为什么Java Enumeration不按顺序返回属性列表？

这是myBundle.properties文件：

#default
 test1=Hello1
 test2=Hello2
 test3=Hello3
 test4=Hello4
 test5=Hello5
 test6=Hello6

这是Java代码：

import java.util.*;

class BundleTest {

    BundleTest() {
        String[] s = returnStringArray(Locale.ENGLISH);
        for(int i=0; i<s.length; i++) {
            System.out.println(s[i]);
        }
    }

    private String[] returnStringArray(Locale language) {
        try {
            ResourceBundle labels = ResourceBundle.getBundle("myBundle",language);
            Enumeration<String> keys = labels.getKeys();
            Vector v = new Vector();
            String key = null;
            while (keys.hasMoreElements()) {
                v.add(keys.nextElement());
            }
            String[] s = new String[v.size()];
            for(int i=0; i<s.length; i++) {
                s[i] = (String)v.elementAt(i);
            }
            return s;
        } catch (MissingResourceException mre) {
            System.out.println("Risorse della lingua non trovate!");
            return null;
        }
    }

    public static void main(String[] args) {
        new BundleTest();
    }
}

但是,令人惊讶的是,当我执行程序时,它会以偶然的顺序返回字符串.为什么枚举这种奇怪的行为？

bash-4.1$java BundleTest
test1
test6
test4
test5
test2
test3