Android基础进阶之EffectJava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<Style> styles) { ... }
}

  //使用
  text.applyStyles(EnumSet.of(Style.BOLD, Style.ITALIC));

Item33: Use EnumMap instead of ordinal indexing(使用EnumMap代替顺序索引)

EnumSet/EnumMap详解

Item34: 使用接口提升枚举的扩展性

code say everything
普通操作加减乘除

// Emulated extensible enum using an interface
public interface Operation {
    double apply(double x, double y);
}
public enum BasicOperation implements Operation {
    PLUS("+") {
        public double apply(double x, double y) { return x + y; }
    },
    MINUS("-") {
        public double apply(double x, double y) { return x - y; }
    },
    TIMES("*") {
        public double apply(double x, double y) { return x * y; }
    },
    DIVIDE("/") {
        public double apply(double x, double y) { return x / y; }
    };

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

现需要扩展取余,异或等方法,不要再BasicOperation 中新加枚举,而是实现公共接口Operation

// Emulated extension enum
public enum ExtendedOperation implements Operation {
    EXP("^") {
        public double apply(double x, double y) {
            return Math.pow(x, y);
        }
    },
    REMAINDER("%") {
        public double apply(double x, double y) {
            return x % y;
        }
    };
 
    private final String symbol;
    ExtendedOperation(String symbol) {
        this.symbol = symbol;
    }
    @Override public String toString() {
        return symbol;
    }
}

Item36: 始终使用override注解

// 你能找到这个bug吗?
public class Bigram {
    private final char first;
    private final char second;
    public Bigram(char first, char second) {
        this.first = first;
        this.second = second;
    }
    public boolean equals(Bigram b) {
        return b.first == first && b.second == second;
    }
    public int hashCode() {
        return 31 * first + second;
    }
    
    public static void main(String[] args) {
        Set<Bigram> s = new HashSet<Bigram>();
        for (int i = 0; i < 10; i++)
            for (char ch = 'a'; ch <= 'z'; ch++)
              s.add(new Bigram(ch, ch));
        System.out.println(s.size());
    }
}

理论上我们希望打印26,因为HashSet没有重复元素,但是得到的值是260
因为没有override equals&hashCode方法,所以默认是使用的"=="比较的地址,得到的都是不同的对象
修复如下

    @Override public boolean equals(Bigram b) {
        return b.first == first && b.second == second;
    }
    @Override public int hashCode() {
        return 31 * first + second;
    }

Item37: 使用标记接口定义类型(待补充)

标记接口:只为了标记某一种类型
如 Serializable 接口,表明可将实例序列化
优点:
1.标记接口定义了一种类型
2.可用于任何扩展的子类
标记注解:
优点:
1.描述信息更丰富,可在使用后添加方法
2.位于FrameWork层,各个应用使用同一套规则