User类，必须要重写hashCode、equals方法
public class User { private String userId; private String name; private Integer age; public String getUserId() { return userId; } public void setUserId(String userId) { this.userId = userId; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; } @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((age == null) ? 0 : age.hashCode()); result = prime * result + ((name == null) ? 0 : name.hashCode()); result = prime * result + ((userId == null) ? 0 : userId.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; User other = (User) obj; if (age == null) { if (other.age != null) return false; } else if (!age.equals(other.age)) return false; if (name == null) { if (other.name != null) return false; } else if (!name.equals(other.name)) return false; if (userId == null) { if (other.userId != null) return false; } else if (!userId.equals(other.userId)) return false; return true; } @Override public String toString() { return "User [userId=" + userId + ", name=" + name + ", age=" + age + "]"; } }1、对User对象的去重
Set<User> set = new HashSet<>(); User user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); set.add(user); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); set.add(user); System.out.println(set.size()); set = new HashSet<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); set.add(user); user = new User(); user.setUserId("111"); user.setName("aa"); user.setAge(11); set.add(user); System.out.println(set.size());执行结果：1、2
第一个部分两个对象的属性值一样，第二个部分第二个user对象的id改变，结果证明可以对对象去重。
2、对List的去重
Set<List<User>> setList = new HashSet<>(); List<User> list = new ArrayList<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(22); set.add(user); list.add(user); setList.add(list); list = new ArrayList<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(22); set.add(user); list.add(user); setList.add(list); System.out.println(setList.size());输出结果：1
两个list的两个user对象全部一样，可以去重复
3、改变其中一个属性的值
Set<List<User>> setList = new HashSet<>(); List<User> list = new ArrayList<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(22); set.add(user); list.add(user); setList.add(list); list = new ArrayList<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(222); set.add(user); list.add(user); setList.add(list); System.out.println(setList.size());输出结果：2
3、改变第二个list里user的顺序
Set<List<User>> setList = new HashSet<>(); List<User> list = new ArrayList<>(); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(22); set.add(user); list.add(user); setList.add(list); list = new ArrayList<>(); user = new User(); user.setUserId("22"); user.setName("bb"); user.setAge(22); set.add(user); list.add(user); user = new User(); user.setUserId("11"); user.setName("aa"); user.setAge(11); list.add(user); setList.add(list); System.out.println(setList.size());输出结果：2
可知：Set对List的去重，保证三个条件：
（1）、重写hashCode、equals方法
（2）、List里边对象的值一致
（3）、List里边的对象的顺序一致
