????????}else?{
	????????????????return?i;
	????????????}
	????????}
	????????return?0;
	????}
	
	
	}

重点:

??public?int?compareTo(java.lang.Object?o)?{
????????if?(o?instanceof?Person){
????????????Person?p?=?(Person)?o;
????????????int?i?=?this.age.compareTo(p.age);
????????????if?(i?==?0){
????????????????return?this.name.compareTo(p.name);
????????????}else?{
????????????????return?i;
????????????}
????????}
????????return?0;
????}

主程序

??public?void?test01(){
????????//?当Person类没有实现Comparable接口时，当向TreeSet中添加Person对象时，报ClassCastException
????????Set??set?=?new?TreeSet();
????????set.add(new?Person("AA",23));
????????set.add(new?Person("DD",13));
????????set.add(new?Person("CC",633));
????????set.add(new?Person("BB",23));
????????for?(Object?str?:set){
????????????System.out.println(str);
????????}
????}

结果:先按照age排序,然后是名称顺序

Person{name=‘DD‘,?age=13}
Person{name=‘AA‘,?age=23}
Person{name=‘BB‘,?age=23}
Person{name=‘CC‘,?age=633}

定制排序:

public?void?test02(){
????TreeSet?set?=?new?TreeSet(new?Comparator()?{
????????public?int?compare(Object?o1,?Object?o2)?{
????????????if?(o1?instanceof?com.atguigu.java.Customer?&&?o2?instanceof?com.atguigu.java.Customer)?{
????????????????com.atguigu.java.Customer?c1?=?(com.atguigu.java.Customer)?o1;
????????????????com.atguigu.java.Customer?c2?=?(com.atguigu.java.Customer)?o2;
????????????????int?i?=?c1.getId().compareTo(c2.getId());
????????????????if?(i?==?0)?{
????????????????????return?c1.getName().compareTo(c2.getName());
????????????????}
????????????????return?i;
????????????}
????????????return?0;
????????}
????});
????set.add(new?com.atguigu.java.Customer("AA",?1003));
????set.add(new?com.atguigu.java.Customer("BB",?1002));
????set.add(new?com.atguigu.java.Customer("GG",?1004));
????set.add(new?com.atguigu.java.Customer("CC",?1001));
????set.add(new?com.atguigu.java.Customer("DD",?1001));

????for?(Object?str?:?set)?{
????????System.out.println(str);
????}
}


?@Test
????public?void?test02(){
????????//?1.创建一个实现了Comparator接口的类对象
????????Comparator?com?=?new?Comparator()?{
????????????//?向TreeSet中添加Customer类的对象，在此compare()方法中，指明是按照Customer
????????????//?的哪个属性排序的。
????????????@Override
????????????public?int?compare(Object?o1,?Object?o2)?{
????????????????if?(o1?instanceof?com.atguigu.java.Customer?&&?o2?instanceof?com.atguigu.java.Customer)?{
????????????????????com.atguigu.java.Customer?c1?=?(com.atguigu.java.Customer)?o1;
????????????????????com.atguigu.java.Customer?c2?=?(com.atguigu.java.Customer)?o2;
????????????????????int?i?=?c1.getId().compareTo(c2.getId());
????????????????????if?(i?==?0)?{
????????????????????????return?c1.getName().compareTo(c2.getName());
????????????????????}
????????????????????return?i;
????????????????}
????????????????return?0;
????????????}
????????};
????????//?2.将此对象作为形参传递给TreeSet的构造器中
????????TreeSet?set?=?new?TreeSet(com);
????????//?3.向TreeSet中添加Comparator接口中的compare方法中涉及的类的对象。
????????set.add(new?com.atguigu.java.Customer("AA",?1003));
????????set.add(new?com.atguigu.java.Customer("BB",?1002));
????????set.add(new?com.atguigu.java.Customer("GG",?1004));
????????set.add(new?com.atguigu.java.Customer("CC",?1001));
????????set.add(new?com.atguigu.java.Customer("DD",?1001));

????????for?(Object?str?:?set)?{
????????????System.out.println(str);
????????}
????}

? ? ? ? ? ? ? ?

Set集合

原文：https://blog.51cto.com/bigdata/2866224