13，定义常量

/**
 * Method One
 */
interface ConstantInterface {
    String SUNDAY = "SUNDAY";
    String MONDAY = "MONDAY";
    String TUESDAY = "TUESDAY";
    String WEDNESDAY = "WEDNESDAY";
    String THURSDAY = "THURSDAY";
    String FRIDAY = "FRIDAY";
    String SATURDAY = "SATURDAY";
}
/**
 * Method Two 
 */

public enum Area
{
  PROVINCE("广州",1);

    private String name;

    private Integer id;

    Area(String name,Integer id)
    {
        this.name = name;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public Integer getId() {
        return id;
    }
}
/**
 * Method Three
 */
class ConstantClassField {
    public static final String SUNDAY = "SUNDAY";
    public static final String MONDAY = "MONDAY";
    public static final String TUESDAY = "TUESDAY";
    public static final String WEDNESDAY = "WEDNESDAY";
    public static final String THURSDAY = "THURSDAY";
    public static final String FRIDAY = "FRIDAY";
    public static final String SATURDAY = "SATURDAY";
}
/**
 * Method Four
 * http://www.ibm.com/developerworks/cn/<a href="http://lib.csdn.net/base/17" class='replace_word' title="Java EE知识库" target='_blank' style='color:#df3434; font-weight:bold;'>Java</a>/l-java-interface/index.html
 */
class ConstantClassFunction {
    private static final String SUNDAY = "SUNDAY";
    private static final String MONDAY = "MONDAY";
    private static final String TUESDAY = "TUESDAY";
    private static final String WEDNESDAY = "WEDNESDAY";
    private static final String THURSDAY = "THURSDAY";
    private static final String FRIDAY = "FRIDAY";
    private static final String SATURDAY = "SATURDAY";
    public static String getSunday() {
        return SUNDAY;
    }
    public static String getMonday() {
        return MONDAY;
    }
    public static String getTuesday() {
        return TUESDAY;
    }
    public static String getWednesday() {
        return WEDNESDAY;
    }
    public static String getThursday() {
        return THURSDAY;
    }
    public static String getFirday() {
        return FRIDAY;
    }
    public static String getSaturday() {
        return SATURDAY;
    }
}
public class TestConstant {
    static final String day = "saturday";
    public static void main(String[] args) {
        System.out.println("Is today Saturday?");
        System.out.println(day.equalsIgnoreCase(ConstantInterface.SATURDAY));
        System.out.println(day.equalsIgnoreCase(ConstantEnum.SATURDAY.name()));
        System.out.println(day.equalsIgnoreCase(ConstantClassField.SATURDAY));
        System.out.println(day.equalsIgnoreCase(ConstantClassFunction
                .getSaturday()));
    }
}