1.?作為Set元素類型的自定義類，需覆寫equals, hashCode假瞬。
Sample:自定義類Person (name, gender, age), 不覆寫equals, hashCode.
hs size不變岭参，均為1。默認(rèn)hashCode基于對象引用，所以即便修改了p1屬性的值能岩，hashset仍視為同一對象募强。
Override equals & hashCode之后株灸，hs size和預(yù)期一樣崇摄，每次修改之后加入，size會加1.
因?yàn)槠綍r項(xiàng)目中慌烧，pojo類大部分是自動生成的（XSD生成）配猫，equals, hashCode已經(jīng)默認(rèn)覆寫過了，所以加入set時很少考慮這個細(xì)節(jié)問題杏死，容易出錯泵肄。
2. Set?also adds a stronger contract on the behavior of the?equals?and?hashCode?operations, allowing?Set?instances to be compared meaningfully even if their implementation types differ. Two?Set?instances are equal if they contain the same elements.
兩個Set包含相同的元素，則equals為true.
注意另外一個點(diǎn)：TreeSet中的元素should be comparable. 可以在實(shí)例化TreeSet的時候指定一個Comparator.
3.??LinkedHashSet, which is implemented as a hash table with a linked list running through it, orders its elements based on the order in which they were inserted into the set (insertion-order).
結(jié)果依次打印0到99.
4. 注意面向接口編程淑翼。聲明中使用Set而不是具體實(shí)現(xiàn)類腐巢，以便只需要修改構(gòu)造函數(shù)就可以改變具體的實(shí)現(xiàn)，從而影響結(jié)果玄括。
//conversion constructor
Collection noDups = new HashSet(c);//去重
???????Collection noDups = new LinkedHashSet(c);//去重冯丙，保留原序
??????//aggregate operations
??????c.stream().collect(Collectors.toSet()); // no duplicates
??????Set set = people.stream().map(Person::getName)
????????????????????????.collect(Collectors.toCollection(TreeSet::new));
