What is the reason for having only one abstract method in a functional interface?

A functional interface in Java contains exactly one abstract method to ensure a clear representation of a functional type. This allows the use of lambda expressions and method references, which can be automatically converted into instances of such interfaces. Having a single abstract method guarantees that the compiler and runtime can correctly associate the lambda expression with the interface without ambiguity.

Example of a functional interface:

@FunctionalInterface
public interface Converter<F, T> {
    T convert(F from);
}

Here, the convert method is the only abstract method, enabling the use of lambda expressions for its implementation.