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The Java Interview Handbook: 300+ Interview Questions

Functional Interfaces

Explore the role of functional interfaces in Java functional programming. Understand how they support lambda expressions with single abstract methods, default, and static methods. Learn to implement common interfaces like Predicate and Consumer, and see their use in modern Java features like records and streams to write concise and expressive code.

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Functional programming in Java relies entirely on functional interfaces.

Let’s explore how these interfaces are structured, why they are required for Lambda expressions, and how they integrate into modern Java development:

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What is a functional interface in Java?

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Next, let’s look at a functional interface that includes default and static methods alongside its single abstract method:

Java
@FunctionalInterface
interface DataProcessor {
    void process(String data);
    default void logProcess() {
        System.out.println("Processing started.");
    }
    static void printSystemStatus() {
        System.out.println("System is ready.");
    }
}

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