Effective Java 学习笔记之枚举

ゞ浴缸里的玫瑰 2022-06-14 06:41 258阅读 0赞

1.用enum代替int常量

1）为了将数据和枚举常量关联起来 得声明实例域，并编写一个带有数据并将数据保存在域中的构造器。  
2）枚举天生就是不可变的，因此所有的域都应该是final的。它们可以是公有的，但是最好做成私有并提供公有的访问方式  
3）枚举类型还允许添加任意的方法和域 并实现任意的接口  
4）提供了所有Object方法的高级实现 实现了Comparable和Serializable接口，并设计了序列化方式

public enum Planet {
    
        MERCURY(3.302E+23, 2.439E6),
        VENUS(4.869e+24, 6.052e6),
        EARTH(5.975e+24, 6.378e6),
        MARS(6.419e+23, 3.393e6),
        JUPITER(1.899e+27, 7.149e7),
        SATURN(5.685e+26, 6.027e7),
        URANUS(8.683e+25, 2.556e7),
        NEPTUNE(1.024e+26, 2.477e7);
    
        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 getMass() {
            return mass;
        }
    
        public double getRadius() {
            return radius;
        }
    
        public double getSurfaceGravity() {
            return surfaceGravity;
        }
    
        public double getSurfaceWeight(double mass) {
            return mass * surfaceGravity;
        }
    
    }

扩展：需要为每个常量添加不同的行为

public enum Operation1 {
    
        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("Unknown op:" + this);
        }
    }

上面的代码存在一些问题，当为枚举类添加新的常量后，忘记为其提供相应apply的实现，编译不会出现错误提示，然而运行结果却不是希望的

因此可以进行修改

提供一个抽象的apply方法,这样每个常量都必须实现这个抽象方法，否则编译就会报错

PLUS {
            @Override
            double apply(double x, double y) {
                return x + y;
            }
        },
        MINUS {
            @Override
            double apply(double x, double y) {
                return x - y;
            }
        },
        TIMES {
            @Override
            double apply(double x, double y) {
                return x * y;
            }
        },
        DIVIDE {
            @Override
            double apply(double x, double y) {
                return x / y;
            }
        };
    
        abstract double apply(double x, double y);

有时候复写toString方法也会非常有用

public enum Operation2 {
        PLUS("+") {
            @Override
            double apply(double x, double y) {
                return x + y;
            }
        },
        MINUS("-")  {
            @Override
            double apply(double x, double y) {
                return x - y;
            }
        },
        TIMES("*")  {
            @Override
            double apply(double x, double y) {
                return x * y;
            }
        },
        DIVIDE("/")  {
            @Override
            double apply(double x, double y) {
                return x / y;
            }
        };
    
        abstract double apply(double x, double y);
    
        private String symbol;
    
        Operation2(String symbol) {
            this.symbol = symbol;
        }
    
        @Override
        public String toString() {
            return symbol;
        }
    }

public class OpeartionTest {
        public static void main(String[] args) {
            double d1 = 1.3;
            double d2 = 2.6;
            for (Operation2 o:Operation2.values()){
                System.out.printf("%f %s %f = %f%n",d1,o,d2,o.apply(d1,d2));
            }
        }
    }

输出结果

1.300000 + 2.600000 = 3.900000
    1.300000 - 2.600000 = -1.300000
    1.300000 * 2.600000 = 3.380000
    1.300000 / 2.600000 = 0.500000

5）枚举类型有一个自动产生的valueOf(String)方法，它将常量的名字转变成常量本身。

Operation2 plus = Operation2.valueOf("PLUS");
    System.out.println(plus);

输出结果

如果在枚举类型中覆盖toString，要考虑编写一个fromString方法，将定制的字符串表示法变回相应的枚举，考虑用map保存

private static Map<String,Operation3> operation3Map = new HashMap<>();
    
    static {
       for (Operation3 o:Operation3.values()){
           operation3Map.put(o.toString(),o);
       }
    }
    
    public static Operation3 fromString(String str){
        return operation3Map.get(str);
    }

测试方法

Operation3 minus = Operation3.fromString("-");
    System.out.println(minus);

输出结果

6）特定于常量的方法有一点不足，它们是的在枚举常量中共享代码变得困难

public enum PayrollDay {
        MONDAY, TUESDAT, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;
    
        /** * 正常工作时间 */
        private static final int HOURS_PER_SHIFT = 8;
    
