43、集合的第二大类：Set

骑猪看日落 2024-03-31 11:18 51阅读 0赞

![c0f24d96f48f42908f361097486f89fa.png][]

一、基本介绍：

1、Set接口的特点：

1）无序（添加和取出的顺序不一致） ，没有索引

2）不允许重复元素，所以最多包含一个null

3） JDK API中Set接口的实现类有：

![f75dc14a084941d4a0bd7b93adaf4053.png][]

2、Set接口的常用方法：

和List接口一样，Set接口也是Collection的子接口，因此，常用方法和Collection接口一样.

3、Set接口的遍历方式：

同Collection的遍历方式一样，因为Set接口是Collection接口的子接口。

**（1）可以使用迭代器**

**（2）增强for**

**（3）不能使用索引的方式来获取．**

package Collection_;
    
    import java.util.HashSet;
    import java.util.Iterator;
    import java.util.Set;
    
    public class SetMethod {
        @SuppressWarnings({"all"})
        public static void main(String[] args) {
            Set set=new HashSet();
            set.add("john");
            set.add("lucky");
            set.add("john");
            set.add("jack");
            set.add(null);
            set.add(null);
            System.out.println("set="+set);
            //set=[lucky, null, john, jack]
            //取出的顺序是固定的，不会说同样的代码每次输出都不一样
    
            set.add("smith");
            System.out.println("set="+set);
            //set=[lucky, null, smith, john, jack]
    
            //遍历
            //1、使用迭代器
            System.out.println("=========使用迭代器=========");
            Iterator iterator=set.iterator();
            while (iterator.hasNext()) {
                Object obj =  iterator.next();
                System.out.println("obj="+obj);
            }
    
            //2、增强for
            System.out.println("=========增强for=========");
            for (Object o :set) {
                System.out.println("o="+o);
            }
    
        }
    }
    //=========使用迭代器=========
    //obj=lucky
    //obj=null
    //obj=smith
    //obj=john
    //obj=jack
    //=========增强for=========
    //o=lucky
    //o=null
    //o=smith
    //o=john
    //o=jack

4、Set接口常用的实现类：有HashSet、TreeSet.

**二、HashSet：**

1、基本介绍：

（1） HashSet实现了Set接口

（2）HashSet底层实际上是HashMap， 看下源码．

public HashSet(){
            map = new HashMap<>();
    }

（3） 可以存放null值，但是只能有一个null.

（4）HashSet不保证元素是有序的，取决于hash后，再确定索引的结果．即，不保证存放元素的顺序和取出顺序一致

（5） 不能有重复元素／对象．

package Collection_;
    
    import java.util.HashSet;
    import java.util.Set;
    @SuppressWarnings({"all"})
    public class HashSet_ {
        public static void main(String[] args) {
            Set hashSet = new HashSet();
            hashSet.add(null);
            hashSet.add(null);
            System.out.println("hashset="+hashSet);
            //hashset=[null]
        }
    }

package Collection_;
    
    import java.util.HashSet;
    import java.util.Set;
    @SuppressWarnings({"all"})
    public class HashSet_ {
        public static void main(String[] args) {
            Set set=new HashSet();
            set.add("john");
            set.add("tom");
    
    
            set =new HashSet();
            System.out.println("set="+set);//set=[]
            set.add("lucy");//ok
            set.add("lucy");//no
            set.add(new Dog("tom"));//ok
            set.add(new Dog("tom"));//ok 不同对象
            System.out.println("set="+set);
            //set=[Dog{name='tom'}, Dog{name='tom'}, lucy]
    
            //经典面试题
            set.add(new String("hsp"));//ok
            set.add(new String("hsp"));//加入不了 看源码
            System.out.println("set="+set);
            //set=[Dog{name='tom'}, hsp, Dog{name='tom'}, lucy]
        }
    }
    class Dog{
        private String name;
    
        public Dog(String name) {
            this.name = name;
        }
    
        @Override
        public String toString() {
            return "Dog{" +
                    "name='" + name + '\'' +
                    '}';
        }
    }

**2、HashSet底层机制说明：（主要研究源码）**

**HashSet底层是HashMap，HashMap底层是（数组＋链表＋红黑树）**

![842152bb72004d5d8f28d23cd1adbdcd.png][]

package Collection_;
    @SuppressWarnings({"all"})
    public class HashSetStructure {
        public static void main(String[] args) {
            //1、创建一个Node类型的数组
            Node[] table=new Node[16];
            System.out.println("table="+table);
    
