通用静态方法过多地约束类型 [英] Generic static method constrains types too much

问题描述

在此答案中，我尝试创建一个静态实用程序方法来生成列表进入地图：

In this answer I attempted to create a static utility method to make a List into a Map:

public static <K, T> Map<K, T> toMapBy(List<T> list,
        Function<? super T, ? extends K> mapper) {
    return list.stream().collect(Collectors.toMap(mapper, Function.identity()));
}

它运作正常。但是，我发现该方法不能在与 list.stream（）。collect（...）表达式相同的上下文中使用。该方法不够灵活。

It works just fine. However, I found that the method cannot be used in all the same contexts as the list.stream().collect(...) expression. The method isn't as flexible.

List<Student> students = Arrays.asList();

Map<Long, Student> studentsById1 = students.stream()
        .collect(Collectors.toMap(Student::getId, Function.identity()));
Map<Long, Student> studentsById2 = toMapBy(students, Student::getId);

Map<Long, Person> peopleById1 = students.stream()
        .collect(Collectors.toMap(Student::getId, Function.identity()));
Map<Long, Person> peopleById2 = toMapBy(students, Student::getId);  // compile error!

在此示例中，学生是一个子类型 Person 并且有一个 getId 方法，返回 Long 。

In this example, Student is a subtype of Person and has a getId method that returns a Long.

最后一个语句失败，不兼容类型：推理变量T具有不兼容的边界... （JDK 1.8。 0_25）。有没有办法定义类型参数，以便静态方法在与它包含的表达式相同的上下文中工作？

The last statement fails with incompatible types: inference variable T has incompatible bounds ... (JDK 1.8.0_25). Is there a way to define the type parameters so that the static method will work in the same contexts as the expression it contains?

推荐答案

您可以为地图的值添加类型参数，以便它们可以与T不同：

You could add a type parameter for the values of the map so they can be different from T:

public static <K, V, T extends V> Map<K, V> toMapBy(List<T> list,
        Function<? super T, ? extends K> mapper) {
    return list.stream().collect(Collectors.toMap(mapper, Function.identity()));
}

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