枚举集 EnumSet类

Java EnumSet 类是用于枚举类型的专用 Set 实现。
它继承了 AbstractSet 类并实现了 Set 接口。

枚举集(EnumSet)示例

import java.util.*;
enum days {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}
public class EnumSetExample {
    public static void main(String[] args) {
    Set set = EnumSet.of(days.TUESDAY, days.WEDNESDAY);
    //Traversing elements
    Iterator iter = set.iterator();
    while (iter.hasNext())
        System.out.println(iter.next());
    }
}

以数字开头的枚举

Java 不允许枚举的名称以数字开头，如 100A、25K。
在这种情况下，我们可以用 _（下划线）或者任何允许的模式添加代码并进行检查。

以名称开头的枚举

public enum BookCode {
10A("Simon Haykin", "Communication System"), 
_42B("Stefan Hakins", "A Brief History of Time"), 
E1("Sedra Smith", "Electronics Circuits"); 
private String author; 
private String title; 
BookCode(String author, String title) { 
   this.author = author; 
   this.title = title; 
} 
public String getName() { 
   String name = name(); 
   if (name.charAt(0) == '') {
       name = name.substring(1, name.length());
   }
   return name;
}
public static BookCode of(String code) {
    if (Character.isDoirt(code.charAt(0))) {
       code = "_" + code;
     }
     return BookCode.valueOf(code);
   }
}

哈希表 Hashtable

Hashtable 是 Java 集合中的一个类，它实现了 Map 接口并扩展了 Dictionary 类

仅包含唯一元素及其同步

import java.util.*;
public class HashtableDemo {
      public static void main(String args[]) {
      //create and populate hash table
      Hashtable map = new Hashtable();
      map.put(101,"C Language");
      map.put(102, "Domain");
      map.put(104, "Databases");
      System.out.println("Values before remove: "+ map);
      //Remove value for key 102
       map.remove(102);
       System.out.println("Values after remove: "+ map);
       }
}

Java EnumMap示例

import java.util.*;
class Book {
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author, String publisher, int quantity) {
     this.id = id;
     this.name = name;
     this.author = author;
     this.publisher = publisher;
     this.quantity = quantity;
}
}
public class EnumMapExample {
   //Creating enum
    public enum Key{
        One, Two, Three
        };
public static void main(String[] args) {
      EnumMap map = new EnumMap(Key.class);
      //Creating Books
      Book b1=new Book(101,"Let us C","Yashwant         Kanetkar","BPB",8);
      Book b2=new Book(102,"Data Communications &      Networking","Forouzan","Mc Graw Hill",4);
      Book b3=new Book(103,"Operating System","Galvin","Wiley",6);
     //Adding Books to Map
      map.put(Key.One, b1);
      map.put(Key.Two, b2);
      map.put(Key.Three, b3);
     //Traversing EnumMap
     for(Map.Entry entry:map.entrySet()){
      Book b=entry.getValue();
      System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);
     }
}
}

Java EnumMap 类是枚举键的专用 Map 实现。
它继承了 Enum 和 AbstractMap 类。

java.util.EnumMap 类的参数。

K：就是这个map维护的key的类型。
V：它是映射值的类型。