Answer.

In Java 8, the concept of default methods was introduced — these are methods with implementations declared inside an interface. They allow adding new methods to interfaces without breaking compatibility with existing implementations.

Syntax:

public interface MyInterface {  
    default void printHello() {  
        System.out.println("Hello!");
    }
}

Features and nuances:

• If a class implements two interfaces with the same default method signature, a conflict arises, and the class must override that method.
• Default methods cannot override methods of the Object class (such as equals, hashCode).
• Interfaces can also declare static methods.

Trick question.

What happens if a class implements two interfaces with the same default method and does not override that method?

Answer: The compiler will throw an error "class... inherits unrelated defaults for ... from types ... and ...", and it will require explicitly implementing that method in the class.

Example:

interface A {  
    default void doSomething() { System.out.println("A"); }
}
interface B {  
    default void doSomething() { System.out.println("B"); }
}
class C implements A, B {} // Compilation error!

Solution:

class C implements A, B {
    @Override
    public void doSomething() {
        A.super.doSomething(); // or B.super.doSomething()
    }
}

Examples of real errors due to ignorance of the nuances of the topic.

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Story

In a team project for API migration, a default method was added to a common interface. The old implementations of the interface did not override the new method, leading to unexpected behavior, as the default implementation was executed instead of the expected logic. As a result, users noticed a drop in functionality.

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Story

When extending a library, one of the developers added a default method with business logic to a common interface. The subsequent addition of a similar method with different behavior in another interface caused an inheritance conflict, preventing the new implementations from compiling.

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Story

A developer attempted to use a default method named hashCode in an interface, expecting it to affect the overriding of the method in subclass implementations, but the compiler did not allow this. This led to a lengthy investigation into the cause of the error and a redesign of the interface structure.