Functional programming is a declarative programming paradigm where one applies pure functions in sequence to solve complex problems. Functional programming mainly focuses on what to solve and uses expressions instead of statements.

Functional Interfaces in Java

Functional interfaces are a key feature of functional programming in Java. They are interfaces that have exactly one abstract method, making them ideal for use with lambda expressions and method references. These interfaces provide target types for lambda expressions and method references, enabling a more functional approach to Java programming.

Lambda Expressions

Lambda expressions are a key feature introduced in Java 8 that enable functional programming. They provide a clear and concise way to represent instances of functional interfaces using an expression. A lambda expression is essentially an anonymous function (a function without a name).

Use cases of lambda expressions in Java functional programming include simplifying the creation of instances of functional interfaces, enabling a more functional style of programming, and making code more readable and expressive.

Without Lambda Expressions

Before Java 8, if you wanted to run some code in a new thread, you typically needed to create an instance of a class that implements the Runnable interface. Here’s how you would do it:

// Without Lambda Expressions
Runnable runnable = new Runnable() {
    @Override
    public void run() {
        System.out.println("Running in a thread");
    }
};

new Thread(runnable).start();

Explanation:

• You create an anonymous class that implements the Runnable interface.
• The run method is overridden to print "Running in a thread."
• This code creates a new thread and starts it, executing the run method.

With Lambda Expressions

With Java 8, you can achieve the same thing more concisely using lambda expressions:

// With Lambda Expressions
Runnable runnable = () -> System.out.println("Running in a thread");
new Thread(runnable).start();

Explanation: