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Lambda expressions, introduced in Java 8, allow you to write more compact, readable, and functional-style code by expressing instances of functional interfaces using a clear and concise syntax.

What Is a Lambda Expression?

A lambda expression is a short block of code that takes in parameters and returns a value. It can be used to implement the abstract method of a functional interface (an interface with a single abstract method).

Syntax of a Lambda Expression

java

(parameters) -> expression

// or

(parameters) -> {
    // multiple statements
    return result;
}

Example:

java

(int a, int b) -> a + b

Functional Interfaces

These are interfaces with only one abstract method. Examples:

• Runnable
• Callable
• Comparator<T>
• Predicate<T>
• Function<T, R>
• Consumer<T>
• Supplier<T>

You can create your own with @FunctionalInterface:

java

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

Using Lambdas with Collections

1. Sorting a list:

java

List<String> names = Arrays.asList("John", "Alice", "Bob");

names.sort((a, b) -> a.compareToIgnoreCase(b));

2. Filtering with Stream API:

java

names.stream()
     .filter(name -> name.startsWith("A"))
     .forEach(System.out::println);

Examples of Common Functional Interfaces

Predicate (returns boolean):

java

Predicate<String> startsWithA = s -> s.startsWith("A");

Function (transforms input to output):

java

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

Consumer (accepts input, returns nothing):

java

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

Supplier (provides a value):

java

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

Lambda vs Anonymous Classes

Before Java 8:

java

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

With Lambda:

java

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

Best Practices

• Use lambdas to simplify boilerplate code.
• Keep lambdas short and focused — use method references if it gets too long.
• Prefer descriptive variable names within lambdas.
• Avoid side-effects within lambda expressions when possible.

Method References (Shorter Syntax)

Instead of:

java

list.forEach(s -> System.out.println(s));

Use:

java

list.forEach(System.out::println);

Lambda expressions make Java code more expressive and align it with modern functional programming trends. Mastering them helps you write cleaner, more efficient code, especially when working with streams and collections.