Lambda Expression in Java was introduced with Java 8, which promises to revolutionize the way you write Java Programs by increasing expressiveness, functionality, and concision. But do you think that these lambda expressions in Java are really as useful as they seem? Are they worth replacing traditional approaches like anonymous classes? Most importantly, do they speed up programs, or are they just overhyped?
In the tutorial, we will take a closer look at Lambda Expressions in Java and learn about their syntax, use cases, performance, and role in Java functional programming. When you have read this article, not only will you know how to use Lambda Expressions in Java, but you will also know why to use them effectively in your Java Program.
Table of Contents:
What Are Lambda Expressions in Java?
Lambda Expression in Java is an anonymous function (a function without a name) through which you can create short, inline implementations of single-method interfaces (functional interfaces). Lambdas were introduced in Java 8 in order to enable function programming where code is data.
Lambdas are distinct from traditional methods in the following ways:
- They don’t have any name or qualifiers (i.e., public, static).
- It can be passed to methods or assigned to a variable.
- It streamlines the use of interfaces like Runnable, Comparator, and event handlers
Example:
Why Were Lambda Expressions Introduced in Java 8?
Developers had utilized anonymous inner classes to create single-method interfaces in Java before Java 8 with resulting in verbose and cluttered code. Lambda expressions addressed core issues:
- Reduction in Verbosity: Java Lambda Expression eliminated the need to write unnecessary boilerplate code.
- Support for functional programming: It allowed Java to adopt modern programming paradigms (such as map/filter/reduction operations via the Stream API).
- API Flexibility: It also allows libraries to build clean, behavior-based APIs (i.e., stream().filter(…))
- Parallelism: It supports easy parallel programming using streams and CompletableFuture
- The aim of Java 8: Java 8 brings Java on par with languages such as Scala and JavaScript that already had function constructs.
Advantages of Lambda Expressions in Java
- Promotes Conciseness: Lambdas reduce code clutter by shortening multiple-line anonymous classes into a single line.
- Readability: The lambdas clarify code intent by emphasizing what is done with the code instead of how it’s done.
- Functional programming support as Lambda permits:
- Higher-order functions: Take functions as parameters (e.g., stream().map(x -> x*2)).
- Immutable data: Process streams without altering source collections.
- Declarative code: Employ chain operations like filter, map, and reduce to manipulate data.
- Enhanced APIs: Java’s predefined functional interfaces (Predicate, Function) and lambda-based Stream API are leveraged for expressive data processing.
Structure and Syntax of Lambda Expression in Java
Lambda expression in Java is a significant feature in Java’s support for functional programming with the goal of making code concise without compromising type safety and readability. Here’s a closer look at the syntax of lambda expressions, lambda rules, and concepts.
1. Basic Syntax and Components
There are three main parts of a lambda expression.
Syntax Variations:
// 1. No parameters
() -> System.out.println("Hello");
// 2. Single parameter (parentheses optional)
s -> s.toUpperCase();
(s) -> s.toUpperCase();
// 3. Multiple parameters
(a, b) -> a + b;
(int x, int y) -> x * y;
// 4. Block body
(name) -> {
String greeting = "Hello, " + name;
System.out.println(greeting);
};
2. Types of Lambda Bodies
The return type and behavior of the Lambda Expression in Java are specified by the lambda body.
a. Expression Body
- Single-line code with implicit return value
- No return statement or braces({}) are needed.
- Suitable for simple operations.
Example:
// Returns the square of a number
Function<Integer, Integer> square = x -> x * x;
// Returns true if a string is empty
Predicate<String> isEmpty = s -> s.trim().isEmpty();
b. Block Body
- Multi-line code contained in braces.
- Needs an explicit return statement if the lambda returns a value.
- Used for complicated logic (i.e., conditionals and loops).
Example:
// Checks if a number is even and positive
Predicate<Integer> isEvenAndPositive = n -> {
if (n <= 0) return false;
return n % 2 == 0;
};
3. Parameter List Rules
a. Empty Parameters
Use () if no arguments are supplied to lambda.
Runnable task = () -> System.out.println("Task executed!");
b. Single Parameter
Parentheses are simply optional in type inference:
// Valid
Consumer<String> print = s -> System.out.println(s);
Consumer<String> print = (s) -> System.out.println(s);
// Invalid (type declared without parentheses)
String s -> s.toUpperCase(); // Compiler error
c. Multiple Parameters
Parentheses are still necessary even when there are implicit types:
// Valid
BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
// Invalid
a, b -> a + b; // Missing parentheses
d. Types: Explicit and Inferred
- Explicit Types: Utilize explicit type parameters where ambiguity is possible.
