Overview

Java, traditionally known for its object-oriented programming capabilities, has embraced functional programming paradigms since the introduction of Java 8. This shift allows developers to write more concise, readable, and maintainable code, particularly for operations that involve collections. Functional interfaces and lambda expressions are central to Java's functional programming support, enabling developers to treat functionality as a method argument, or code as data.

Key Concepts

1. Functional Interfaces: An interface with exactly one abstract method. These serve as the type for lambda expressions and method references.
2. Lambda Expressions: A concise way to represent an anonymous function that can be passed to methods as parameters. They are used primarily to define inline implementation of a functional interface.
3. Streams API: A key feature introduced in Java 8 that supports functional-style operations on streams of elements, such as map-reduce transformations.

Common Interview Questions

Basic Level

1. What is a functional interface in Java?
2. Can you write a simple lambda expression in Java?

Intermediate Level

3. How does the java.util.function package support functional programming?

Advanced Level

4. How can lambda expressions and functional interfaces improve the performance of a Java application?

Detailed Answers

1. What is a functional interface in Java?

Answer: A functional interface in Java is an interface that has exactly one abstract method, but it can have multiple default or static methods. Functional interfaces provide the target types for lambda expressions and method references. The @FunctionalInterface annotation, while not mandatory, is used to indicate that an interface is intended to be a functional interface. This annotation is useful for compile-time checking to ensure the interface meets the requirements of a functional interface.

Key Points:
- Must have exactly one abstract method.
- Can have multiple default or static methods.
- @FunctionalInterface annotation is optional but recommended for clarity.

Example:

@FunctionalInterface
interface Greeting {
    void sayHello(String name);
}

// Usage with a lambda expression
Greeting greeting = (name) -> System.out.println("Hello, " + name);
greeting.sayHello("Java");

2. Can you write a simple lambda expression in Java?

Answer: Lambda expressions in Java provide a clear and concise way to implement a single method of a functional interface, without the need for an anonymous class. A lambda expression consists of a set of parameters, an arrow token (->), and a body.

Key Points:
- Syntax: (parameters) -> expression or (parameters) -> { statements; }
- Can access final variables from the enclosing scope.
- Enhances readability and reduces boilerplate code.

Example:

// A simple functional interface
interface MathOperation {
    int operation(int a, int b);
}

// Usage with lambda expressions
MathOperation addition = (a, b) -> a + b;
MathOperation subtraction = (a, b) -> a - b;

System.out.println("10 + 5 = " + addition.operation(10, 5));
System.out.println("10 - 5 = " + subtraction.operation(10, 5));

3. How does the java.util.function package support functional programming?

Answer: The java.util.function package provides a rich set of functional interfaces to facilitate functional programming in Java. These interfaces represent common function shapes, including predicates, functions, suppliers, and consumers, and are extensively used in the Stream API to support complex data processing queries.

Key Points:
- Predicates (Predicate<T>) for boolean-valued logic.
- Functions (Function<T,R>) for transformations.
- Suppliers (Supplier<T>) for providing instances.
- Consumers (Consumer<T>) for operating on a single input.

Example:

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;

List<String> names = Arrays.asList("Java", "Kotlin", "Scala");
Predicate<String> startsWithJ = name -> name.startsWith("J");

names.stream()
    .filter(startsWithJ)
    .forEach(System.out::println); // Prints names starting with "J"

4. How can lambda expressions and functional interfaces improve the performance of a Java application?

Answer: Lambda expressions and functional interfaces can lead to more efficient Java applications, primarily through improved readability, reduced boilerplate code, and better utilization of multicore processors. With the Streams API, lambda expressions enable parallel execution of code with minimal effort, allowing for significant performance improvements for operations on large data sets.

Key Points:
- Enhanced readability and maintainability.
- Reduced boilerplate code.
- Facilitates parallel execution of collection operations using streams, potentially improving performance on multicore systems.

Example:

import java.util.Arrays;
import java.util.List;

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
int sum = numbers.parallelStream().mapToInt(Integer::intValue).sum();

System.out.println("Sum: " + sum); // Efficient parallel summing

Each of these examples and explanations provides insights into how Java supports functional programming, making it easier for developers to write efficient, concise, and readable code.