泛型java中的动态数据类型

Question

我有一个用例，客户端发送列表。任务是迭代并执行该函数并将其保存在 TypedSafeMap 中。 伪客户端代码：

Function<String, Integer> firstFn = x -> x.length();
Function<String, String> secondFn = x -> x.substring(0);

client.runTheseFunctions(Arrays.asList(firstFn, secondFn));

在代码中的runtTheseFunctions内部，任务是执行这些函数并将其保存在一个TypedSafeMap中，其中key是函数结果类型的数据类型，value是functions.apply()的返回值；

下面的代码

public static void runTheseFunctions(List<Function<Employee, ?>> lst, Employee o) {
           lst.stream().forEach( x -> {
             typedSafeMap.put(????, x.apply(o));  
        //The key is nothing but the datatype of the x.apply(o).
        //How do I add this in runtime here. Generics is all compile time safety.
           });
    }

Answer 1

增强@Yonas 答案：

private static Map<?, ? extends Object> runTheseFunctions(List<Function<String, ? extends Object>> list, String o) {
    return list.stream()
            .map(f -> f.apply(o))
            .collect(Collectors.toMap(result -> result.getClass(), Function.identity()));
}

这将只调用一次f.apply(o)。

Answer 2

如果函数列表包含两个或多个具有相同输出类型的函数（例如：String getFirstName、String getLastName、toMap 将失败。所以另一种选择是：

var map = list.stream().collect(groupingBy(
    f -> f.apply(e).getClass(), 
    mapping(f -> f.apply(e), toList())
));

Answer 3

这是您想要实现的目标的示例，您可以将其用于测试。我假设 Employee 类的一个简单实现，只是为了给你一个想法：

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;

class Employee {
    String name;

    public Employee(String name) {
        this.name = name;
    }

    public int length() {
        return name.length();
    }

    public String substring(int index) {
        return name.substring(index);
    }
}

public class Test {

    public static void main(String[] args) {

        Employee e = new Employee("Marco");

        Function<Employee, Integer> firstFn = x -> x.length();
        Function<Employee, String> secondFn = x -> x.substring(0);

        runTheseFunctions(Arrays.asList(firstFn, secondFn), e);

    }

    public static void runTheseFunctions(List<Function<Employee, ?>> lst, Employee o) {

        Map<Class, Object> typedSafeMap = new HashMap<>();

        lst.stream().forEach(x -> {

            Object result = x.apply(o);

            typedSafeMap.put(x.apply(o).getClass(), x.apply(o));
            // The key is nothing but the datatype of the x.apply(o).
            // How do I add this in runtime here. Generics is all compile time safety.
        });

        typedSafeMap.entrySet().forEach(entry -> System.out.println(entry.getKey() + " - " + entry.getValue()));
    }

}

这是输出：

class java.lang.String - Marco
class java.lang.Integer - 5

Answer 4

您可以实现您的“runTheseFunctions”方法，如下所示：

    public static void runTheseFunctions(List<Function<Employee, ?>> lst, Employee o) {
        Map<Class<?>, Object> typedSafeMap = new HashMap<>();

        lst.stream().forEach(x -> {
            Object value = x.apply(o);
            typedSafeMap.put(value.getClass(), value);
        });

        System.out.println(typedSafeMap);
    }

Answer 5

public static void runTheseFunctions(List<Function<Employee, ?>> lst, Employee o) {
    lst.stream().collect(Collectors.toMap(f -> f.apply(o).getClass(), f -> f.apply(o)));
}