- Inferred Types: Compiler infers the type according to the function interface.
Example:
// Explicit type declaration (redundant here, but valid)
Comparator<String> comparator = (String a, String b) -> a.compareTo(b);
// Inferred types (compiler knows a and b are Strings)
Comparator<String> comparator = (a, b) -> a.compareTo(b);
4. Valid vs Invalid Lambdas
Functional Interfaces in Java
Functional interfaces in Java form the basis of Java’s lambda expressions. They simply enable Java to support functional programming paradigms by using a type-safe representation of lambda expressions.
A functional interface is an interface with a single abstract method. It is used as the “type” for lambda expressions in order to declare the method signature to be implemented by lambda.
Basic Principles:
- It should have just one abstract method (other than default and static methods).
- You can annotate it with @FunctionalInterface for checking during compilation.
Examples: Runnable, Comparator, Predicate.
@FunctionalInterface
interface Greeting {
void greet(String message); // Single abstract method
}
Built-in Functional Interfaces in Java
The java.util.function package in Java provides standard function interfaces for common use:
Example:
Predicate<Integer> isEven = n -> n % 2 == 0;
Function<String, Integer> lengthMapper = s -> s.length();
Consumer<String> printer = s -> System.out.println(s);
How Lambdas Work with Functional Interfaces
The lambda expression provides an in-place implementation of the abstract method of the functional interface.
Step-by-Step:
1. Define a Functional Interface:
@FunctionalInterface
interface Calculator {
int compute(int a, int b);
}
2. Assign a Lambda to It:
Calculator add = (a, b) -> a + b;
Calculator multiply = (a, b) -> a * b;
3. Use the Lambda:
System.out.println(add.compute(2, 3)); // Output: 5
System.out.println(multiply.compute(2, 3)); // Output: 6
The @FunctionalInterface Annotation
- Purpose: Ensures the interface has a single abstract method.
- Compile-Time Check: Generates an error if the rule is violated.
- Recommended but not mandatory: Improves code readability and safety.
Example:
@FunctionalInterface
interface StringFormatter {
String format(String s);
// Default methods are allowed!
default void log(String s) {
System.out.println("Logging: " + s);
}
}
Lambda Expressions vs Anonymous Classes
Before Java 8 and Lambda Expressions, anonymous classes in Java were used to define interfaces with a single method. Anonymous classes provided a mechanism to create in-place implementations, but in a clumsy and verbose form. Lambda expressions aim to simplify it by providing a readable and concise form that is also functional.
Let us consider the following major differences between Lambda Expressions and Anonymous Classes in Java:
1. Code Readability and Conciseness
a. Anonymous Class Example
Before Java 8, we would typically use anonymous classes to declare functional interfaces like Runnable:
Output:
b. Lambda Expression Example
In Java 8, we can accomplish the same thing with a Lambda Expression and reduce boilerplate code:
Output:
Java Lambda expression reduces redundant syntax and make the code cleaner and more readable.
One question typically arises regarding performance that “Are Lambda Expressions in Java faster than Anonymous Classes?”
- Java Anonymous Classes create an inner class during runtime with a slight memory and execution overhead.
- Lambda expressions in Java typically use the invokedynamic bytecode instruction and hence are generally more space-efficient.
But in real life, the difference is largely academic, and in practice, you should make the decision on the basis of code clarity and not on pure performance.
3. Key Differences between Anonymous Classes and Lambda Expressions in Java
When to Use Anonymous Classes in Java?
- Whenever there are multiple methods in the implementation
- When using local variables that are not effectively final (Lambda Expressions in Java require local variables to be final or effectively final).
- When anonymous classes create a new class file and hence are simpler to debug with the aid of debug tools.
When to Use Lambda Expressions in Java?
- When using function interfaces (interfaces with a single abstract method).
- When writing concise and to-the-point implementations
- When you wish to employ the Java Stream API for functional programming.
Using Lambda Expressions with Java Collections
Java Collections are the most frequently used functionalities in Java, and Lambda Expressions made it easy to use them in a concise and readable format and made it efficient. Verbose for-loops and anonymous classes are no longer necessary in order to iterate, filter, sort, and map.
Let’s talk about the use of Lambda Expressions with Java Collections.
