How to make a lambda expression define toString in Java 8?

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Short answer, you can’t. @FunctionalInterfaces cannot be used to “override” methods from Object.

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());
    }
}

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

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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