effect java 学习摘要(5) - 枚举和注解

enum枚举类型代替int常量

枚举enum示例 :

public enum Apple{

FUJI(1),
PIPPIN(2),
GRANNY_SMITH(3);

private int category;

Apple(int category) {
this.category = category;
}
}
public enum Orange{NAVEL, TEMPLE, BLOOD}

枚举本质上是int值

枚举类型为类型安全的枚举模式

策略枚举 : 利用抽象方法, 避免switch过于复杂,冗余

public enum PayrollDay {

MONDAY(PayType.WEEKDAY),
TUESDAY(PayType.WEEKDAY),
WEDNESDAY(PayType.WEEKDAY),
THURSDAY(PayType.WEEKDAY),
FRIDAY(PayType.WEEKDAY),
SATURDAY(PayType.WEEKEND),
SUNDAY(PayType.WEEKEND);

private final PayType payType;

PayrollDay(PayType payType) {
this.payType = payType;
}

double pay(double hoursWorked, double payRate) {
return payType.pay(hoursWorked, payRate);
}

private enum PayType {

WEEKDAY {
@Override
double overtimePay(double hours, double payRate) {
return hours <= HOURS_PER_SHIT ? 0 : (hours - HOURS_PER_SHIT) * payRate / 2;
}
},
WEEKEND {
@Override
double overtimePay(double hours, double payRate) {
return hours * payRate / 2;
}
};

private static final int HOURS_PER_SHIT = 8;

abstract double overtimePay(double hours, double payRate);

double pay(double hoursWorked, double payRate) {
double basePay = hoursWorked + payRate;
return basePay + overtimePay(hoursWorked, payRate);
}
}
}

用实例代替序数

```
public enum Apple{

FUJI(1),
PIPPIN(2),
GRANNY_SMITH(3);

private int category;

Apple(int category) {
this.category = category;
}
}
```

用EnumSet替代位域

```
public static class Text {
public static final int STYLE_BOLD = 1 << 0;
public static final int STYLE_ITALIC = 1 << 1;

public enum Style {BOLD ,ITALIC;}
}
```

use an EnumMap to associate data with an enum

```
public class Herb {

public enum Type {ANNUAL, PERENNIAL, BIENNIAL}

final String name;
final Type type;

public Herb(String name, Type type) {
this.name = name;
this.type = type;
}

@Override
public String toString() {
return name;
}
}

public void associate() {
Map<Herb.Type , Set<Herb>> hrebsByType = new EnumMap<Herb.Type, Set<Herb>>(Herb.Type.class);
for (Herb.Type t : Herb.Type.values()) {
hrebsByType.put(t, new HashSet<Herb>());
}
for (Herb h : garden) {
hrebsByType.get(h.type).add(h);
}
}
```

用接口模拟可伸缩的枚举

虽然无法编写可扩展的枚举类型, 却可以通过编写接口以及实现该接口的基础枚举类型

注解

通过Retention和Target注解进行注解, 被称为元注解(meta-annotation)

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Test {
}