1. Iterating Over Collections Using forEach()
Before Java 8, iteration on a collection would typically be done with an explicit loop:
Traditional Approach (Before Java 8)
Output:
Using Lambda Expression with forEach()
Output:
Primary Advantages
- It removes the need for explicit iteration.
- Enhances code readability and expressiveness.
2. Sorting Collections Using Lambda Expressions
Sorting collections using a Comparator was previously done using anonymous classes:
Traditional Approach (Before Java 8)
Output:
Sorting with Lambda Expression (Java 8+)
Sorting becomes more convenient using the Lambda Expression of Java:
Output:
Or we can also do it with a method reference:
names.sort(String::compareTo);
Principal Advantages
- Eliminates boilerplate code (no need to implement Comparator).
- Improves readability and maintainability.
3. Filtering Elements from Collections Using Streams
Java 8 introduced the Stream API in conjunction with Java Lambda Expressions for filtering data.
Filtering List Before Java 8
Output:
Filtering List Using Lambda and Streams
Output:
Key Benefits:
- More expressive and concise.
- It employs functional programming for better readability.
Method References in Java
Java Method References provide a readable and concise means of referring to methods without invoking them. Java Method References were introduced in Java 8 and simplify Lambda Expressions further by calling a method that is already implemented in an object or class. It simplifies the code without compromising on functionality and makes it more readable and maintainable.
Syntax of Method Reference
ClassName::methodName
Example: Lambda Expression vs. Method Reference
1. Using a Lambda Expression
Output:
Key Benefit: Shortens words and improves readability
Types of Method References
There are four types of method references in Java:
When to Use Method References Over Lambda Expressions?
Example: When NOT to Use Method Reference
If there are further operations to be performed inside the lambda function, a method reference will not be suitable.
list.forEach(name -> {
System.out.println("Processing: " + name);
});
Since System.out::println does not take “Processing: “, a lambda is required.
Variable Capture in Lambda Expressions
Variable Capture in Lambda Expressions is how a lambda can refer to the variables in the enclosing scope. Java has strict rules, especially for local variables, to promote uniformity.
1. Final and Effectively Final Variables
- The local variables must be final or effectively final (not modified after initialization).
- Changing a captured local variable within or outside the lambda is a compiler error.
Example (Valid Capture of an Effectively Final Variable):
Output:
2. Instance and Static Variable Capture
Unlike local variables, instance and static variables can be modified in a lambda.
Example (Modifying an Instance Variable in Lambda):
Output:
3. Variable Capture in Closures
A closure allows a lambda to have access to the variables of the outer scope even after the outer scope has ended.
Example (Lambda Capturing a Local Variable After Method Returns):
Output:
Major Points
- The local variables need to be final or effectively final.
- Instance and static fields can be modified in lambdas.
- Closures permit a lambda to use variables even after the enclosing function has returned.
It ensures thread-safety and determinism when using lambda functions in Java..
Lambda Expressions in Multithreading
Multithreading in Java allows multiple threads to be executed concurrently to achieve better efficiency in the execution of the parallel operations. Lambda expressions make it easier to use threads in a compact format for defining code in Runnable, Callable, and other function interfaces.
1. Using Lambda Expressions with Threads
Threads have long been implemented with anonymous inner classes:
Output:
- The code is too verbose and redundant.
- Let’s simplify it with Lambda Functions
Output:
Here, Java Lambda Expression minimizes boilerplate code in order to keep it clean.
2. Using Lambda Expressions with ExecutorService
Java relies on ExecutorService to control thread pools. Lambda functions simplify submitting tasks:
Output:
Lambda simplifies the process of passing tasks to thread pools without the requirement to introduce separate Runnable classes.
3. Using Lambda Expressions with Callable (Returning a Result)
Using Callable, lambda functions simplify asynchronous computations:
Output:
Lambdas simplify returning results in multithreading using Callable.
Exception Handling in Lambda Expressions
Exception Handling of Lambda expressions in Java needs to deal with checked exceptions properly since they can’t throw them unless they are explicitly thrown.
1. Handling Checked Exceptions in Lambda Expressions
A checked exception (like IOException) must be handled in the lambda:
Output:
Checked exceptions need to be caught in the lambda since they may not be thrown outside the lambda.
2. Wrapper Methods for Exception Handling
In order not to clutter lambda expressions with try-catch, a helper function may enclose exceptions:
Output:
Code readability and exception handling are made easier through wrapper methods.
