Java Lambda Expressions

Lambda Expressions in Java 8

1. What is a Lambda Expression and why was it introduced in Java 8?

A Lambda Expression is a concise way to implement a Functional Interface (an interface with a single abstract method).

Why introduced:

• Reduces boilerplate code (especially Anonymous Inner Classes)
• Enables functional programming
• Supports the Streams API

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2. How is Lambda Expression related to Functional Interface?

A Lambda Expression can be used only where a Functional Interface is expected:

Lambda = Implementation of the Functional Interface’s single abstract method

Runnable r = () -> System.out.println("Running");

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Syntax & Usage

3. Lambda for a Functional Interface with one parameter

// Functional Interface
interface Greeting { void say(String name); }

// Lambda Implementation
Greeting g = name -> System.out.println("Hello " + name);

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4. Lambda for a Functional Interface with no parameters

interface Task { void run(); }

Task t = () -> System.out.println("Task Executed");

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5. When can parentheses, braces, and return keyword be omitted in lambda?

Element	Can be omitted when	Example
Parentheses	Only one parameter, type is inferred	name -> ...
Braces {}	Only one statement in lambda body	x -> x * x
return	Only one expression is returned	x -> x * x

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Variable & Scope

6. What is an effectively final variable in lambda?

A local variable not modified after initialization is considered effectively final. Lambdas can only use effectively final variables from the enclosing method.

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7. Why can’t lambda modify local variables from outside scope?

Local variables in methods are stored on the stack, while lambdas may execute later in heap. To avoid unpredictable behavior, Java allows only reading, not modifying, of externally captured variables.

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Comparison & Reasoning

8. Difference between Lambda Expression and Anonymous Inner Class

Feature	Lambda Expression	Anonymous Inner Class
Target	Only Functional Interfaces	Any Interface/Abstract Class
Syntax	Short & clean	Verbose
Runtime Form	Uses invokedynamic, lightweight	Creates extra class file, heavier
Readability	High	Lower

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9. How does lambda improve code readability?

Lambdas remove boilerplate code such as:

new Interface() { public void method() { ... }}

And replace it with:

() -> ...

This makes code shorter, cleaner, and intention-focused.

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10. Benefit of using lambda with Streams API

Lambdas allow Streams to express what to do, not how to do it, enabling functional, less error-prone processing.

list.stream().filter(x -> x > 10).forEach(System.out::println);

• Cleaner code
• No manual loops
• Easy parallelization

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Bonus: Method References (::)

11. What are Method References and when do we use them?

Method Reference is a shorthand for calling an existing method instead of writing a lambda.

list.forEach(System.out::println);   // instead of: x -> System.out.println(x)

Use when:

• The lambda only calls a method
• Makes code more readable

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