package com.broadengate.fangxing;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
/**
* 应该都知道 hashset 不允许放入重复的数据,那么他是根据什么来判断是否是重复的数据呢?
* 在hashset中用到了hash算法,因为hashset底层也是用的Hashmap来存取数据
* ,在hashmap底层则是用的数组来存取,而在hashmap放数据的时候会用到hashcode
* ,这就是hashmap存入的数据没有顺序的原因,他就是根据hashcode来存放的
*
* 如果不重写hashcode 则默认是使用Object里面的,是根据对象的引用地址来生成的hashcode,所以在没创建一个对象的时候他的引用地址都不一样,
*
*/
public class HashCodeTest {
public static void main(String[] args) {
HashCodeTest hashCodeTest = new HashCodeTest();
Collection coll = new /* ArrayList() */HashSet();
Person person = hashCodeTest.new Person("1", 1);
Person person1 = hashCodeTest.new Person("1", 1);
Person person2 = hashCodeTest.new Person("1", 1);
Person person3 = hashCodeTest.new Person("1", 1);
Person person4 = hashCodeTest.new Person("1", 1);
Person person5 = hashCodeTest.new Person("1", 1);
coll.add(person5);
coll.add(person4);
coll.add(person);
coll.add(person2);
coll.add(person3);
coll.add(person1);
// 如果重写了hashcode方法和eq方法, 如果这里把放入内存中的对象的值改变,则他的hashcode值就会变,
// jvm在根据hashcode算出来的区域去找person对象就找不到那个对象,所以就不能移除
person.setAge(2);
coll.remove(person);
System.out.println(coll.size());
}
class Person {
private String name;
private int age;
public Person() {
}
public Person(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 int hashCode() { final int prime = 31; int result =
* 1; result = prime * result + getOuterType().hashCode();
* 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 (!getOuterType().equals(other.getOuterType())) return
* false; 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; }
*
* private HashCodeTest getOuterType() { return
* HashCodeTest.this; }
**/
}
}