Lambda Expression Example in Java

Introduction

Lambda expressions (Java 8+) provide a concise way to pass behaviour as data. They are widely used with collections, streams and event handlers.

Syntax

(parameters) -> expression
(parameters) -> { statements; }

Example Program

import java.util.*;

@FunctionalInterface
interface MathOperation {
    int operate(int a, int b);
}

public class LambdaDemo {
    public static void main(String[] args) {
        // Runnable lambda
        Runnable task = () -> System.out.println("Running in a lambda");
        task.run();

        // Comparator lambda
        List<String> names = new ArrayList<>(Arrays.asList("Charlie", "Alice", "Bob"));
        names.sort((a, b) -> a.compareToIgnoreCase(b));
        System.out.println("Sorted: " + names);

        // Custom functional interface
        MathOperation add = (x, y) -> x + y;
        MathOperation multiply = (x, y) -> x * y;
        System.out.println("Add      : " + add.operate(6, 7));
        System.out.println("Multiply : " + multiply.operate(6, 7));
    }
}

Sample Output

Running in a lambda
Sorted: [Alice, Bob, Charlie]
Add      : 13
Multiply : 42

Best Practices

• Keep lambdas short; extract complex logic into named methods.
• Prefer method references (String::compareToIgnoreCase) when they improve readability.
• Annotate custom single-method interfaces with @FunctionalInterface.

Common Mistakes

• Using lambdas where the target type is ambiguous (compiler cannot infer the functional interface).
• Modifying non-final outer variables from inside a lambda.

How It Works

Here is a step-by-step walkthrough of how the Java program for Lambda Expression Example in Java runs, line by line:

1. interface MathOperation { — performs part of the program’s logic.
2. int operate(int a, int b); — declares or assigns a value the program uses.
3. public class LambdaDemo { — performs part of the program’s logic.
4. public static void main(String[] args) { — defines a function used by the program.
5. Runnable task = () -> System.out.println("Running in a lambda"); — writes a line of output shown in the sample output.
6. task.run(); — performs part of the program’s logic.
7. List<String> names = new ArrayList<>(Arrays.asList("Charlie", "Alice", "Bob")); — declares or assigns a value the program uses.
8. names.sort((a, b) -> a.compareToIgnoreCase(b)); — performs part of the program’s logic.

After running it, compare your console output with the Sample Output above. Try changing the values and re-running the program to see how the result changes — experimenting is the fastest way to understand the logic.