Android基础进阶之EffectiveJava翻译系列(第五章:枚举和注解)

Java1.5中提供的两种新类型

Item30: 用枚举替代int型常量

枚举:一系列常量类型的集合
没有枚举前大量定义的常量如下

    // The int enum pattern - severely deficient!
    public static final int APPLE_FUJI = 0;
    public static final int APPLE_PIPPIN = 1;
    public static final int APPLE_GRANNY_SMITH = 2;
    public static final int ORANGE_NAVEL = 0;
    public static final int ORANGE_TEMPLE = 1;
    public static final int ORANGE_BLOOD = 2;

首先调试不方便,我们只会打印出数字,然后回归代码
其次没有命名空间做区别,命名累赘
使用枚举后

    public enum Apple { FUJI, PIPPIN, GRANNY_SMITH }
    public enum Orange { NAVEL, TEMPLE, BLOOD }

再来看一个星球的例子

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
    }
}

再来看一个算数的例子

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

总结:枚举比int型常量更安全和具有可读性

Item31: Use instance fields instead of ordinals(使用实例字段代替序号)

所有的枚举都有
ordinal 方法(从0开始计数),如果我们想用序号的int值(从1开始计数),不要直接修改ordinal 方法

//bad
public enum Ensemble {
    SOLO, DUET, TRIO, QUARTET, QUINTET,
    SEXTET, SEPTET, OCTET, NONET, DECTET;
    public int numberOfMusicians() { return ordinal() + 1; }
}

虽然上述代码能正常使用,但是却会导致一场噩梦
如果我们改变了枚举中的常量顺序,之前的序号就会一团糟
优化代码如下

public enum Ensemble {
    SOLO(1), DUET(2), TRIO(3), QUARTET(4), QUINTET(5),
    SEXTET(6), SEPTET(7), OCTET(8), DOUBLE_QUARTET(8),
    NONET(9), DECTET(10), TRIPLE_QUARTET(12);
    
    private final int numberOfMusicians;
    
    Ensemble(int size) { 
        this.numberOfMusicians = size; 
    }
    
    public int numberOfMusicians() { 
        return numberOfMusicians; 
    }
}

Item32: Use EnumSet instead of bit fields(用EnumSet替代位字段)

如我们有一段代码

// Bit field enumeration constants - OBSOLETE!
public class Text {
  public static final int STYLE_BOLD = 1 << 0; // 1
  public static final int STYLE_ITALIC = 1 << 1; // 2
  public static final int STYLE_UNDERLINE = 1 << 2; // 4
  public static final int STYLE_STRIKETHROUGH = 1 << 3; // 8
  // Parameter is bitwise OR of zero or more STYLE_ constants
  public void applyStyles(int styles) { ... }
}
  //使用
  text.applyStyles(STYLE_BOLD | STYLE_ITALIC);

用EnumSet优化如下:

// EnumSet - a modern replacement for bit fields
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