2021-01-07 HashSet

package com.llb.hashSet;

import java.util.Objects;

/**
 * 哈希值
 *   是JDK根据对象的地址值或者属性值，算出来的int类型的整数
 *     hashCode()获取哈希值
 *   特点
 *     如果没有重写hashCode()方法，那么是根据对象的地址值计算出的哈希值
 *     如果重写了hashCode()方法，一般都是通过对象的属性值计算出哈希值
 *   JDK7
 *     底层结构  哈希表（数组+链表）
 *       1，创建一个默认长度16，默认加载因子0.75的数组（16*0.17=12的时候扩容，扩容为原来的2倍），数组名table
 *     添加元素
 *       2，根据元素的哈希值跟数组的长度计算出存入的位置
 *       3，判断当前位置是否为null，如果是null直接存入
 *       4，如果存入的位置不为null，表示有元素，则调用equals方法比较属性值
 *       5，如果一样则不存，不一样则存入数组，新元素存入表中，老元素挂在新元素下面，和新元素形成一个链表
 *   JDK8
 *     底层结构  数组+红黑树
 *       当链表长度超过8，则将链表换成红黑树
 *       步骤
 *         计算哈希值
 *         计算应存入的索引
 *         是否为空
 *           为空，直接存
 *           不为空，判断结构
 *             链表
 *             红黑树
 */
public class MyHashSet {
    public static void main(String[] args) {
        Student hinata = new Student("hinata", 18);
        Student naruto = new Student("naruto", 18);
        //object类中  根据对象的地址值计算出来的哈希值
        System.out.println(hinata.hashCode());//356573597  重写后921755666
        System.out.println(naruto.hashCode());//1735600054 重写后1727067586

    }
}
class Student{
    private String name;
    private Integer age;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(name, student.name) &&
                Objects.equals(age, student.age);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

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

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public Student() {
    }
}



package com.llb.hashSet;

import java.util.HashSet;
import java.util.Objects;

public class MyHashSetExe {
    public static void main(String[] args) {
        //自定义对象要存入hashSet集合中，需要重写hashCode()和equals()方法
        HashSet<Studen00t> hashSet = new HashSet<>();
        Studen00t aaa = new Studen00t("aaa", 18);
        Studen00t bbb = new Studen00t("bbb", 18);
        Studen00t ccc = new Studen00t("ccc", 18);
        Studen00t ddd = new Studen00t("ccc", 18);
        hashSet.add(aaa);
        hashSet.add(bbb);
        hashSet.add(ccc);
        hashSet.add(ddd);
        for (Studen00t studen00t : hashSet) {
            System.out.println(studen00t);
        }
    }
}
class Studen00t{
    private String name;
    private Integer age;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Studen00t studen00t = (Studen00t) o;
        return Objects.equals(name, studen00t.name) &&
                Objects.equals(age, studen00t.age);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

    @Override
    public String toString() {
        return "Studen00t{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

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

    public Studen00t(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public Studen00t() {
    }
}