【Java基礎】列舉和註解

cryAllen發表於2016-06-28
Java

在Java1.5版本中，引入了兩個型別：列舉型別enum type和註解型別annotation type。

Num1：用enum代替int常量

列舉型別enum type是指由一組固定的常量組成合法值的型別。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

示例程式碼：

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));
    }
}

總而言之，正是因為列舉型別要用在集合Set中，所以沒有理由用位域來表示它。

Num3：註解優先於命名模式

命名模式有兩個缺點：

  • 文字拼寫錯誤會導致失敗，且沒有任何提示。
  • 無法確保它們只用於相應的程式元素上。
  • 它們沒有提供將引數值與程式元素關聯起來的好方法。

針對以上幾個問題，註解很好地解決了所有這些問題。

/**
 * 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註解型別的宣告就是它自身通過Retention和Target註解進行了註解。註解型別宣告中的這張註解被稱作”元註解“meta-annotaition。

那麼宣告的Test註解，則稱作為”標記註解”marker annotation。因為它沒有引數，只是“標註”被註解的元素。

示例程式碼：

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() {
    }
}

另外一種註解方式：

@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);
    }
}

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