Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids changing state and mutable data. Java introduced functional programming features in Java 8, primarily through the introduction of lambda expressions, the Stream API, and functional interfaces. These features enable developers to write code in a more functional style, which can lead to more concise and readable code. Here are some key aspects of functional programming in Java:

1. Lambda Expressions:

   • Lambda expressions in Java allow you to define anonymous functions (closures) concisely. They are often used to define functions inline or as arguments to higher-order functions.
   • Example of a lambda expression:
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     (a, b) -> a + b

2. Functional Interfaces:

   • Functional interfaces are interfaces that have exactly one abstract method. They are used to represent functions and are a fundamental part of functional programming in Java.
   • Java provides predefined functional interfaces like Consumer, Supplier, Function, Predicate, etc., in the java.util.function package.

3. Streams API:

   • The Streams API in Java allows you to process sequences of data in a functional style. You can filter, map, reduce, and perform various transformations on data using streams.
   • Example of using a stream to filter and map elements:
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     List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5); List<Integer> squaredNumbers = numbers.stream() .filter(n -> n % 2 == 0) .map(n -> n * n) .collect(Collectors.toList());

4. Immutability:

   • Functional programming encourages immutability, which means once data is assigned, it cannot be changed. In Java, you can create immutable objects by making fields final and not providing setter methods.

5. First-Class and Higher-Order Functions:

   • Java supports first-class functions, meaning functions can be assigned to variables, passed as arguments, and returned as values.
   • Higher-order functions are functions that take other functions as arguments or return them as results. You can achieve this using functional interfaces and lambda expressions.

6. Recursion:

   • Functional programming often emphasizes the use of recursion instead of loops for repetitive tasks.

Here’s a simple example of using lambda expressions and functional interfaces in Java:

This is just a basic introduction to functional programming in Java.