            //2、创建结点
            Node john=new Node("jonh",null);
            table[2]=john;
            Node jack=new Node("jack",null);
            john.next=jack;//将jack结点挂载到john
            Node rose=new Node("Rose",null);
            jack.next=rose;//将rose结点挂载到jack
            System.out.println("table="+table);
        }
    }
    class Node{//结点，存储数据，可以指向下一个结点，从而形成链表
        Object item;//存放数据
        Node next;//指向下一个结点
    
        public Node(Object item, Node next) {
            this.item = item;
            this.next = next;
        }
    
        @Override
        public String toString() {
            return "Node{" +
                    "item=" + item +
                    ", next=" + next +
                    '}';
        }
    }

![2315fa4c68914a448b1301f5d6118fb7.png][]

![5f7a5261385d428ea6f223e548f026f0.png][]![bfec004772844942987f08824630a690.png][]

*注意：equals的判断标准并不能简单地认为是两个字符串比较，因为String已经重写了equals方法，所以equals的判断标准和String没有关系，要视具体情况而定*

//源码部分，非战斗人员，请做好撤退准备
    package Collection_;
    
    import javax.swing.tree.TreeNode;
    import java.util.HashMap;
    import java.util.HashSet;
    @SuppressWarnings({"all"})
    public class HashSetSource {
        public static void main(String[] args) {
            HashSet hashSet=new HashSet();
            hashSet.add("java");
            hashSet.add("php");
            hashSet.add("java");
            System.out.println("set="+hashSet);
            /*
            追源码：
            //1、执行HashSet()
            public HashSet() {
                    map = new HashMap<>();
                }
            //2、执行add方法
            public boolean add(E e) {
                    return map.put(e, PRESENT)==null;
                    //PRESENT的源码： private static final Object PRESENT = new Object();主要起到占位的作用
                }
            //3、执行put方法，该方法会执行hash(key)，得到key对应的hash值，
            //（hash(key)与hashcode不一样，hash(key)里面还包含了算法）相关算法： (h = key.hashCode()) ^ (h >>> 16)
            public V put(K key, V value) {//是PRESENT的值
                    return putVal(hash(key), key, value, false, true);
                }
            //4、执行 putVal（核心代码）
             final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                               boolean evict) {
                    Node<K,V>[] tab; Node<K,V> p; int n, i;//定义了辅助变量
                    //table是HashMap的一个数组，类型是Node[]
                    //if语句表示如果当前table是null，或者大小=0
                    //就是第一次扩容到16个空间
                    if ((tab = table) == null || (n = tab.length) == 0)
                        n = (tab = resize()).length;
                    //（1）根据key，得到hash去计算该key应该存放到table表的哪个索引位置
                    //并把这个位置的对象，赋给p
                    //（2）判断p是否为null
                    //（2.1）如果p为null，表示还没有存放元素，就创建一个Node(key="java",value=PRESENT)
                    //（2.2）否则，就放在该位置tab[i] = newNode(hash, key, value, null);
                    if ((p = tab[i = (n - 1) & hash]) == null)
                        tab[i] = newNode(hash, key, value, null);
                    else {
                    //一个开发技巧提示：在需要局部变量（辅助变量）时，再创建
                        Node<K,V> e; K k;
                        //如果当前索引位置对应的链表的第一个元素和准备添加的key的hash值一样
                        //并且满足下面两个条件之一：
                        //（1）准备加入的key和p指向的Node的结点的key是同一个对象
                        //（2）p指向的Node结点的key的equals()和准备加入的key比较后相同
                        //就不能加入
                        if (p.hash == hash &&
                            ((k = p.key) == key || (key != null && key.equals(k))))
                            e = p;
                            //再判断p是不是一棵红黑树
                            //如果是一棵红黑树，就调用putTreeVal来进行添加
                        else if (p instanceof TreeNode)
                            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
                        else {
                        //如果table对应索引位置，已经是一个链表，就使用for循环比较
                        //（1）依次和该链表的每一元素比较后，都不相同，就加入到该链表的最后
                        //    注意在把元素添加到链表后，立即判断该链表是否已经达到8个结点
                        //    就调用treeifyBin()对当前这个链表进行树化（转成红黑树）
                        //    注意，在转成红黑树时，要进行判断，判断条件
                        //    if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
                        //            resize();
                        //    如果上面条件成立，先table扩容
                        //    只有上面条件不成立时，才进行转成红黑树
                        //（2）依次和该链表的每一个元素比较过程中，如果有相同情况，就直接break
                            for (int binCount = 0; ; ++binCount) {
                                if ((e = p.next) == null) {
                                    p.next = newNode(hash, key, value, null);
                                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                                        treeifyBin(tab, hash);
                                    break;
                                }
                                if (e.hash == hash &&
                                    ((k = e.key) == key || (key != null && key.equals(k))))
                                    break;
                                p = e;
                            }
                        }
                        if (e != null) { // existing mapping for key
                            V oldValue = e.value;
                            if (!onlyIfAbsent || oldValue == null)
                                e.value = value;
                            afterNodeAccess(e);
                            return oldValue;
                        }
                    }
                    ++modCount;
                    //size就是我们每加入一个结点Node(k,v,h,next),size++
                    if (++size > threshold)
                        resize();
                    afterNodeInsertion(evict);
                    return null;
                }
                */
        }
    }