3. Exception Handling in Streams and Functional Interfaces
If we use lambda expressions in Java Streams, it’s not easy to manage exceptions. It’s better to put them in a utility function:
Output:
Lambda functions applied to streams require proper handling of exceptions not to spoil the entire stream.
Advanced Topics in Java Lambda Expressions
1. Java Lambdas and Closures
A closure is a function with the ability to trap enclosing scope variables even after the scope is closed. Java lambda expressions are closures but with limitations.
How Closures in Java Lambdas Work
The local variables in the enclosing method can be captured effectively by lambda expressions and stored in a state held even after the method has returned.
Example: Lambda Capturing a Local Variable (Closure)
Output:
The lambda still maintains access to num even after createClosure() returns.
2. Serialization of Lambda Expressions
Serialization is used to represent an object in the form of a byte stream. Anonymous classes are serializable if they implement Serializable. Lambda expressions are not serializable in the default way except when they are specially annotated.
Why Lambdas are Not Serializable by Default
- Lambda expressions are implementation-dependent and can internally differ across various JVM implementations.
- In contrast to normal objects, lambda expressions in Java lack a class name and hence are less serializable.
Creating a Serializable Lambda
You must explicitly define an interface that implements Serializable:
Output:
The lambdas need to be made serializable explicitly in order to allow serialization.
3. Debugging Lambda Expressions (Stack Traces, Logging)
As lambda expressions lack a particular class name, debugging can be challenging sometimes. Some of the common debugging issues are:
1. Learning Stack Traces for Lambda Expressions in Java
If there’s an exception in a lambda, stack traces exhibit synthetic method names like:
Exception in thread "main" java.lang.ArithmeticException: / by zero
at LambdaDebugging.lambda$main$0(LambdaDebugging.java:8)
at java.base/java.util.Arrays.forEach(Arrays.java:1510)
- lambda$main$0 is an anonymous lambda method inside the main method.
Solution: Add descriptive logs to track execution in lambdas.
Output:
Visibility during debugging is enhanced through logging in lambdas.
2. Naming Conventions to Enhance Debugging
Rather than inlining complex logic in lambdas, use method references:
Output:
Named methods enhance stack traces.
4. Recursive Lambdas
Recursion is calling a function that calls itself. Lambda expressions cannot recurse automatically since they have no name. They can be assigned to a variable and called to call themselves.
Using a Recursive Lambda to Calculate the Factorial
Output:
A lambda may not refer to itself directly but can employ an anonymous inner class workaround.
Using Java Lambda Expressions effectively requires knowing the best practices for keeping your code readable, maintainable, and efficient. The following are the most significant best practices and efficiency tips to keep in mind:
1. Use Method References Where Possible
Method calls (ClassName::methodName) are easier to read than inlined lambda expressions.
a. Using a lambda unnecessarily:
list.forEach(str -> System.out.println(str));
b. Use a method reference instead:
list.forEach(System.out::println);
Why? Since method references are concise and also promote efficiency by avoiding unnecessary creation of a lambda expression in Java.
2. Keep Lambda Expressions Short and Readable
A Lambda Expression in Java has to be concise and designed to do a particular task.
a. Nested logic in a lambda:
list.forEach(str -> {
if (str.startsWith("A")) {
System.out.println(str.toUpperCase());
} else {
System.out.println(str.toLowerCase());
}
});
b. Split logic into another approach for better reading
list.forEach(LambdaBestPractices::processString);
private static void processString(String str) {
System.out.println(str.startsWith("A") ? str.toUpperCase() : str.toLowerCase());
}
Why? Because it is more readable and easier to debug with a lambda
3. Refrain from State Mutation within Lambdas
The lambdas should be stateless in order to avoid concurrency issues.
a. Modifying external state in a lambda (not thread-safe):
b. Use stream operations instead:
List<String> names = Arrays.asList("Ayaan", "Alam", "Abhinav");
List<String> results = new ArrayList<>();
names.forEach(name -> results.add(name.toUpperCase())); // Not thread-safe
List<String> results = names.stream()
.map(String::toUpperCase)
.collect(Collectors.toList());
Why? Since the operations on streams do not modify external state and hence make the code both efficient and thread-safe.
