【Java基础】枚举和注解

╰半夏微凉° 2022-06-17 12:29 169阅读 0赞

在Java1.5版本中，引入了两个类型：枚举类型`enum type`和注解类型`annotation type`。

# Num1：用enum代替int常量 #

枚举类型`enum type`是指由一组固定的常量组成合法值的类型。比如：

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; // In kilograms private final double radius; // In meters private final double surfaceGravity; // In m / s^2 // Universal gravitational constant in m^3 / kg s^2 private static final double G = 6.67300E-11; // Constructor 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; // F = ma } } public class WeightTable { public static void main(String[] args) { double earthWeight = Double.parseDouble(args[0]); double mass = earthWeight / Planet.EARTH.surfaceGravity(); for (Planet p : Planet.values()) System.out.printf("Weight on %s is %f%n", p, p.surfaceWeight(mass)); } }

# Num2：用EnumSet代替位域 #

示例代码：

public class Text { public enum Style { BOLD, ITALIC, UNDERLINE, STRIKETHROUGH } // Any Set could be passed in, but EnumSet is clearly best public void applyStyles(Set<Style> styles) { // Body goes here } // Sample use public static void main(String[] args) { Text text = new Text(); text.applyStyles(EnumSet.of(Style.BOLD, Style.ITALIC)); } }

总而言之，正是因为枚举类型要用在集合`Set`中，所以没有理由用位域来表示它。

# Num3：注解优先于命名模式 #

命名模式有两个缺点：

*  文字拼写错误会导致失败，且没有任何提示。
 *  无法确保它们只用于相应的程序元素上。
 *  它们没有提供将参数值与程序元素关联起来的好方法。

针对以上几个问题，注解很好地解决了所有这些问题。

/** * Indicates that the annotated method is a test method. Use only on * parameterless static methods. */ @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface Test { }

Test注解类型的声明就是它自身通过`Retention`和`Target`注解进行了注解。注解类型声明中的这张注解被称作”元注解“`meta-annotaition`。

那么声明的Test注解，则称作为”标记注解”`marker annotation`。因为它没有参数，只是“标注”被注解的元素。

示例代码：

public class Sample { @Test public static void m1() { } // Test should pass public static void m2() { } @Test public static void m3() { // Test Should fail throw new RuntimeException("Boom"); } public static void m4() { } @Test public void m5() { } // INVALID USE: nonstatic method public static void m6() { } @Test public static void m7() { // Test should fail throw new RuntimeException("Crash"); } public static void m8() { } }

另外一种注解方式：

@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface ExceptionTest { Class<? extends Exception>[] value(); }

使用方式

public class Sample2 { @ExceptionTest(ArithmeticException.class) public static void m1() { // Test should pass int i = 0; i = i / i; } @ExceptionTest(ArithmeticException.class) public static void m2() { // Should fail (wrong exception) int[] a = new int[0]; int i = a[1]; } @ExceptionTest(ArithmeticException.class) public static void m3() { } // Should fail (no exception) // Code containing an annotation with an array parameter - Page 174 @ExceptionTest({ IndexOutOfBoundsException.class, NullPointerException.class }) public static void doublyBad() { List<String> list = new ArrayList<String>(); // The spec permits this method to throw either // IndexOutOfBoundsException or NullPointerException list.addAll(5, null); } }