java - 如何实现泛型函数方法参数？

Question

目前，我必须根据要传递到方法中的 Function 类型定义不同的方法，例如:

addCustomParameter(orderLinePrefix + "quantity=", concatenateLong(orderLines, OrderPart::getQuantity));
addCustomParameter(orderLinePrefix + "surcharge=", concatenateDouble(orderLines, OrderPart::getSurcharge));
addCustomParameter(orderLinePrefix + "dropship=", concatenateBoolean(orderLines, OrderPart::isDropship));

方法如下所示:

@Nonnull
    public <T> String concatenateBoolean(@Nonnull final Collection<T> items, @Nonnull final Function<T, Boolean> mapper)
    {
        return items.stream()
                .map(mapper)
                .map(String::valueOf)
                .collect(joining(","));
    }

    @Nonnull
    public <T> String concatenateLong(@Nonnull final Collection<T> items, @Nonnull final Function<T, Long> mapper)
    {
        return items.stream()
                .map(mapper)
                .map(String::valueOf)
                .collect(joining(","));
    }

    @Nonnull
    public <T> String concatenateDouble(@Nonnull final Collection<T> items, @Nonnull final Function<T, Double> mapper)
    {
        return items.stream()
                .map(mapper)
                .map(String::valueOf)
                .collect(joining(","));
    }

如何将上述 3 个方法重构为一个接受任何类型并返回字符串的方法？

Answer 1

为映射器使用通配符返回类型:

@Nonnull
public <T> String concatenateAny(@Nonnull final Collection<T> items, @Nonnull final Function<? super T, ?> mapper)
{
    return items.stream()
            .map(mapper)
            .map(String::valueOf)
            .collect(joining(","));
}