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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。
代码语言：javascript复制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
代码语言：javascript复制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。
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制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。