Java中的hash

String类的HashCode

package demo3;
/*
 * 对象的哈希值，普通的十进制整数
 * 父类Object，方法public int hashCode()计算int整数
 */
public class HashDemo {

    public static void main(String[] args) {
        Person p = new Person();
        int i = p.hashCode();
        System.out.println(i);
        
        String s1 = new String("abc");
        String s2 = new String("abc");
        // h = 31 * h + val[i];
        System.out.println(s1.hashCode());//96354(31*0+97=97，31*97+98=3105，31*3105+99=31354)
        System.out.println(s2.hashCode());//96354
        String s3="abc";
        String s4="abc";
        String s5="abd";
        String s6 = "ccc";
        System.out.println(s3.hashCode());//96354
        System.out.println(s4.hashCode());//96354
        System.out.println(s5.hashCode());//96355
        System.out.println(s6.hashCode());//98307(31*0+99=99,31*99+99=3168,31*3168+99=98307)
        //String 的hashCode()源码
         /*public int hashCode() {
                int h = hash;
                if (h == 0 && value.length > 0) {
                    char val[] = value;

                    for (int i = 0; i < value.length; i++) {
                        h = 31 * h + val[i];
                    }
                    hash = h;
                }
                return h;
            }*/
    }
}

哈希表的实现

HashSet存储过程

HashSet存储String类型

public static void function() {
        HashSet<String> set = new HashSet<String>();
        set.add(new String("abc"));
        set.add(new String("abc"));
        set.add(new String("bbc"));
        set.add(new String("bbc"));
        set.add(null);
        set.add(null);
        System.out.println(set);//[null, bbc, abc]
    }

HashSet存储自定义对象，并重写HashCode和equals方法

package demo3;

import java.util.HashSet;

/*
 * HashSet集合的自身特点：
 * 底层数据结构，哈希表
 * 存储，取出都比较快
 * 线程不安全，运行速度
 * 
 * 哈希表（链表数组结合体）
 * 
 */
public class HashSetDemo1 {

    public static void main(String[] args) {
//        function();
        function1();
        
    }
    //存储String类型
    public static void function() {
        HashSet<String> set = new HashSet<String>();
        set.add(new String("abc"));
        set.add(new String("abc"));
        set.add(new String("bbc"));
        set.add(new String("bbc"));
        set.add(null);
        set.add(null);
        System.out.println(set);//[null, bbc, abc]
    }
    //存储自定义Person类型(由于是new出来的所以不是同一个对象，所以变成了可以存储重复了)
    public static void function1() {
        HashSet<Person> setPerson = new HashSet<Person>();
        setPerson.add(new Person("a",20));
        setPerson.add(new Person("b",10));
        setPerson.add(new Person("b",10));
        setPerson.add(new Person("c",15));
        setPerson.add(new Person("d",19));
        setPerson.add(new Person("e",17));
        System.out.println(setPerson);
    }
}

重写HashCode和equals方法

@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)//内存地址一样
            return true;
        if (obj == null)//是否为孔
            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;
    }