Java中定义常量(Constant) 的几种方法

 

 

常量使用目的

1，为什么要将常亮提取出来？ 2，提取出来怎么定义，定义在interface中，还是class中？有什么区别？

1）常亮提取出来有利于代码阅读，而且下次再做这种判断不用手写或复制，直接通过常量类就能得到，能够避免有的地方是username,有的地方是name来判断同一个逻辑含义相同变量问题。不过我觉得提取出来并不会有利于代码性能提升，因为常量分配在内存的常亮池中，所以内存不会出现多个相同常量字符串。总的来说提取常量主要是为了避免魔法数字和提高代码可读性。

2）常亮定义在final的class中，防止被其它类继承和实例化。定义在interface中性质也是一样，static final属性。但是interface看起来就是会被继承的。

interface中定义常量

采用接口(Interface)的中变量默认为static final的特性。

public interface ConstantInterface {  
    String SUNDAY = "SUNDAY";  
    String MONDAY = "MONDAY";  
    String TUESDAY = "TUESDAY";  
    String WEDNESDAY = "WEDNESDAY";  
    String THURSDAY = "THURSDAY";  
    String FRIDAY = "FRIDAY";  
    String SATURDAY = "SATURDAY";  
}

Enum定义常量

enum ConstantEnum {  
        SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY  
    }

 

普通类中定义常量

采用了在普通类中使用static final修饰变量的方法。

public 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";  
    }

也可以使用get方法

public 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;  
        }  
    }

 

final class定义常量

public final 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";  
   }

 

推荐，使用final class定义，表面意思更清楚。虽然为了简单编码，浪尖采用的是interface。