Module 2: Functional Interfaces & Lambda Expressions

Phase

Phase 1 – Foundation

Target Audience

• Java developers moving from Java 7 to Java 8
• Developers who want to write clean, modern Java code
• Beginners learning Java 8 fundamentals

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1. Objective of This Module

By the end of this module, you will:

• Understand what Functional Interfaces are
• Learn why Lambdas exist
• Write Lambda Expressions confidently
• Use built-in functional interfaces in real scenarios

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2. What Is a Functional Interface?

A Functional Interface is an interface that:

• Has exactly one abstract method
• Can have default and static methods
• Acts as the foundation for Lambda expressions

Example

@FunctionalInterface
public interface Calculator {
    int add(int a, int b);
}

This interface can be implemented using a Lambda.

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3. @FunctionalInterface Annotation

• Optional but recommended
• Helps the compiler enforce the rule of one abstract method
• Improves readability and intent

@FunctionalInterface
interface Printer {
    void print(String message);
}

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4. Why Lambda Expressions Were Introduced

Before Java 8, behavior was passed using:

• Anonymous inner classes
• Verbose syntax

Java 7 Style

Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("Running...");
    }
};

Java 8 Style

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

✔ Less code
✔ More readable
✔ Clear intent

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5. Lambda Expression Syntax

Basic Syntax

(parameters) -> expression

Examples

() -> System.out.println("Hello");

name -> name.toUpperCase();

(a, b) -> a + b;

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6. Built-in Functional Interfaces

Java 8 provides commonly used functional interfaces in java.util.function.

Predicate<T>

• Takes input
• Returns boolean

Predicate<Integer> isEven = n -> n % 2 == 0;

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Function<T, R>

• Takes input
• Returns output

Function<String, Integer> length = s -> s.length();

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Consumer<T>

• Takes input
• Returns nothing

Consumer<String> printer = s -> System.out.println(s);

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Supplier<T>

• Takes no input
• Returns output

Supplier<Double> random = () -> Math.random();

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7. Lambda vs Anonymous Class

Feature	Anonymous Class	Lambda
Boilerplate	High	Low
Readability	Low	High
this reference	Refers to class	Refers to enclosing scope
Reusability	Poor	Better

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8. Effectively Final Variables

• Lambdas can access local variables
• Variables must be effectively final

int x = 10;
Runnable r = () -> System.out.println(x);

❌ This will fail:

x++;

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9. Real-World Use Cases

• Sorting collections
• Validation logic
• Callbacks
• Event handling
• Passing behavior to methods

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10. Hands-On Exercises

Exercise 1

Create a functional interface for:

• User validation

Exercise 2

Use Predicate to:

• Filter even numbers from a list

Exercise 3

Use Consumer for:

• Logging messages

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11. Common Mistakes

• Creating unnecessary functional interfaces
• Overusing lambdas for complex logic
• Ignoring readability

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12. Summary

• Functional Interfaces enable Lambdas
• Lambdas reduce boilerplate
• Built-in interfaces cover most use cases

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13. What’s Next?

➡️ Module 3: Method References & Code Readability

Keep going 🚀