How do I use the Consumer functional interface in Java?

The Consumer<T> interface in Java is a functional interface from the java.util.function package. It represents an operation that accepts a single input argument and does not return any result. It is commonly used for operations where a value is passed in and some side effect occurs (e.g., printing, modifying state, or logging).

Steps to Use a Consumer:

Functional Interface: Since Consumer is a functional interface, you can use it with lambda expressions, method references, or anonymous classes.
Method: It has a single abstract method:
- void accept(T t): Performs the operation on the given input.

Example Usage

Here are several ways we can use the Consumer<T> interface:

1. Using Lambda Expressions

package org.kodejava.util.function;

import java.util.function.Consumer;

public class ConsumerExample {
   public static void main(String[] args) {
      Consumer<String> printConsumer = s -> System.out.println(s);

      // Output: Hello, Consumer!
      printConsumer.accept("Hello, Consumer!");
   }
}

2. Using Method References

package org.kodejava.util.function;

import java.util.function.Consumer;

public class ConsumerExample2 {
   public static void main(String[] args) {
      // Referencing the println method
      Consumer<String> printConsumer = System.out::println;

      // Output: Hello, Method Reference!
      printConsumer.accept("Hello, Method Reference!");
   }
}

3. Using Anonymous Classes

package org.kodejava.util.function;

import java.util.function.Consumer;

public class ConsumerExample3 {
   public static void main(String[] args) {
      Consumer<String> printConsumer = new Consumer<String>() {
         @Override
         public void accept(String t) {
            System.out.println(t);
         }
      };

      // Output: Hello, Anonymous Class!
      printConsumer.accept("Hello, Anonymous Class!");
   }
}

4. Using with `andThen` for Chaining

The Consumer interface provides a default method andThen that allows chaining multiple Consumers in sequence.

package org.kodejava.util.function;

import java.util.function.Consumer;

public class ConsumerExample4 {
   public static void main(String[] args) {
      Consumer<String> printConsumer = s -> System.out.println("Printing: " + s);
      Consumer<String> lengthConsumer = s -> System.out.println("Length: " + s.length());

      // Chaining Consumers
      Consumer<String> chainedConsumer = printConsumer.andThen(lengthConsumer);
      chainedConsumer.accept("Hello, Chaining!");
      // Output:
      // Printing: Hello, Chaining!
      // Length: 16
   }
}

5. Using with Collections

Consumer is commonly used with the forEach method of Java collections.

package org.kodejava.util.function;

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

public class ConsumerExample5 {
   public static void main(String[] args) {
      List<String> names = Arrays.asList("Alice", "Bob", "Carol");

      // Using forEach with Consumer
      Consumer<String> printName = name -> System.out.println("Hello, " + name + "!");
      names.forEach(printName);

      // Output:
      // Hello, Alice!
      // Hello, Bob!
      // Hello, Carol!
   }
}

6. A Real-World Example

We might use a Consumer<T> in logging operations, updating GUI elements, or applying modifications to a list of objects.

package org.kodejava.util.function;

import java.util.function.Consumer;

public class LoggingExample {
   public static void main(String[] args) {
      Consumer<String> logger = message -> System.out.println("[LOG] " + message);
      logger.accept("Application started.");
      logger.accept("Processing user request.");
      logger.accept("Application terminated.");
   }
}

Summary

Use Consumer<T> to perform operations on a single input argument.
It can be implemented using lambdas, method references, or anonymous classes.
It is often used with the forEach method of collections or in places where side effects (like logging or output) are important.

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