Functional Interfaces in Java

Functional Interfaces in Java | Generated by AI

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The java.util.function package in Java provides a collection of functional interfaces that are designed to support functional programming, introduced in Java 8. These interfaces each have a single abstract method, making them compatible with lambda expressions and method references. This response explains how to use some of the most common functional interfaces in this package—Function<T, R>, Predicate<T>, Consumer<T>, and Supplier<T>—with practical examples.

What Are Functional Interfaces?

A functional interface is an interface with exactly one abstract method. The java.util.function package offers predefined functional interfaces for common tasks, so you don’t need to create your own. These interfaces are widely used with lambda expressions, method references, and the Stream API to write concise and expressive code.

Here’s how to use the key interfaces:

1. `Function<T, R>`: Transforming Input to Output

The Function<T, R> interface represents a function that takes an input of type T and produces an output of type R. Its abstract method is apply.

Example: Get the Length of a String

import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> stringLength = s -> s.length();
        System.out.println(stringLength.apply("Hello")); // Outputs: 5
    }
}

Explanation: The lambda expression s -> s.length() defines a Function that takes a String (T) and returns an Integer (R). The apply method executes this logic.

2. `Predicate<T>`: Testing a Condition

The Predicate<T> interface represents a boolean-valued function that takes an input of type T. Its abstract method is test.

Example: Check if a Number is Even

import java.util.function.Predicate;

public class Main {
    public static void main(String[] args) {
        Predicate<Integer> isEven = n -> n % 2 == 0;
        System.out.println(isEven.test(4)); // Outputs: true
        System.out.println(isEven.test(5)); // Outputs: false
    }
}

Explanation: The lambda n -> n % 2 == 0 defines a Predicate that returns true if the input is even. The test method evaluates this condition.

3. `Consumer<T>`: Performing an Action

The Consumer<T> interface represents an operation that takes an input of type T and returns no result. Its abstract method is accept.

Example: Print a String

import java.util.function.Consumer;

public class Main {
    public static void main(String[] args) {
        Consumer<String> printer = s -> System.out.println(s);
        printer.accept("Hello, World!"); // Outputs: Hello, World!
    }
}

Explanation: The lambda s -> System.out.println(s) defines a Consumer that prints its input. The accept method performs the action.

4. `Supplier<T>`: Generating a Result

The Supplier<T> interface represents a supplier of results, taking no input and returning a value of type T. Its abstract method is get.

Example: Generate a Random Number

import java.util.function.Supplier;
import java.util.Random;

public class Main {
    public static void main(String[] args) {
        Supplier<Integer> randomInt = () -> new Random().nextInt(100);
        System.out.println(randomInt.get()); // Outputs a random integer between 0 and 99
    }
}

Explanation: The lambda () -> new Random().nextInt(100) defines a Supplier that generates a random integer. The get method retrieves the value.

Using Functional Interfaces with Streams

These interfaces shine in the Java Stream API, where they enable concise data processing. Here’s an example that filters, transforms, and prints a list of strings:

Example: Process a List of Strings

import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;

public class Main {
    public static void main(String[] args) {
        List<String> strings = Arrays.asList("a", "bb", "ccc", "dddd");

        Predicate<String> longerThanTwo = s -> s.length() > 2;       // Filter strings longer than 2
        Function<String, String> toUpperCase = s -> s.toUpperCase(); // Convert to uppercase
        Consumer<String> printer = s -> System.out.println(s);       // Print each result

        strings.stream()
               .filter(longerThanTwo)   // Keeps "ccc" and "dddd"
               .map(toUpperCase)        // Converts to "CCC" and "DDDD"
               .forEach(printer);       // Outputs: CCC, DDDD (on separate lines)
    }
}

Explanation:
- filter uses a Predicate to keep strings with length > 2.
- map uses a Function to transform strings to uppercase.
- forEach uses a Consumer to print each result.

Using Method References

You can make this even shorter with method references:

strings.stream()
       .filter(s -> s.length() > 2)
       .map(String::toUpperCase)      // Method reference for Function
       .forEach(System.out::println); // Method reference for Consumer

Composing Functional Interfaces

Some interfaces allow composition for more complex operations:

Function Composition: Use andThen or compose.

Function<String, Integer> toLength = s -> s.length();
Function<Integer, String> toString = i -> "Length is " + i;
Function<String, String> combined = toLength.andThen(toString);
System.out.println(combined.apply("Hello")); // Outputs: Length is 5

Predicate Combination: Use and, or, or negate.

Predicate<String> isLong = s -> s.length() > 5;
Predicate<String> startsWithA = s -> s.startsWith("A");
Predicate<String> isLongAndStartsWithA = isLong.and(startsWithA);
System.out.println(isLongAndStartsWithA.test("Avocado")); // Outputs: true

Summary

Here’s when and how to use these interfaces:

Function<T, R>: Transform an input into an output (e.g., apply).
Predicate<T>: Test a condition (e.g., test).
Consumer<T>: Perform an action on an input (e.g., accept).
Supplier<T>: Generate a value without input (e.g., get).
With Streams: Combine them for powerful data processing.
Composition: Chain or combine them for complex logic.

You can implement these interfaces using lambda expressions (e.g., s -> s.length()) or method references (e.g., String::toUpperCase). They enable a functional programming style in Java, making your code more concise, readable, and reusable, especially with the Stream API.

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