QUESTION 75
Given:
3. import java.util.*;
4. public class Mapit {
5.   public static void main(String[] args) {
6.     Set<Integer> set = new HashSet<Integer>();
7.     Integer i1 = 45;
8.     Integer i2 = 46;
9.     set.add(i1);
10.    set.add(i1);
11.    set.add(i2); System.out.print(set.size() + " ");
12.    set.remove(i1); System.out.print(set.size() + " ");
13.    i2 = 47;
14.    set.remove(i2); System.out.print(set.size() + " ");
15.  }
16. }

public class Mapit {
    public static void main(String[] args) {
        Set<Integer> set = new HashSet<Integer>();
        Integer i1 = 45;
        Integer i2 = 46;
        
        System.out.println(i2.hashCode());
        
        set.add(i1);
        set.add(i1);
        set.add(i2);
        System.out.print(set.size() + " ");
        set.remove(i1);
        System.out.print(set.size() + " ");
        i2 = 47;
        set.remove(i2);
        System.out.print(set.size() + " ");
        
        System.out.println(i2.hashCode());
    }
}

------解决方案--------------------

因为i2 = 47时，java的boxing机制使得i2的引用指向了新的Integer对象47,所以hashcode发生了变化，set调用remove方法时比较运算就不会等于Integer对象46的hashcode,所以reomve操作并没有删除set里的46。HashSet的remove方法不是返回boolean值嘛，你可以输出看看。