迭代函数

Question

是这样的可能在Java中吗？

for (Object o : objects) {
  for (Function f : functions) {
    f(o);
  }
}

我只是调用的函数了一把，但我需要撰写他们，就像这样：

for (Object o : objects) {
  for (Function f : functions) {
    for (Function g : functions) {
      f(g(o));
    }
  }
}

和我想避免写出几百个函数调用行。

我试过研究函数指针和函子，但没有发现任何相关的。

Answer 1

您不能使用f(g(o))语法，但可以使用（与合适的接口）f.call(g.call(o))。

public interface UnaryFunction<Arg, Ret> {
    Ret call(Arg arg);
}

用法示例（这是接近你可以在Java中，函数子至少要等到倒闭使其进入语言）：

public class Exp implements UnaryFunction<Double, Double> {
    public Double call(Double arg) {
        return Math.exp(arg);
    }
}

---

如果你不希望创建一个数不胜数类，基于反射的方法可能更好地工作（例如对于double - > double功能java.lang.Math，但很容易适应其他场景）：

public class MathUnary implements UnaryFunction<Double, Double> {
    private final Method method;

    public MathUnary(String funName) {
        try {
            method = Math.class.getMethod(funName, double.class);
        } catch (NoSuchMethodException exc) {
            throw new IllegalArgumentException(exc);
        }
        if (method.getReturnType() != double.class)
            throw new IllegalArgumentException();
    }

    public Double call(Double arg) {
        try {
            return (Double) method.invoke(null, arg);
        } catch (IllegalAccessException exc) {
            throw new AssertionError(exc);
        } catch (InvocationTargetException exc) {
            throw new AssertionError(exc);
        }
    }
}

（异常消息已省去为了简洁，很明显，我把他们在生产代码。）

示例用法：

MathUnary[] ops = {
    new MathUnary("sin"), new MathUnary("cos"), new MathUnary("tan")
};

for (UnaryFunction<Double, Double> op1 : ops) {
    for (UnaryFunction<Double, Double> op2 : ops) {
        op1.call(op2.call(arg));
    }
}

Answer 2

Java并没有真正做仿函数完全是，但你可以得到相当接近与接口。我reccommend也许想这样的事情。

public interface Function {
    Object doWork(Object o);
}

public class Function1 implements Function {
    public Object doWork(Object o) {
        ...
    }
}

...

然后在你的代码，你会创建一个包含功能1，函数2 ...对象的数组或列表和运行的东西，看起来很像你的代码。

for (Object o : objects) {
      for (Function f : functionList) {
          f.doWork(o);
      }
}

或者，对于两个嵌套层次：

for (Object o : objects) {
      for (Function f : functionList1) {
            for (Function g : functionList2) {
                f.doWork(g.doWork(o));
            }
      }
}

Answer 3

@Seth - 这里是您的泛型例子。由于在运行时不存在仿制药，我不明白为什么你害怕的“灵活性”的损失。如果您使用泛型，那么你只是使用对象。

如果您想根据G的回归F型的行为改变，那么你就只需要声明的F到做一些事情，如F，易peasy。

//=== Function.java

public interface Function<ReturnType, Type> {
    ReturnType doWork(Type arg);
}

//=== SomethingWeird.java

import java.util.*;

// yo dawg, i heard you liked functions.  so i put a function in yo'
// function, so you can derive while you derive.
public class SomethingWeird {
    public static <FReturnType, FType, GType> List<FReturnType> collateOrSomething(
        Iterable<GType> objects,
        Iterable<Function<FReturnType, FType>> fList,
        Iterable<Function<FType, GType>> gList
    ) {
        List<FReturnType> results = new ArrayList<FReturnType>();
        for (GType garg : objects) {
            for (Function<FReturnType, FType> f : fList) {
                for (Function<FType, GType> g : gList) {
                    results.add(f.doWork(g.doWork(garg)));
                }
            }
        }
        return results;
    }
}

//=== SomethingWeirdTest.java

import java.util.*;

import org.junit.*;
import static org.junit.Assert.*;

public class SomethingWeirdTest {
    // this is kinda silly, and...
    public static class F1 implements Function<Integer, Double> {
        @Override
        public Integer doWork(Double arg) {
            return arg.intValue();
        }

    }

    // ...this has all kinds of autoboxing madness, but...
    public static class F2 implements Function<Integer, Double> {
        @Override
        public Integer doWork(Double arg) {
            double ceil = Math.ceil(arg);
            return (int) ceil;
        }       
    }


    // ...why you'd want to do something like this is quite beyond me...
    public static class G1 implements Function<Double, String> {
        @Override
        public Double doWork(String arg) {
            return Math.PI * arg.length();
        }
    }

    // ...ditto this...
    public static class G2 implements Function<Double, String> {
        @Override
        public Double doWork(String arg) {
            return Math.E * arg.length();
        }

    }

    // oh, yeah, it was so we could test this weird thing
    @Test
    public void testCollateOrSomething() {
        List<String> data = Arrays.asList("x", "xx", "xxx");
        List<Function<Integer, Double>> fList = Arrays.asList(new F1(), new F2());
        List<Function<Double, String>> gList = Arrays.asList(new G1(), new G2());
        List<Integer> results = SomethingWeird.collateOrSomething(data, fList, gList);

        assertEquals(12, results.size());

        // x
        assertEquals(3, (int) results.get(0));
        assertEquals(2, (int) results.get(1));
        assertEquals(4, (int) results.get(2));
        assertEquals(3, (int) results.get(3));

        // xx
        assertEquals(6, (int) results.get(4));
        assertEquals(5, (int) results.get(5));
        assertEquals(7, (int) results.get(6));
        assertEquals(6, (int) results.get(7));

        // xxx
        assertEquals(9, (int) results.get(8));
        assertEquals(8, (int) results.get(9));
        assertEquals(10, (int) results.get(10));
        assertEquals(9, (int) results.get(11));
    }
}

Answer 4

也许你可以尝试流畅的介面，将让你们这帮这些结合在一起。这可能是一个不错的设计，但我无法从你的例子告诉我们。