使用枚举值来表示二元运算符或函数
我正在寻找一种优雅的方式来使用 Java 枚举中的值来表示操作或函数。我的猜测是，因为这是 Java，所以不会有一个好的方法来做到这一点，但无论如何，这里就是这样。我的枚举看起来像这样：
public enum Operator {
LT,
LTEQ,
EQEQ,
GT,
GTEQ,
NEQ;
...
}
其中 LT 表示 < （小于），LTEQ 表示 <= （小于或等于）等等 - 你明白了。现在我想实际使用这些枚举值来应用运算符。我知道我可以只使用一大堆 if 语句来做到这一点，但这是丑陋的 OO 方式，例如：
int a = ..., b = ...;
Operator foo = ...; // one of the enum values
if (foo == Operator.LT) {
return a < b;
}
else if (foo == Operator.LTEQ) {
return a <= b;
}
else if ... // etc
我想要做的是剪掉这个结构并使用某种一流的函数甚至是多态性，但我不太确定如何实现。类似：
int a = ..., b = ...;
Operator foo = ...;
return foo.apply(a, b);
或者甚至
int a = ..., b = ...;
Operator foo = ...;
return a foo.convertToOperator() b;
但据我所知，我认为不可能返回运算符或函数（至少，在不使用某些第三方库的情况下）。有什么建议吗？
I'm looking for an elegant way to use values in a Java enum to represent operations or functions. My guess is, since this is Java, there just isn't going to be a nice way to do it, but here goes anyway. My enum looks something like this:
public enum Operator {
LT,
LTEQ,
EQEQ,
GT,
GTEQ,
NEQ;
...
}
where LT means < (less than), LTEQ means <= (less than or equal to), etc - you get the idea. Now I want to actually use these enum values to apply an operator. I know I could do this just using a whole bunch of if-statements, but that's the ugly, OO way, e.g.:
int a = ..., b = ...;
Operator foo = ...; // one of the enum values
if (foo == Operator.LT) {
return a < b;
}
else if (foo == Operator.LTEQ) {
return a <= b;
}
else if ... // etc
What I'd like to be able to do is cut out this structure and use some sort of first-class function or even polymorphism, but I'm not really sure how. Something like:
int a = ..., b = ...;
Operator foo = ...;
return foo.apply(a, b);
or even
int a = ..., b = ...;
Operator foo = ...;
return a foo.convertToOperator() b;
But as far as I've seen, I don't think it's possible to return an operator or function (at least, not without using some 3rd-party library). Any suggestions?
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这不仅是可能的，而且在经常引用的 Effective Java，第二版中用作示例乔什·布洛赫。
我不想侵犯他的版权，会在网上寻找类似的示例...好吧，我记得的代码可以在我之前链接的网站上免费获得。单击“下载本书中使用的代码示例”，然后查看
effective2/examples/Chapter6/Item30/Operation.java。Not only is this possible, it's used as an example in the frequently referenced Effective Java, Second Edition by Josh Bloch.
I don't want to step on his copyright, will look for a similar example online...Okay, the code I remembered is freely available at the website I linked earlier. Click "Download the code samples used in this book", then look at
effective2/examples/Chapter6/Item30/Operation.java.如果 foo 是对象并且 Operator 是基类并且具体运算符是实现 apply 的具体类，则可以执行此操作。
it is possible to do this if foo is object and Operator is base class and concrete operators are concrete classes that implement apply.