10. What is functional interface - Java 8 Interview

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In this series we will be covering all Java interview questions and answers. This is going to be comprehensive list of Java interview questions which can help experienced Java developers to prepare for an interview as well as upskilling the Java knowledge.

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Title: Functional Interfaces in Java 8: An In-Depth Interview Explanation

Introduction:
In the realm of Java programming, the concept of functional interfaces has garnered significant attention with the release of Java 8. Functional interfaces serve as the foundation for the introduction of lambda expressions, a powerful feature that enables developers to write concise and expressive code. This interview article aims to provide a comprehensive understanding of functional interfaces in Java 8, exploring their definition, characteristics, and practical applications.

Section 1: The Essence of Functional Interfaces
1.1 What is a Functional Interface?
A functional interface is an interface that contains a single abstract method, also known as the functional method. This fundamental requirement distinguishes functional interfaces from regular interfaces and enables them to be used in conjunction with lambda expressions.

1.2 The Role of Functional Interfaces in Java 8
Java 8 introduced lambda expressions as a means to simplify the writing of anonymous classes and promote functional programming paradigms. Functional interfaces provide the necessary foundation for utilizing lambda expressions effectively by enabling them to be assigned to functional interface variables or passed as method parameters.

1.3 Characteristics of Functional Interfaces
To delve deeper into functional interfaces, we explore their key characteristics, including the presence of a single abstract method, compatibility with lambda expressions, default and static methods, and their relationship with other types, such as SAM types.

Section 2: Understanding Lambda Expressions
2.1 What are Lambda Expressions?
Lambda expressions are concise, anonymous functions that can be treated as first-class citizens in Java. They provide a compact syntax for defining behavior that can be assigned to functional interface variables.

2.2 Lambda Expression Syntax
This subsection delves into the syntax of lambda expressions, demonstrating how they are composed of parameters, an arrow token, and a body, and how they can be used to implement functional interfaces.

2.3 Capturing Variables in Lambda Expressions
Lambda expressions can capture variables from their surrounding scope. We explore the concepts of effectively final variables, closures, and potential issues related to capturing variables in lambda expressions.

2.4 Type Inference in Lambda Expressions
Java 8 introduced improved type inference, which allows the compiler to infer the types of parameters in lambda expressions. We discuss the benefits and limitations of type inference and examine scenarios where explicit type declarations may be required.

Section 3: Working with Functional Interfaces and Lambda Expressions
3.1 Built-in Functional Interfaces
Java 8 introduced several built-in functional interfaces, such as Predicate, Function, Consumer, and Supplier, among others. We explore their purpose, characteristics, and practical use cases, providing code examples to illustrate their usage.

3.2 Implementing Custom Functional Interfaces
Developers can create their own functional interfaces to encapsulate specific behaviors. This section explains the process of defining custom functional interfaces and demonstrates how they can be used with lambda expressions.

3.3 Method References
Method references provide an additional syntax for creating lambda expressions by referring to existing methods. We discuss the four types of method references and illustrate their usage in different scenarios.

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