3、练习题：

（1）![d5b91bb153164cafb68815d2ca612fc4.png][]

//我的代码：

package Collection_;
    
    import java.util.HashSet;
    import java.util.Iterator;
    
    @SuppressWarnings({"all"})
    public class HashSetExercise {
        public static void main(String[] args) {
            HashSet hashSet = new HashSet();
            Employee[] employees = new Employee[4];
            employees[0]=new Employee("jack", 18);
            employees[1]=new Employee("tom", 19);
            employees[2]=new Employee("rose", 20);
            employees[3]=new Employee("rose", 20);
            hashSet.add(employees[0]);
            for (int i = 1; i < employees.length; i++) {
                int tmp=0;
                Iterator iterator=hashSet.iterator();
                while (iterator.hasNext()) {
                    Object next =  iterator.next();
                    if (employees[i].getAge() == hashSet.hashCode()|| employees[i].getName().equals(hashSet.hashCode())) {
                        break;
                    }
                }
                hashSet.add(employees[i]);
            }
            for (Object o :hashSet) {
                System.out.println(o);
            }
        }
    }
    class Employee{
        private String name;
        private int age;
    
        public Employee(String name, int age) {
            this.name = name;
            this.age = age;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        @Override
        public String toString() {
            return "Employee{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

//老师的代码：

package Collection_;
    
    import java.util.HashSet;
    import java.util.Objects;
    @SuppressWarnings({"all"})
    public class HashSetExercise {
        public static void main(String[] args) {
            HashSet hashSet = new HashSet();
            hashSet.add(new Employee("milan",18));
            hashSet.add(new Employee("jack",28));
            hashSet.add(new Employee("milan",18));
            System.out.println("hashSet="+hashSet);
        }
    }
    class Employee{
        private String name;
        private int age;
    
        public Employee(String name, int age) {
            this.name = name;
            this.age = age;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        @Override
        public String toString() {
            return "Employee{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
        //重写equals()方法和hashCode()方法：
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Employee employee = (Employee) o;
            return age == employee.age && Objects.equals(name, employee.name);
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(name, age);
        }
    }
    //hashSet=[Employee{name='milan', age=18}, Employee{name='jack', age=28}]

·重写equals()方法和hashCode()方法：

alt+insert---->![b03c145c1aa94307bc83e5b384e5a88b.png][]\---->下一步---->![005e3bf9ddda426a9e286d99631ed1c2.png][]\---->下一步---->

![81f4f8bc0b494547bf8c52859195efb3.png][]

（2）![0c65ad514c0a4b9985b091b9439a6ef0.png][]

//我的代码

package Collection_;
    
    import java.util.HashSet;
    import java.util.Objects;
    
    @SuppressWarnings({"all"})
    public class HashSetExercise02 {
        public static void main(String[] args) {
            HashSet hashSet=new HashSet();
            MyDate jack=new MyDate(2000,1,1);
            MyDate tom=new MyDate(2001,2,2);
            hashSet.add(new Employee("jack",20000,jack));
            hashSet.add(new Employee("tom",30000,tom));
            hashSet.add(new Employee("jack",20000,jack));
            System.out.println(hashSet);
        }
    }
    class Employee{
        private String name;
        private double sal;
        private MyDate birthday;
    
        public Employee(String name, double sal, MyDate birthday) {
            this.name = name;
            this.sal = sal;
            this.birthday = birthday;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public double getSal() {
            return sal;
        }
    
        public void setSal(double sal) {
            this.sal = sal;
        }
    
        public MyDate getBirthday() {
            return birthday;
        }
    
        public void setBirthday(MyDate birthday) {
            this.birthday = birthday;
        }
    
