1. 前言

1. 无意有次需求是遍历Properties对象，但突然就不知道怎么遍历了，因此写文章记录一下。

2. 正片

1. Properties 是键值对 存储的，key—value，因此遍历方式跟Map基本一致。

1. keySet()方式

public class demo {
    public static void main(String[] args) {
        Properties properties = new Properties();
        properties.put("1", "11");
        properties.put("2", "22");
        properties.put("3", "31");
        properties.put("4", "41");

        Set<Object> keySet = properties.keySet();
        for (Object key : keySet) {
            System.out.println(key + ": " + properties.get(key));
        }
    }
}

2. entrySet()方式

public class demo {
    public static void main(String[] args) {
        Properties properties = new Properties();
        properties.put("1", "11");
        properties.put("2", "22");
        properties.put("3", "31");
        properties.put("4", "41");

        Set<Map.Entry<Object, Object>> entries = properties.entrySet();
        for (Map.Entry<Object, Object> map : entries) {
            System.out.println(map.getKey() + ":" + map.getValue());
        }
    }
}

3. propertyNames()方式

public class demo {
    public static void main(String[] args) {
        Properties properties = new Properties();
        properties.put("1", "11");
        properties.put("2", "22");
        properties.put("3", "31");
        properties.put("4", "41");

		// 就是迭代器
        Enumeration<?> enumeration = properties.propertyNames();
        while (enumeration.hasMoreElements()) {
            Object key = enumeration.nextElement();
            System.out.println(key + ":" + properties.get(key));
        }
    }
}

4. stringPropertyNames()方式

1. stringPropertyNames() 是把key全部获取封装到Set中，之后直接用增强foreach来操作，但是只是针对key，value都是String类型的情况下，这个是重点。

2. 下面是正常使用情况：

   public class demo {
       public static void main(String[] args) {
           Properties properties = new Properties();
           properties.put("1", "11");
           properties.put("2", "22");
           properties.put("3", "31");
           properties.put("4", "41");
   
           Set<String> keys = properties.stringPropertyNames();
           for (String key : keys) {
               System.out.println(key + ":" + properties.get(key));
           }
       }
   }
   

   

3. 如果我把key，value 改成不是String 类型的，如下：

   public class demo {
       public static void main(String[] args) {
           Properties properties = new Properties();
           properties.put(1, "11");
           properties.put("2", 22);
           properties.put("3", 31);
           properties.put(4, "41");
   
           Set<String> keys = properties.stringPropertyNames();
           System.out.println("keys的size：" + keys.size());
           for (String key : keys) {
               System.out.println(key + ":" + properties.get(key));
           }
       }
   }
   

   
   由上可以看出，key，value如果有一个不是String类型，则不会接受。我们从源码角度来看：

   public Set<String> stringPropertyNames() {
           Hashtable<String, String> h = new Hashtable<>();
           enumerateStringProperties(h);
           return h.keySet();
   }
   
   
    private synchronized void enumerateStringProperties(Hashtable<String, String> h) {
           if (defaults != null) {
               defaults.enumerateStringProperties(h);
           }
           for (Enumeration<?> e = keys() ; e.hasMoreElements() ;) {
               Object k = e.nextElement();
               Object v = get(k);
               // 这里就是关键了
               if (k instanceof String && v instanceof String) {
                   h.put((String) k, (String) v);
               }
           }
       }