如何在Java 8中创建一个lambda表达式来定义toString？ [英] How to make a lambda expression define toString in Java 8?

问题描述

我不希望有一个正常的lambda实现一个方法，并将它的toString重新定义为一个附加值。我希望lambda表达式只实现toString方法。
我知道我的表达不是很好，但我相信你会理解这个例子。

I don't want to have a normal lambda implementing a method and redefine it's toString as a added value. I want the lambda expression implement only the toString method. I know I am not expressing it very well but I am sure you will understand me with this example.

public class LambdaToStringTest {

    public interface ToStringInterface {
        public abstract String toString();
    }

    public static void main(String[] args) {
        print("TRACE: %s", (ToStringInterface)()->someComputation()); // <<-- ERROR: The target type of this expression must be a functional interface
    }

    private static void print(String format, Object... args) {
        System.out.println(String.format(format, args));
    }

}

如果我更改名称，它会编译该方法然后它不会覆盖toString，因此print方法将不会打印预期的内容。

It compiles if I change the name of the method but then it does not override toString so print method will not print what is expected.

这是尝试定义仅在需要时评估lambdas的日志子系统（当它真的要打印时）但与非lambda参数兼容。我知道其他方法来实现它，但我想知道为什么我不能这样做，如果有解决方法或我做错了什么，

This is an attempt to define a log subsystem that evaluates lambdas only when needed (when it is really going to be printed) but being compatible with non-lambda arguments. I know other ways to achieve it but I wonder why I can't do it this way and if there is a workaround or I am doing something wrong,

推荐答案

简短回答，你不能。 @FunctionalInterface s不能用于覆盖来自 Object 的方法。

Short answer, you can't. @FunctionalInterfaces cannot be used to "override" methods from Object.

但是，您可以使用虚拟扩展方法实现 Formattable 。注意：下面的代码是UNTESTED：

You can implement Formattable however, with a virtual extension method. Note: code below is UNTESTED:

@FunctionalInterface
public interface ToStringInterface
    extends Formattable
{
    String asString();

    @Override
    default void formatTo(Formatter formatter, int flags, int width, int precision)
    {
        formatter.format("%s", this);
        // Or maybe even better:
        formatter.out().append(this.asString());
    }
}

我建议使用此解决方案，因为您使用 String.format（）使用此接口。

I propose this solution since you are using String.format() which makes use of this interface.

或者您可以定义自己的界面。或者甚至为这个接口编写一个包装器，它在 .asString（）中调用 .toString（）。选择很多。

Or you can just define your own interface. Or even write a wrapper for this interface which calls .toString() in .asString(). Choices are many.

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