Java Collection - 如何从属性获取键列表

propertyNames() method in Properties class combines the keys in the
properties list with the keys in the defaults properties list passed into the constructor.
  

import java.io.FileInputStream;
import java.util.Enumeration;
import java.util.Properties;

public class MainClass {
  public static void main(String args[]) throws Exception {
    Properties p = new Properties();
    p.load(new FileInputStream("test.txt"));
    Enumeration e = p.propertyNames();

    for (; e.hasMoreElements();) {
      System.out.println(e.nextElement());

    }
  }
}