枚举和注解

30.用enum代替int类型

枚举类型指由一组固定的常量组成合法值的类型，没有枚举型前用的是声明一组常量。在安全和使用上没有帮助，将Apple传到orange也没有问题，还会用==比较，而且是编译器常量，如果改变，必须重新编译，不重新编译也没问题，行为就不确定了

public static final int APPLE_FUJI = 0;public static final int APPLE_PIPPIN = 1;public static final int APPLE_GRANNY_SMITH = 2;public static final int PRANGE_NAVEL = 0;public static final int ORANGE_TEMPLE = 1;public static final int ORANGE_BLOOD = 2;

这是int枚举模式

public enum Apple{FUJI,PIPPIN,GRANNY_SMITH}public enum Orange{NAVEL,TEMPLE,BLOOD}

本质是int型，是通过公有的静态final域为每个枚举常量导出实例的类，没有可以访问的构造器，枚举类型是真正的final，是单例和泛型化的

public enum Planet {//枚举常量后面括号中的数值就是传递给构造器的参数MERCURY(3.302e+23,2.439e6),VENUS(4.859e+24,2.439e6),EARTH(3.302e+23,2.439e6),MARS(3.302e+23,2.439e6),JUPITER(3.302e+23,2.439e6),SATUNE(3.302e+23,2.439e6),URANUS(3.302e+23,2.439e6),NEPTUNE(3.302e+23,2.439e6);private final double mass;private final double radius;private final double surfaceGravity;private static final double G = 6.67300E-11;Planet(double mass, double radius) {this.mass = mass;this.radius = radius;surfaceGravity = G *mass /(radius *radius);}public double mass(){return mass;}public double radius(){return radius;}public double surfaceGravity(){return surfaceGravity;}public double surfaceWeight(double mass){return mass *surfaceGravity;}}

public class WeightTable {public static void main(String[] args) {double earthWeight = 1;double mass = earthWeight /Planet.EARTH.surfaceGravity();//静态的values方法,按照声明顺序返回值数组for (Planet planet : Planet.values()) {System.out.println(planet.surfaceWeight(mass));}}}

//计算加减乘除，通过启用枚举的值来实现，代码脆弱，如果添加新的枚举值，switch里不添加相应的条件，试图用新的运算 ，会运行失败public enum Operation {PLUS,MINUS,TIMES,DIVIDE;double apply(double x,double y){switch (this) {case PLUS:return x+y;case MINUS:return x-y;case TIMES:return x*y;case DIVIDE:return x/y;}throw new AssertionError();}}

//将不同的行为与每个常量连接起来，在枚举类型中声明一个抽象方法，在常量的类主体中，用具体的方法覆盖每个常量的抽象apply方法public enum Operation {PLUS{double apply(double x,double y){return x+y;}},MINUS{double apply(double x,double y){return x-y;}},TIMES{double apply(double x,double y){return x*y;}},DIVIDE{double apply(double x,double y){return x/y;}};abstract double apply(double x,double y);}

public enum Operation {PLUS("+"){double apply(double x,double y){return x+y;}},MINUS("-"){double apply(double x,double y){return x-y;}},TIMES("*"){double apply(double x,double y){return x*y;}},DIVIDE("/"){double apply(double x,double y){return x/y;}};private final String symbol;private Operation(String symbol) {this.symbol=symbol;}//覆盖toString方法返回与该操作相关联的符号@Overridepublic String toString(){return symbol;}abstract double apply(double x,double y);}

//常量被创建后，Operation常量从静态代码块中被放入到stringToEnum的map中private static final Map<String, Operation> stringToEnum = new HashMap<>();static{for (Operation op:values()) {stringToEnum.put(op.toString(), op);}}//将定制的字符串表示法变回相应的枚举public static Operation fromString(String symbol){return stringToEnum.get(symbol);}

每添加一个枚举常量，强制选择一种策略，策略枚举

//将加班工资计算移到一个私有的嵌套枚举中enum PayrollDay {MONDAY(PayType.WEEKDAY),TUESDAY(PayType.WEEKDAY),WENESDAY(PayType.WEEKDAY),THURSDAY(PayType.WEEKDAY),FRIDAY(PayType.WEEKDAY),SATURDAY(PayType.WEEKEND),SUNDAY(PayType.WEEKEND);private final PayType payType;PayrollDay(PayType payType) {this.payType =payType;}private enum PayType{WEEKDAY{double overTimePay(double hours,double payRate){return hours <= HOURS_PER_SHIFT ? 0:(hours-HOURS_PER_SHIFT)*payRate/2;}},WEEKEND{double overTimePay(double hours,double payRate){return hours * payRate /2;}};private static final int HOURS_PER_SHIFT = 8;abstract double overTimePay(double hours,double payRate);double pay(double hoursWorked,double payRate){double basePay = hoursWorked * payRate;return basePay + overTimePay(hoursWorked, payRate);}}}

枚举中的switch语句适合于给外部的枚举类型增加特定于常量的行为。

//假设Operation枚举不受自己的控制，希望它有个实例方法来返回每个运算的反运算public static Operation inverse(Operation op){switch (op) {case PLUS:return Operation.MINUS;case MINUS:return Operation.PLUS;case TIMES:return Operation.DIVIDE;case DIVIDE:return Operation.MINUS;default:throw new AssertionError(op);}

每当需要一组固定常量的时候使用枚举，包括在编译时就知道的所有可能值的其他集合
31.用实例域代替序数