05.hashCode方法和equals方法的优化

hashCode方法优化:

• 如果让hashCode()方法返回一个固定值,那么每个新添加的元素都要调用equals(Object obj)方法比较,那么效率较低
• 只需要让不同属性的值的元素产生不同的哈希值,那么就可以不再调用equals方法比较提高效率

package com.itheima_02;

public class Person {
    String name;
    int age;
    
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
    
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.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;
        Person other = (Person) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    
/*  @Override
    public boolean equals(Object obj) {
        System.out.println("--------");
        
        //提高效率
        if(this == obj) {
            return true;
        }
        
        //提高程序健壮性
        if(this.getClass() != obj.getClass()) {
            return false;
        }
        
        //向下转型
        Person p = (Person)obj;
        
        if(this.name.equals(p.name)) {
            return false;
        }
        
        if(this.age != p.age) {
            return false;
        }
        
        return true;
    }*/
}

package com.itheima_02;

import java.util.HashSet;

public class HashSetDemo {
    public static void main(String[] args) {
        //创建集合对象
        HashSet hs = new HashSet();
        //创建元素对象
        Person p = new Person("zhangsan",18);
        Person p2 = new Person("lisi",19);
        Person p3 = new Person("lisi",19);
        //添加元素对象
        hs.add(p);
        hs.add(p2);
        hs.add(p3);
        //遍历集合
        for (Person person : hs) {
            System.out.println(person);
        }
    }
}