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🧠 What is a Functional Interface?

A functional interface in Java is an interface with exactly one abstract method. These are the backbone of lambda expressions and method references, introduced in Java 8 to bring functional programming features to the language.

Every functional interface can have multiple default or static methods, but only one abstract method.

✅ Common Functional Interfaces from JDK

Here are some of the most commonly used functional interfaces directly from the Java Standard Library:

1. Runnable (java.lang.Runnable)

java

Runnable r = () -> System.out.println("Running a thread!");
new Thread(r).start();

• Represents a task that can be executed concurrently.
• Has the method: void run()

2. Callable (java.util.concurrent.Callable)

java

Callable<String> c = () -> "Callable result";

• Similar to Runnable, but returns a result and may throw an exception.
• Has the method: V call() throws Exception

3. Comparator<T> (java.util.Comparator)

java

Comparator<String> comparator = (s1, s2) -> s1.length() - s2.length();

• Used to compare two objects.
• Has the method: int compare(T o1, T o2)

4. Consumer<T> (java.util.function.Consumer)

java

Consumer<String> printer = s -> System.out.println(s);
printer.accept("Hello, Consumer!");

• Takes a single input and returns nothing.
• Has the method: void accept(T t)

5. Supplier<T> (java.util.function.Supplier)

java

Supplier<Double> randomSupplier = () -> Math.random();
System.out.println(randomSupplier.get());

• Provides a result without taking any input.
• Has the method: T get()

6. Predicate<T> (java.util.function.Predicate)

java

Predicate<String> isEmpty = s -> s.isEmpty();
System.out.println(isEmpty.test("")); // true

• Accepts one input and returns a boolean.
• Has the method: boolean test(T t)

7. Function<T, R> (java.util.function.Function)

java

Function<String, Integer> lengthMapper = s -> s.length();
System.out.println(lengthMapper.apply("Function")); // 8

• Accepts one input and produces a result.
• Has the method: R apply(T t)

8. BiFunction<T, U, R> (java.util.function.BiFunction)

java

BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
System.out.println(add.apply(5, 3)); // 8

• Takes two inputs and returns a result.
• Has the method: R apply(T t, U u)

📦 Where Are These Used?

These interfaces are widely used in:

• Streams API
• Concurrency utilities
• Lambda expressions
• Collections operations
• Asynchronous and reactive programming

🔚 Conclusion

Java 8's java.util.function package brought a robust set of predefined functional interfaces to streamline functional programming. Mastering these interfaces is essential for writing clean, concise, and modern Java code.