        /** * 计算工资 * * @param hoursWorked 工作时间 * @param payRate 薪资 * @return 工资 */
        double pay(double hoursWorked, double payRate) {
            double basePay = hoursWorked * payRate;
    
            double overtimePay;
    
            switch (this) {
                case SATURDAY:
                case SUNDAY:
                    overtimePay = hoursWorked * payRate / 2;
                default:
                    overtimePay = hoursWorked <= HOURS_PER_SHIFT ? 0 : (hoursWorked - HOURS_PER_SHIFT) * payRate / 2;
            }
            return basePay + overtimePay;
        }
    }

当添加一个元素而没有添加相应的case时就会出现问题

可以使用枚举策略来解决，即讲加班工资的计算转移到一个私有的嵌套枚举中，将这个枚举的实例传到外部枚举的构造器中，这样外部枚举就可以将工资计算委托给内部枚举不需要再用switch进行转化了

public enum PayrollDay2 {
    
        MONDAY(PayType.PAY_WEEKDAY),
        TUESDAY(PayType.PAY_WEEKDAY),
        WEDNESDAY(PayType.PAY_WEEKDAY),
        THURSDAY(PayType.PAY_WEEKDAY),
        FRIDAY(PayType.PAY_WEEKDAY),
        SATURDAY(PayType.PAY_WEEKEND),
        SUNDAY(PayType.PAY_WEEKEND);
    
        private final PayType payType;
    
        PayrollDay2(PayType payType) {
            this.payType = payType;
        }
    
        public double pay(double hoursWork, double payRate) {
            return this.payType.pay(hoursWork, payRate);
        }
    
        private enum PayType {
            PAY_WEEKDAY {
                @Override
                double overtimePay(double hoursWork, double payRate) {
                    return hoursWork <= HOUR_SHIFT_TIME ? 0 : (hoursWork - HOUR_SHIFT_TIME) * payRate / 2;
                }
            },
            PAY_WEEKEND {
                @Override
                double overtimePay(double hoursWork, double payRate) {
                    return hoursWork * payRate;
                }
            };
    
            private static final int HOUR_SHIFT_TIME = 8;
    
            abstract double overtimePay(double hoursWork, double payRate);
    
            public double pay(double hoursWork, double payRate) {
                double basePay = hoursWork * payRate;
                double overtimePay = overtimePay(hoursWork, payRate);
                return basePay + overtimePay;
            }
        }
    }

2.用实例域代替序数

public enum Ensemble {
        SOLO(1),
        DUET(2),
        TRIO(3),
        QUARTET(4),
        QUEINTET(5),
        SEXTET(6),
        SEPTET(7);
    
        private final int numberOfMusicians;
    
        Ensemble(int size) {
            this.numberOfMusicians = size;
        }
    
        public int getNumberOfMusicians() {
            return numberOfMusicians;
        }
    
        //public int numberOfMusicians() { 
        // //ordinal():返回每个枚举常量在类型中的数字位置
        // return ordinal() + 1;
        //}
    }

3.用接口模拟可伸缩的枚举

public interface Operation { 
        double apply(double x, double y);
    }
    public enum BaseOperation implements Operation {
        PLUS {
            @Override
            public double apply(double x, double y) {
                return x + y;
            }
        }, MINUS {
            @Override
            public double apply(double x, double y) {
                return x - y;
            }
        }, TIMES {
            @Override
            public double apply(double x, double y) {
                return x * y;
            }
        }, DIVIDE {
            @Override
            public double apply(double x, double y) {
                return x / y;
            }
        };
    }
    
    public enum ExtendOperation implements Operation {
        MOD {
            @Override
            public double apply(double x, double y) {
                return x % y;
            }
        },
        EXP {
            @Override
            public double apply(double x, double y) {
                return Math.pow(x, y);
            }
        };
    }

测试方法

public class ExtendOperationTest {
        public static void main(String[] args) {
            double d1 = 1.3;
            double d2 = 2.6;
            test(ExtendOperation.class, d1, d2);
            test1(Arrays.asList(ExtendOperation.values()), d1, d2);
        }
    
        private static <T extends Enum<T> & Operation> void test(Class<T> opSet, double d1, double d2) {
            for (Operation o : opSet.getEnumConstants()) {
                System.out.printf("%f %s %f = %f%n", d1, o, d2, o.apply(d1, d2));
            }
        }
    
        private static void test1(Collection<? extends Operation> opSet, double d1, double d2) {
            for (Operation o : opSet) {
                System.out.printf("%f %s %f = %f%n", d1, o, d2, o.apply(d1, d2));
            }
        }
    }

输出结果

1.300000 MOD 2.600000 = 1.300000
    1.300000 EXP 2.600000 = 1.978120
    1.300000 MOD 2.600000 = 1.300000
    1.300000 EXP 2.600000 = 1.978120

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