4. Use Functional Interfaces Instead of Anonymous Classes
Functional interfaces enable you to write concise code compared to anonymous classes.
a. Using an anonymous class:
Comparator<Integer> comparator = new Comparator<>() {
@Override
public int compare(Integer a, Integer b) {
return Integer.compare(a, b);
}
};
b. Using a Java lambda expression instead:
Comparator<Integer> comparator = Integer::compare;
Why? Lambdas make the code less verbose and more readable.
- Lambdas minimize the amount of boilerplate code but incur some overhead in the form of object creation and boxing/unboxing.
- Use primitive-specialized function interfaces (IntConsumer, LongPredicate, etc.) to avoid unnecessary autoboxing:
IntConsumer printDouble = num -> System.out.println(num * 2);
printDouble.accept(5); // Output: 10
Why? It prevents boxing of int to Integer in the interest of memory conservation.
Common Problems and How to Avoid Them
Despite their benefits, lambda expressions have some pitfalls. Here’s how to avoid them:
1. Forgetting That Lambda Parameters Must Be Effectively Final
Lambda expressions can refer to final or effectively final variables in the surrounding scope.
Changing a local variable in a lambda (compile-time error):
int counter = 0;
Runnable r = () -> counter++; // ERROR: Variable used in lambda should be final or effectively final
Solution: Use an array or an AtomicInteger for a mutable state
AtomicInteger counter = new AtomicInteger(0);
Runnable r = () -> counter.incrementAndGet();
2. Using Lambdas When Method References Are Less Readable
If the lambda is calling an existing method, use method references.
a. Lambda calling an existing method:
list.forEach(name -> System.out.println(name));
b. Replace with a method reference:
list.forEach(System.out::println);
Why? Because the method references are cleaner and slightly quicker.
3. Catching Unexpected Catches of Outer Scope Variables
Lambda functions take variables in the local scope and have the potential to create memory leaks if not handled properly.
Capturing this can suppress garbage collection by retaining it in an instance method
class LambdaCaptureExample {
Runnable r = () -> System.out.println(this);
}
Solution: Refrain from unnecessary captures or use static methods.
4. Inadequate Exception Handling in Lambdas
Lambda expressions in Java should not throw checked exceptions except when they are explicitly handled
Lambda throwing a checked exception without handling:
Consumer<String> fileReader = fileName -> Files.readAllLines(Path.of(fileName)); // ERROR
Solution: Catch the exceptions in the lambda.
Consumer<String> fileReader = fileName -> {
try {
Files.readAllLines(Path.of(fileName));
} catch (IOException e) {
e.printStackTrace();
}
};
5. Lambdas for Complex Logic
Lambdas are not intended to be replacements for normal procedures for complicated operations..
Overcomplicated lambda:
list.sort((a, b) -> a.length() == b.length() ? a.compareTo(b) : Integer.compare(a.length(), b.length()));
Extract to a method for clarity:
list.sort(LambdaBestPractices::compareStrings);
private static int compareStrings(String a, String b) {
return a.length() == b.length() ? a.compareTo(b) : Integer.compare(a.length(), b.length());
}
Conclusion
With this, we have completed the Java Lambda Expression Tutorial in which we have learned what lambda expressions are in Java and how these Lambda Expressions make functional programming a lot easier by ensuring code is concise, readable, and performant. They make collections, streams, multithreading, and event handling better by minimizing boilerplate code. We also discussed its syntax, method references, variable capture, and exception handling, and advanced topics such as best practices and pitfalls to achieve maximum performance.
After gaining a solid knowledge about Lambdas, you can now write clean, maintainable, and high-performance Java applications. Use them wisely to ensure your code is more efficient and productive!
Lambda Expression in Java – FAQs
What is a Lambda Expression in Java?
Lambda Expression is a short form for defining the functional interfaces without the necessity to define them in explicit classes.
Why are Lambda Expressions used in Java?
Lambda Expression in Java generally minimizes boilerplate code, makes code easier to read, and allows easier use of the constructs of functional programming, specifically for streams, collections, and event handling.
Can Lambda Expressions replace Anonymous Classes completely?
No, lambda expressions may be utilized in place of just single-method interfaces, but anonymous classes may be implemented with multiple methods or may extend classes.
Are Lambdas Objects or Functions?
Lambdas are objects themselves but are used as functions. All the lambda expressions in Java are an implementation of a functional interface.
How do Lambdas Improve API Design?
- Streamlines Callbacks: Eliminates boilerplate anonymous classes
- Enhances readability: Reduces boilerplate code
- Encapsulating behavior: Enabling functional programming paradigms