Lambda Expressions

Syntax

(parameters) -> expression

Examples

interface Greeter {
    String greet(String name);
}

// Old way: anonymous class
Greeter oldStyle = new Greeter() {
    @Override
    public String greet(String name) {
        return "Hello, " + name;
    }
};

// New way: lambda expression
Greeter newStyle = name -> "Hello, " + name;

System.out.println(newStyle.greet("Fola"));  // Hello, Fola

import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Consumer;

Function<Integer, Integer> square = x -> x * x;
System.out.println(square.apply(5));  // 25

Predicate<Integer> isEven = n -> n % 2 == 0;
System.out.println(isEven.test(4));    // true

Consumer<String> printer = s -> System.out.println("Value: " + s);
printer.accept("Hello");                // Value: Hello

import java.util.function.Function;

Function<Integer, String> classify = n -> {
    if (n < 0) {
        return "negative";
    } else if (n == 0) {
        return "zero";
    } else {
        return "positive";
    }
};

System.out.println(classify.apply(-5));  // negative
System.out.println(classify.apply(0));    // zero

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<>(java.util.List.of("Charlie", "Alice", "Bob"));

names.sort((a, b) -> a.compareTo(b));
System.out.println(names);  // [Alice, Bob, Charlie]

names.sort((a, b) -> b.length() - a.length());  // longest name first
System.out.println(names);

Best practices

• Use lambdas for short, simple implementations of a functional interface - switch to a full method or class once the logic gets long or complex
• Prefer built-in functional interfaces from java.util.function (Function, Predicate, Consumer, Supplier) over defining your own where possible
• Keep lambda parameter names short but meaningful, especially in stream chains where context is often clear from position
• Remember a lambda can only implement an interface with exactly one abstract method (a functional interface) - not any arbitrary interface

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation