Java集合HashSet的hashcode技巧引起的内存泄漏问题

发布时间：2021-12-18 17:04:44 所属栏目：教程来源：互联网

导读：package apistudy; import Java.util.HashSet; import java.util.Set; /** * Created on 2010-6-7 * pDescription: [HashSet的hashcode方法引起的内存泄漏]/p * @author shixing_11@sina.com * @version 1.0 */ public class HashSetTest { public static vo

package apistudy;
import Java.util.HashSet;
import java.util.Set;
/**
* Created on 2010-6-7
* <p>Description: [HashSet的hashcode方法引起的内存泄漏]</p>
* @author         shixing_11@sina.com
* @version        1.0
*/
public class HashSetTest
{
    public static void main(String[] args)
    {
        Set<Person> set = new HashSet<Person>();
        Person p1 = new Person("唐僧","pwd1",25);
        Person p2 = new Person("孙悟空","pwd2",26);
        Person p3 = new Person("猪八戒","pwd3",27);
        set.add(p1);
        set.add(p2);
        set.add(p3);
        System.out.println("总共有:"+set.size()+" 个元素!"); //结果：总共有:3 个元素!
        p3.setAge(2);    //修改p3的年龄,此时p3元素对应的hashcode值发生改变
        set.remove(p3); //此时remove不掉，造成内存泄漏
        set.add(p3);     //重新添加，居然添加成功
        System.out.println("总共有:"+set.size()+" 个元素!"); //结果：总共有:4 个元素!
        for (Person person : set)
        {
            System.out.println(person);
        }

    }
}
运行结果：

总共有:3 个元素!

总共有:4 个元素!

猪八戒-->pwd3-->2

孙悟空-->pwd2-->26

唐僧-->pwd1-->25

猪八戒-->pwd3-->2

总结：
HashSet中，在对应元素添加进set集合后，不要再去修改元素的值，否则对应元素的hashcode值发生变化，此时如果调用
集合的remove(),contains()方法，将不会得到正确的结果。remove()方法并不能正确remove掉对应的元素，造成内存泄漏。
Person.java

package apistudy;
public class Person
{
    private String username;
    private String password;
    private int age;
    public Person(String username, String password, int age)
    {
        this.username = username;
        this.password = password;
        this.age = age;
    }
    public String getUsername()
    {
        return username;
    }
    public void setUsername(String username)
    {
        this.username = username;
    }
    public String getPassword()
    {
        return password;
    }
    public void setPassword(String password)
    {
        this.password = password;
    }
    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 + age;
        result = prime * result + ((password == null) ? 0 : password.hashCode());
        result = prime * result + ((username == null) ? 0 : username.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 (password == null)
        {
            if (other.password != null)
                return false;
        }
        else if (!password.equals(other.password))
            return false;
        if (username == null)
        {
            if (other.username != null)
                return false;
        }
        else if (!username.equals(other.username))
            return false;
        return true;
    }
    @Override
    public String toString()
    {
        return this.username+"-->"+this.password+"-->"+this.age;
    }

}

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