        @Override
        public String toString() {
            return "Employee{" +
                    "name='" + name + '\'' +
                    ", sal=" + sal +
                    ", birthday=" + birthday +
                    '}';
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Employee employee = (Employee) o;
            return Objects.equals(name, employee.name) && Objects.equals(birthday, employee.birthday);
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(name, birthday);
        }
    }
    class MyDate{
        private int year;
        private int month;
        private int day;
    
        public MyDate(int year, int month, int day) {
            this.year = year;
            this.month = month;
            this.day = day;
        }
    
        public int getYear() {
            return year;
        }
    
        public void setYear(int year) {
            this.year = year;
        }
    
        public int getMonth() {
            return month;
        }
    
        public void setMonth(int month) {
            this.month = month;
        }
    
        public int getDay() {
            return day;
        }
    
        public void setDay(int day) {
            this.day = day;
        }
    
        @Override
        public String toString() {
            return "MyDate{" +
                    "year=" + year +
                    ", month=" + month +
                    ", day=" + day +
                    '}';
        }
    }
    //[Employee{name='jack', sal=20000.0, birthday=MyDate{year=2000, month=1, day=1}}, Employee{name='tom', sal=30000.0, birthday=MyDate{year=2001, month=2, day=2}}]

**三、LinkedHashSet**

![115c387406404fec9bd95b1e9f8c57e8.png][]

1、基本介绍：

（1）LinkedHashSet 是 HashSet的子类

（2）LinkedHashSet 底层是一个 LinkedHashMap，底层维护了一个数组＋双向链表

（3）LinkedHashSet 根据元素的 hashCode 值来决定元素的存储位置，同时使 用链表维护元素的次序，这使得元素看起来是以插入顺序保存的。

![b62fa0ba2d484016858bb5c09bbc850d.png][]

（4）LinkedHashSet 不允许添重复元素

**2、LinkedHashSet底层机制：**

package Collection_;
    
    import java.util.LinkedHashSet;
    import java.util.Set;
    @SuppressWarnings({"all"})
    public class LinkedHashSetSource {
        public static void main(String[] args) {
            Set set=new LinkedHashSet();
            set.add(new String("AA"));
            set.add(456);
            set.add(456);
            set.add(new Customer("刘",1001));
            set.add(123);
            set.add("HSP");
        }
    }
    class Customer{
        private String name;
        private int no;
    
        public Customer(String name, int no) {
            this.name = name;
            this.no = no;
        }
    }

![1497f19b18d84803baf53ad5170709d9.png][]

3、练习题：![094fd7063d324b41a52c262abf6c4fc8.png][]

//我的代码：

package Collection_;
    
    import java.util.LinkedHashSet;
    import java.util.Objects;
    import java.util.Set;
    @SuppressWarnings({"all"})
    public class LinkedHashSet_ {
        public static void main(String[] args) {
            Set linkedHashSet=new LinkedHashSet();
            linkedHashSet.add(new Car("奥拓",1000));
            linkedHashSet.add(new Car("奥迪",300000));
            linkedHashSet.add(new Car("法拉利",10000000));
            linkedHashSet.add(new Car("保时捷",70000000));
            linkedHashSet.add(new Car("奥迪",300000));
            System.out.println("linkedHashSet="+linkedHashSet);
        }
        public static class Car {
            private String name;
            private double price;
    
            public Car(String name, double price) {
                this.name = name;
                this.price = price;
            }
    
            public String getName() {
                return name;
            }
    
            public void setName(String name) {
                this.name = name;
            }
    
            public double getPrice() {
                return price;
            }
    
            public void setPrice(double price) {
                this.price = price;
            }
    
            @Override
            public String toString() {
                return "Car{" +
                        "name='" + name + '\'' +
                        ", price=" + price +
                        '}';
            }
    
            @Override
            public boolean equals(Object o) {
                if (this == o) return true;
                if (o == null || getClass() != o.getClass()) return false;
                Car car = (Car) o;
                return Double.compare(car.price, price) == 0 && Objects.equals(name, car.name);
            }
    
            @Override
            public int hashCode() {
                return Objects.hash(name, price);
            }
        }
    }
    //linkedHashSet=[Car{name='奥拓', price=1000.0}, Car{name='奥迪', price=300000.0}, Car{name='法拉利', price=1.0E7}, Car{name='保时捷', price=7.0E7}]

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[81f4f8bc0b494547bf8c52859195efb3.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/bef39103aed844a5ae21afb0a2a9e596.png
[0c65ad514c0a4b9985b091b9439a6ef0.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/40709369ef504dc7938c1d196c2e353d.png
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