Java重温学习笔记，Hashtable 类、Properties 类

一、Hashtable是原始的java.util的一部分， 是一个Dictionary具体的实现 。

然而，Java 2 重构的Hashtable实现了Map接口，因此，Hashtable现在集成到了集合框架中。它和HashMap类很相似，但是它支持同步。像HashMap一样，Hashtable在哈希表中存储键/值对。当使用一个哈希表，要指定用作键的对象，以及要链接到该键的值。

import java.util.*;

public class MyDemo {

   public static void main(String args[]) {
      Hashtable balance = new Hashtable();
      balance.put("Zara", new Double(3434.34));
      balance.put("Mahnaz", new Double(123.22));
      balance.put("Ayan", new Double(1378.00));
      balance.put("Daisy", new Double(99.22));
      balance.put("Qadir", new Double(-19.08));

      // Show all balances in hash table.
      Enumeration names = balance.keys();
      while(names.hasMoreElements()) {
         String str = (String) names.nextElement();
         System.out.println(str + ": " + balance.get(str));
      }
      System.out.println();
      // Deposit 1,000 into Zara's account
      double bal = ((Double)balance.get("Zara")).doubleValue();
      balance.put("Zara", new Double(bal+1000));
      System.out.println("Zara's new balance: " +
      balance.get("Zara"));
   }
}

输出如下：

%JAVA_HOME%injava "MyDemo" ...
Qadir: -19.08
Zara: 3434.34
Mahnaz: 123.22
Daisy: 99.22
Ayan: 1378.0

Zara's new balance: 4434.34

二、Properties 继承于 Hashtable。表示一个持久的属性集.属性列表中每个键及其对应值都是一个字符串。

Properties 类被许多 Java 类使用。例如，在获取环境变量时它就作为 System.getProperties() 方法的返回值。

import java.util.*;
 
public class MyDemo {
 
   public static void main(String args[]) {
      Properties capitals = new Properties();
      Set states;
      String str;
      
      capitals.put("Illinois", "Springfield");
      capitals.put("Missouri", "Jefferson City");
      capitals.put("Washington", "Olympia");
      capitals.put("California", "Sacramento");
      capitals.put("Indiana", "Indianapolis");
 
      // Show all states and capitals in hashtable.
      states = capitals.keySet(); // get set-view of keys
      Iterator itr = states.iterator();
      while(itr.hasNext()) {
         str = (String) itr.next();
         System.out.println("The capital of " +
            str + " is " + capitals.getProperty(str) + ".");
      }
      System.out.println();
 
      // look for state not in list -- specify default
      str = capitals.getProperty("Florida", "Not Found");
      System.out.println("The capital of Florida is "
          + str + ".");
   }
}

输出如下：

%JAVA_HOME%injava "MyDemo" ...
The capital of Missouri is Jefferson City.
The capital of Illinois is Springfield.
The capital of Indiana is Indianapolis.
The capital of California is Sacramento.
The capital of Washington is Olympia.

The capital of Florida is Not Found.

本文参考：

https://www.runoob.com/java/java-properties-class.html

https://www.runoob.com/java/java-hashTable-class.html