In Java, functional interfaces are a central feature introduced in Java 8 to support functional programming. A functional interface is an interface with exactly one abstract method, which makes it compatible with lambda expressions. These interfaces provide target types for lambda expressions and method references, allowing for more concise and readable code.

The `java.util.function` package contains several commonly used functional interfaces. Some key examples include:

1. **Predicate<T>**: Represents a boolean-valued function of one argument, typically used for filtering or matching conditions.

2. Function<T, R>: Represents a function that accepts one argument and produces a result, useful for transforming data.

3. Consumer<T>: Represents an operation that accepts a single input argument and returns no result, often used for operations like printing or logging.

4. Supplier<T>: Represents a supplier of results, providing a method to generate values without taking any arguments.

5.UnaryOperator<T> andBinaryOperator<T>: Specializations of `Function` for cases where the input and output types are the same, often used for operations like incrementing a number.

Functional interfaces enable functional programming patterns in Java, encouraging a more declarative style and facilitating parallel processing by enabling developers to pass behaviors (lambda expressions) as parameters, thereby increasing the expressiveness and flexibility of the code. This makes Java more versatile for both sequential and parallel processing.