Lambda Expressions

Advanced ~20 min read

Lambda expressions were added in Java 8. They provide a clear and concise way to represent one method interface using an expression. They enable Functional Programming in Java.

Syntax

The simplest lambda expression contains a single parameter and an expression:

parameter -> expression

For more parameters or complex logic:

(param1, param2) -> { 
    // code block 
    return something; 
}

Functional Interface

A Lambda expression can be used wherever the type is a Functional Interface. A functional interface is an interface with only one abstract method.

Common examples: Runnable, Comparator, ActionListener.

Before Java 8 (Anonymous Class)

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

After Java 8 (Lambda)

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

Full Example

import java.util.ArrayList;

interface MathOperation {
    int operation(int a, int b);
}

public class LambdaExample {
    public static void main(String[] args) {

// 1. Implementing an interface using Lambda
        // Defines addition
        MathOperation addition = (int a, int b) -> a + b;

// Defines subtraction
        MathOperation subtraction = (a, b) -> a - b;

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

// 2. Using Lambda with Collections (ForEach)
        System.out.println("\n--- List Iteration ---");
        ArrayList<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

// Old way: for(String n : names) ...
        // Lambda way:
        names.forEach((n) -> System.out.println(n));
    }

// Helper method
    private static int operate(int a, int b, MathOperation mathOp) {
        return mathOp.operation(a, b);
    }
}

Output

Click Run to execute your code

Summary

Lambdas make code more concise and readable.
They remove the boilerplate of Anonymous Inner Classes.
Syntax: (parameters) -> { body }.

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