java 枚举 扩展性_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
}
//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)
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 s = new HashSet();
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