Question

这样的事情可以用Java做吗？

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

我只是在调用一些函数，但我需要编写它们，就像这样：

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

我想避免写出数百行函数调用。

我尝试过研究函数指针和仿函数，但没有发现任何相关内容。

Answer 1

您不能使用f(g(o))语法，但可以使用（使用合适的界面）f.call(g.call(o))。

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

示例用法（这与Java中的仿函数一样接近，至少在闭包进入语言之前）：

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

如果您不想创建数量众多的类，基于反射的方法可能会更好（double - ＆gt; double java.lang.Math中的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); } } }函数，但很容易适应到其他场景）：

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));
    }
}

（为简洁起见，省略了异常消息。显然，我会将它们放入生产代码中。）

样本用法：

{{1}}

Answer 2

也许你可以尝试fluent interface让你将这些团结在一起。这可能是一个很好的设计，但我无法从你的例子中说出来。

Answer 3

Java并不是真正完美的编写器，但是你可以通过一个接口非常接近。我推荐也许会尝试这样的事情。

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

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

...

然后在您的代码中，您将创建一个包含Function1，Function2 ...对象的数组或列表，并运行与您的代码非常相似的内容。

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

或者，对于两个嵌套级别：

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

Answer 4

@Seth - 这是你的泛型的例子。由于仿制药在运行时不存在，我不明白为什么你担心失去“灵活性”。如果你使用泛型，那么你只是使用对象。

如果你希望F的行为根据G的返回类型而变化，那么你只需要声明你的F做类似F的事情，轻松自如。

//=== 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));
    }
}

迭代函数

4 个答案: