public class HashSetTest { @Test public void test(){ HashSet set = new HashSet(); Person p1 = new Person(1001, "AA"); Person p2 = new Person(1002, "BB"); set.add(p1); set.add(p2); p1.name = "CC"; //remove()会判断哈希值,因为此时已经修改了p1.name,大概率会和之前的p1不同,所有删除不了 boolean remove = set.remove(p1); System.out.println(remove);//false System.out.println(set);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}] //第一次的1001CC的哈希值实际上是1001AA的哈希值,所以哈希值不一样 set.add(new Person(1001,"CC")); System.out.println(set);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}] //虽然此次的1001AA和第一次的1001AA的哈希值一样,但是equals()比较的时候,由于第一次1001AA已经改为1001CC,所有能添加进去 set.add(new Person(101,"AA")); System.out.println(set);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}, Person{id=101, name='AA'}] }}