Default methods in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a default method in Java interfaces?

A default method is a method defined in an interface with a body. It allows interfaces to have method implementations without breaking existing classes that implement the interface.

beginner

How do you declare a default method in a Java interface?

Use the default keyword before the method name in the interface. For example:<br>

default void sayHello() { System.out.println("Hello"); }

intermediate

Why were default methods introduced in Java 8?

They were introduced to allow adding new methods to interfaces without breaking existing implementations. This helps evolve interfaces over time.

beginner

Can a class override a default method from an interface?

Yes, a class that implements the interface can provide its own implementation of the default method, overriding the interface's version.

intermediate

What happens if two interfaces provide default methods with the same signature and a class implements both?

The class must override the conflicting default method to resolve the ambiguity, otherwise the code will not compile.

Which keyword is used to declare a default method in a Java interface?

A static

B abstract

C final

D default

The default keyword is used to declare a method with a body inside an interface.

What is the main benefit of default methods in interfaces?

A Allow interfaces to have method implementations without breaking existing classes

B Make all interface methods static

C Force all implementing classes to override methods

D Prevent interfaces from having any methods

Default methods let interfaces add new methods with implementations without breaking existing code.

If a class implements two interfaces with the same default method signature, what must it do?

A Override the method to resolve the conflict

B Use the method from the first interface only

C Use the method from the second interface only

D Nothing, it compiles fine without changes

The class must override the conflicting default method to avoid ambiguity.

Can default methods call other methods inside the interface?

A Only static methods can be called

B No, default methods cannot call any methods

C Yes, they can call other interface methods

D Only methods from Object class can be called

Default methods can call other methods declared in the interface.

Which Java version introduced default methods?

A Java 7

B Java 8

C Java 6

D Java 9

Default methods were introduced in Java 8 to enhance interfaces.

Explain what default methods are and why they are useful in Java interfaces.

Think about how interfaces evolved to add new features without breaking old code.

Describe how Java handles conflicts when a class implements two interfaces with the same default method.

Consider what happens when two parents give the same instruction.