effective java (六：枚举和注解)

枚举

 

什么时候需要使用枚举？

当程序中需要使用到一组常量（整型，字符串）时，就应该使用枚举对这些常量进行封装。

如，星期一到星期天、性别分男和女等

 

使用枚举可以带来哪些好处：

类型安全，防止传入错误的参数值

可读性好，比int值强

不易出错，比硬编码字符串到程序各个角落好

枚举实例可以具备行为，操作起来更加方便

 

使用枚举替换int常量和字符串常量

 

	public enum Sex {
		MALE, FEMALE;
	} 

 

用实例域替代枚举自身的序数

 

如果需要使用枚举常量的序数，最好自己定义，不要使用默认的ordinal实现

 

public enum Priority {
		STEP_A(0, "洗脸"), 
		STEP_B(1, "刷牙"), 
		STEP_C(2, "抽烟"), 
		STEP_D(2, "WC"), 
		STEP_E(3, "上班");
		
		private int ordinal;
		private String task;
		
		Priority(int ordinal, String task) {
			this.ordinal = ordinal;
			this.task = task;
		}
		
		public int getOridnal() {
			return this.ordinal;
		}
		
		@Override
		public String toString() {
			return this.ordinal + ": " + this.task;
		}
	}
	
	//test
	public static void main(String[] args) {
		for(Priority p : Priority.values()) {
			System.out.println(p);
		}
	}

  

 

EnumSet

通过集合接收几个枚举常量

 

package effective.chapter6;

import java.util.EnumSet;

public class Text {
	public enum Style {
		BOLD, ITALIC, UNDERLINE, STRIKETHROUGH;
	}
	
	public static void applyStyle(EnumSet<? extends Style> styles) {
		for(Style s : styles) {
			System.out.println("apply style: " + s);
		}
	}
	
	//test
	public static void main(String[] args) {
		Text.applyStyle(EnumSet.of(Style.BOLD, Style.STRIKETHROUGH));
	}
}

 

 

EnumMap

枚举类型作为Map的key进行使用

 

描述两个阶段映射到一个阶段过渡

package effective.chapter6;

import java.util.EnumMap;
import java.util.Map;

/**
 * 嵌套枚举定义
 */
public enum Phase {
	SOLID("固体"), LIQUID("液体"), GAS("汽体");
	
	private String status;
	
	Phase(String status) {
		this.status = status;
	}
	
	@Override
	public String toString() {
		return status;
	}
	
	//不同阶段转换的过程
	public enum Transition {
		MELT(SOLID, LIQUID), 
		FREEZE(LIQUID, SOLID), 
		BOIL(LIQUID, GAS);
		
		private Phase src;
		private Phase dst;
		Transition(Phase src, Phase dst) {
			this.src = src;
			this.dst = dst;
		}
		
		//key: 起始状态
		//value: [key:结束状态，value:转变过程]
		private static final Map<Phase, Map<Phase,Transition>> m = 
				new EnumMap<Phase, Map<Phase,Transition>>(Phase.class);
		
		static {
			for(Phase p : Phase.values())
				m.put(p, new EnumMap<Phase,Transition>(Phase.class));
			for(Transition trans : Transition.values())
				m.get(trans.src).put(trans.dst, trans);
		}
		
		public static Transition from(Phase src, Phase dst) {
			return m.get(src).get(dst);
		}
		
		@Override
		public String toString() {
			return this.src + "->" + this.dst;
		}
		
	}
}

 

public class TestNestEnum {
	public static void main(String[] args) {
		Transition trans = Phase.Transition.from(Phase.SOLID, Phase.LIQUID);
		System.out.println(trans);
	}
}

 

结果：固体->液体

 

通过接口扩展枚举的功能

 

public interface Operation {
	double aplly(double x, double y);
}

 

public enum BasicOperation implements Operation {
	PLUS("+"){
		@Override
		public double aplly(double x, double y) {
			return x + y;
		}
	},
	MINUS("-"){
		@Override
		public double aplly(double x, double y) {
			return x - y;
		}
	},
	TIMES("*"){
		@Override
		public double aplly(double x, double y) {
			return x * y;
		}
	},
	DIVIDE("/"){
		@Override
		public double aplly(double x, double y) {
			return x / y;
		}
	},
	EXP("^") {
		@Override
		public double aplly(double x, double y) {
			return Math.pow(x, y);
		}
	},
	REMAINDER("%"){
		@Override
		public double aplly(double x, double y) {
			return x % y;
		}
	};
	
	private final String symbol; 

	BasicOperation(String symbol) {
		this.symbol = symbol;
	}
	
	@Override
	public String toString(){
		return symbol;
	}

	@Override
	public double aplly(double x, double y) {
		throw new RuntimeException("You must implements method in each enum instance!");
	}
}

 

class TestEnumInterface {
	public static void main(String[] args) {
		double x = 2.17;
		double y = 3.02;
		//数组转换为集合
		testEnum(Arrays.asList(BasicOperation.values()), x, y);
	} 
	
