Overview

Lambda expressions were introduced in Java 8 as a significant feature to bring functional programming aspects into Java. They provide a clear and concise way to represent one method interface using an expression. Lambda expressions are especially useful in collections library, where they can be used to iterate, filter, and extract data from collections succinctly.

Key Concepts

1. Functional Interfaces: A key concept for lambda expressions, a functional interface is any interface with a single abstract method (SAM), allowing lambda expressions to provide the implementation of the abstract method.
2. Syntax of Lambda Expressions: Understanding how lambda expressions are written and their syntax is crucial. The basic format is (argument) -> { body }.
3. Stream API: Introduced alongside lambda expressions, the Stream API supports operations on collections, such as map-reduce transformations, which are often used with lambda expressions for more efficient and readable code.

Common Interview Questions

Basic Level

1. What is a lambda expression in Java 8?
2. How do you use a lambda expression to create a simple thread?

Intermediate Level

3. How do lambda expressions relate to functional interfaces in Java 8?

Advanced Level

4. Can you explain how lambda expressions interact with the scope of variables in Java?

Detailed Answers

1. What is a lambda expression in Java 8?

Answer: A lambda expression is a short block of code which takes in parameters and returns a value. Lambda expressions are similar to methods, but they do not need a name and can be implemented right in the body of a method.

Key Points:
- Lambda expressions help to treat functionality as a method argument, or code as data.
- They allow us to express instances of single-method interfaces (functional interfaces) more compactly.
- They are used extensively in working with collections and the Stream API.

Example:

// Example of using a lambda expression with the Runnable interface
Runnable r = () -> System.out.println("A simple lambda expression example");
Thread t = new Thread(r);
t.start();

2. How do you use a lambda expression to create a simple thread?

Answer: Lambda expressions can simplify the syntax when creating instances of single-method interfaces, such as Runnable, used with threads.

Key Points:
- Reduces boilerplate code compared to anonymous class.
- Enhances readability and maintainability.
- Makes it easier to use functional programming concepts in Java.

Example:

// Using lambda expression to create a thread
Thread thread = new Thread(() -> {
    for(int i = 0; i < 5; i++) {
        System.out.println("Child Thread: " + i);
    }
});
thread.start();

3. How do lambda expressions relate to functional interfaces in Java 8?

Answer: Lambda expressions are essentially a way to provide the inline implementation of the abstract method defined in a functional interface. Java 8 allows lambda expressions to be used as an instance of any interface with a single abstract method, thereby making it a functional interface.

Key Points:
- A functional interface in Java is an interface that contains only one abstract method.
- Lambda expressions can be used to represent an instance of a functional interface.
- Java 8 includes many built-in functional interfaces in the java.util.function package.

Example:

@FunctionalInterface
interface StringLength {
    int getLength(String s);
}

// Using a lambda expression to implement the StringLength functional interface
public class LambdaDemo {
    public static void main(String[] args) {
        StringLength len = (str) -> str.length();
        System.out.println("Length of string 'Hello, World!' is: " + len.getLength("Hello, World!"));
    }
}

4. Can you explain how lambda expressions interact with the scope of variables in Java?

Answer: Lambda expressions interact with their surrounding scope in a similar way to anonymous classes, but with a few differences. They can access final or effectively final variables from the enclosing scope. Unlike anonymous classes, lambda expressions do not have their own this reference; this refers to the enclosing object where the lambda is defined.

Key Points:
- Lambda expressions can capture final or effectively final variables from the surrounding scope.
- They do not introduce a new level of scoping (unlike inner classes).
- this in a lambda expression refers to the enclosing instance where the lambda is defined, not the lambda itself.

Example:

public class LambdaScopeExample {
    private int instanceVar = 42;

    public void testLambdaScope() {
        int localVar = 26; // Effectively final

        Runnable r = () -> {
            System.out.println("Instance var: " + instanceVar);
            System.out.println("Local var: " + localVar);
            // localVar = 27; // Uncommenting this line will cause a compilation error
        };
        r.run();
    }

    public static void main(String[] args) {
        new LambdaScopeExample().testLambdaScope();
    }
}