在JDK1.5 之前，我们定义常量都是： public static fianl.... 。现在好了，有了枚举，可以把相关的常量分组到一个枚举类型里，而且枚举提供了比常量更多的方法。

public enum Color {

  RED, GREEN, BLANK, YELLOW

} 

enum Signal {

        GREEN, YELLOW, RED

    }

    public class TrafficLight {

        Signal color = Signal.RED;

        public void change() {

            switch (color) {

            case RED:

                color = Signal.GREEN;

                break;

            case YELLOW:

                color = Signal.RED;

                break;

            case GREEN:

                color = Signal.YELLOW;

                break;

            }

        }

    }

public static String postjson = "applicaton/json; charset=utf-8";
public static String postform = "application/x-www-form-urlencoded; charset=utf-8";