	//泛型参数接收枚举集合
	public static void testEnum(Collection<? extends Operation> opSet, double x, double y) {
		for(Operation op : opSet)
			System.out.printf("%f %s %f = %f%n", x, op, y, op.aplly(x, y));
	}
}

 

结果：

2.170000 + 3.020000 = 5.190000
2.170000 - 3.020000 = -0.850000
2.170000 * 3.020000 = 6.553400
2.170000 / 3.020000 = 0.718543
2.170000 ^ 3.020000 = 10.377874
2.170000 % 3.020000 = 2.170000

 

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

 

注解

 

标记注解

package effective.chapter6;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)//配置‘@AnnoTest’应该被保留到运行阶段,在运行阶段通过反射定位此注解
@Target(ElementType.METHOD)//配置‘@AnnoTest’可使用的目标：用在方法上，而不是用在字段或者类上
public @interface MyAnno {

}

 

package effective.chapter6;

/**
 * 将注解标注到方法上
 */
public class Sample {
	@MyAnno
	public static void m1() {}
	
	public static void m2() {}
	
	@MyAnno
	public static void m3() {
		throw new RuntimeException("Boommmm");
	}
	
	public static void m4() {}
	
	@MyAnno
	public void m5() {}
	
	public void m6() {}
	
	@MyAnno
	public static void m7() {
		throw new RuntimeException("Boommmm again");
	}
	
	public static void m8(){}
	
}

 

package effective.chapter6;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * 读取注解，只处理那些标记了特定注解的方法
 * 在Sample类中使用@MyAnno注解对若干方法进行标记
 * 1.反射Sample类中的方法
 * 2.判断方法是否被@MyAnno标记了
 * 3.如果标记了，则以静态方法反射调用此方法
 * 4.统计被正确调用的静态方法个数，与出现异常的方法个数
 * 5.未标记@MyAnno注解的方法不会得到执行
 */
public class Test {
	public static void main(String[] args) throws Exception {
		int tests = 0;
		int passed = 0;
		
		Class<?> testClass = Class.forName("effective.chapter6.Sample");
		for(Method m : testClass.getDeclaredMethods()) {
			//判断方法上的注解是否为@MyAnno
			if(m.isAnnotationPresent(MyAnno.class)) {
				tests++;
				try{
					//反射调用静态方法
					m.invoke(null);
					passed++;
				} catch (InvocationTargetException wrappedExc) {
					Throwable exc = wrappedExc.getCause();
					System.out.println(m + "failed:" + exc);
				} catch (Exception e) {
					System.out.println("Invalid @MyAnno:" + m);
				}
			}
		}
		
		System.out.printf("passed: %d, failed %d%n", passed, tests - passed);
	}
}

 

注解中定义数组参数

package effective.chapter6;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

//配置‘@AnnoTest’应该被保留到运行阶段,在运行阶段通过反射定位此注解
@Retention(RetentionPolicy.RUNTIME)
//配置‘@AnnoTest’可使用的目标：用在方法上，而不是用在字段或者类上
@Target(ElementType.METHOD)
public @interface MyAnno {
	Class<? extends Exception>[] values();//接收一组异常类
}

 

package effective.chapter6;

import java.util.ArrayList;
import java.util.List;

/**
 * 将注解标注到方法上
 * 给注解的数组参数添加配置
 */
public class Sample {
	
	@MyAnno(values = {IndexOutOfBoundsException.class, NullPointerException.class})
	public static void badMethod() {
		List<String> list = new ArrayList<String>();
		list.addAll(5,null);
	}
	
}

 

package effective.chapter6;

import java.lang.reflect.Method;

/**
 * 对抛出特定异常的方法进行测试
 */
public class Test {
	public static void main(String[] args) throws Exception {
		int tests = 0;
		int passed = 0;
		
		Class<?> testClass = Class.forName("effective.chapter6.Sample");
		for(Method m : testClass.getDeclaredMethods()) {
			//判断方法上的注解是否为@MyAnno
			if(m.isAnnotationPresent(MyAnno.class)) {
				tests++;
				try{
					//反射调用静态方法
					m.invoke(null);
					System.out.printf("Tests %s failed: no exception%n", m);
				} catch (Throwable wrappedExc) {
					Throwable excCause = wrappedExc.getCause();
					Class<? extends Exception>[] excTypes = m.getAnnotation(MyAnno.class).values();
					int oldPassed = passed;
					for(Class<? extends Exception> excType : excTypes) {
						if(excType.isInstance(excCause)) {
							passed++;
							break;
						}
					}
					if(passed == oldPassed)
						System.out.printf("Test %s failed: %s %n", m, excCause);
					else
						System.out.println("Test ok!");
				} 
			}
		}
		System.out.printf("passed: %d, failed %d%n", passed, tests - passed);
	}
}

 

 

建议：坚持使用@overrider注解，避免非法错误！