阅读JDK8 HashSet源码应该了解什么

一、HashSet的底层由HashMap实现。

二、transient关键字：做用是控制变量的序列化，在变量声明前加上该关键字，能够阻止该变量被序列化到文件中，在被反序列化后，transient 变量的值被设为初始值，如 int 型的是 0，对象型的是 null。

三、序列化：将 Java 对象序列化为二进制文件。https://www.ibm.com/developerworks/cn/java/j-lo-serial/index.html

四、spliterator方法：JDK 1.8 中添加，建立集合的分片迭代器。

public static void main(String[] args) throws Exception {
  Employee emp = new Employee();
  emp.name = "Tom";
  Employee emp2 = new Employee();
  emp2.name = "Jerry";
  Collection<Employee> set = new HashSet<>();
  set.add(emp);
  set.add(emp2);

  Spliterator<Employee> spl = set.spliterator();

  while (spl.tryAdvance(new Consumer<Employee>() {
    public void accept(Employee employee) {
      employee.name += "_09";
    }
  }));

  for (Employee employee : set) {
    System.out.println(employee.name);
  }

  Spliterator<Employee> spl2 = set.spliterator();
  while (spl2.tryAdvance(e -> e.name += "_10"));

  for (Employee employee : set) {
    System.out.println(employee.name);
  }
}

五、Cloneable接口：一个标记性接口，是浅拷贝，意味着若是你的类中有数组，Collection,对象，那么仅仅拷贝的是这些变量的引用，并无将数组，集合中的成员对象复制成新的。

public class Employee implements Cloneable {
  public int age;
  public String name;

  public Employee clone() {
    Employee ee = null;
    try {
      ee = (Employee) super.clone();
    } catch (CloneNotSupportedException e) {
      e.printStackTrace();
    }
    return ee;
  }
}

public static void main(String[] args) {
  Employee ee = new Employee();
  ee.age = 21;
  ee.name = "clone";

  Employee ee1 = ee.clone();
  ee1.name = "clone1";

  System.out.println(ee.name);
  System.out.println(ee.age);
  
  System.out.println(ee1.name);
  System.out.println(ee1.age);
}