Java 8:Lambda函数和通用通配符 [英] Java 8 : Lambda Function and Generic Wildcards

问题描述

我有以下课程

class Book implement Borrowable  {
    @Override
    public String toString(Function<? extends Borrowable
                            , String> format) {
          return format.apply(this);    
    }
}

这给了我一个错误，我不能在此(Book对象)上使用应用".

This gives me an error that i cannot use "apply" on this(Book object).

我当前的格式化程序是

My current formatter is

 Function<Book, String> REGULAR_FORMAT = book -> "name='" + book.name + '\'' +
        ", author='" + book.author + '\'' +
        ", year=" + book.year;

我不想创建类型为lambda的函数

I don't want to make the lambda function of the type

Function<Borrowable, String>

因为我将无法访问未借阅的Book成员.

as I would lose access to the members of Book not exposed by Borrowable.

推荐答案

Function<? extends Borrowable, String>类型表示能够接受扩展Borrowable的 some 类型的函数.这并不意味着它接受Book.可能最好的解决方案是为Borrowable引入通用参数:

The Function<? extends Borrowable, String> type means function that able to accept some type which extends Borrowable. It does not mean that it accepts Book. Probably the best solution is to introduce the generic parameter for Borrowable:

public interface Borrowable<T> {
    public String toString(Function<? super T, String> format);
}

并在Book中指定它:

public class Book implements Borrowable<Book> {
    @Override
    public String toString(Function<? super Book, String> format) {
        return format.apply(this);
    }
}

这与Comparable界面的工作方式类似.

It's similar to how the Comparable interface works.

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