ArrayList,HashSet的比较

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1,ArrayList有序可重复,HashSet,无序,不可重复,先判断有没有相等的对象,再放入.

2,如果两个对象的equals对象相等,就要让它们的hashcode值相等

/*只有类的实例对象要被采用哈希算法进行存储和检索的时候,这个类才需要按照要求覆盖hashCode
      方法.虽然程序可能暂时不会用到当前类的hashCode方法,但是为它
      提供一个hashCode方法也不会有什么不好,可能以后就能用到这个方法了,
      所以,通常要求hashCode方法的和equals方法一并被同时覆盖.*/

3,内存泄漏问题:

练习实例:

package cn.itcast.day1;

import java.io.*;
import java.util.*;

public class ReflextTest2 {

public static void main(String[] args) throws Exception {

Collection collections = new HashSet();
  ReflectPoint pt1 = new ReflectPoint(3,3);
  ReflectPoint pt2 = new ReflectPoint(5,5);
  ReflectPoint pt3 = new ReflectPoint(3,3);

  collections.add(pt1);
  collections.add(pt2);
  collections.add(pt3);
  collections.add(pt1);


  pt1.y = 7;   //引起内存泄漏的地方
  collections.remove(pt1);
  System.out.println(collections.size());
}

}

package cn.itcast.day1;

public class ReflectPoint {
private int x ;
public int y ;
public String str1 = "ball";
public String str2 = "basketball";
public String str3 = "itcast";
public ReflectPoint(int x, int y) {
  super();
  this.x = x;
  this.y = y;
}

  public int hashCode() {
  final int prime = 31;
  int result = 1;
  result = prime * result + x;
  result = prime * result + y;
  return result;
}

public boolean equals(Object obj) {
  if (this == obj)
   return true;
  if (obj == null)
   return false;
  if (getClass() != obj.getClass())
   return false;
  ReflectPoint other = (ReflectPoint) obj;
  if (x != other.x)
   return false;
  if (y != other.y)
   return false;
  return true;
}

public String toString(){
return str1 + ":" + str2 + ":" + str3;
}
}