Java Consumer Tutorial with Examples

Consumer
Consumer + Method reference
Consumer.andThen

1. Consumer

No ADS

In Java 8, Consumer is a functional interface, representing an operator that accepts an input parameter and returns nothing.

Java Functional Interface

Consumer

@FunctionalInterface
public interface Consumer<T> {
    void accept(T t);

    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> {
            accept(t);
            after.accept(t);
        };
    }
}

Example:

ConsumerEx1.java

package org.o7planning.ex;

import java.util.function.Consumer;

public class ConsumerEx1 {
    
    public static void main(String[] args)  {

        // Create a Consumer object directly
        Consumer<String> greeter = name -> System.out.println("Hello " + name);
        
        greeter.accept("Tran");   // Hello Tran
    }
}

Expanding the above example, we use Consumer in Stream.forEach method.

// A method of Stream class
public void forEach(Consumer<? super T> action);

ConsumerEx2.java

package org.o7planning.ex;

import java.util.function.Consumer;
import java.util.stream.Stream;

public class ConsumerEx2 {
    
    public static void main(String[] args)  {

        // Create a Consumer object directly
        Consumer<String> greeter = name -> System.out.println("Hello " + name);  
        
        Stream<String> names = Stream.of("Tran", "Nguyen", "Pham");
        
        names.forEach(greeter);
    }
}

Output:

Hello Tran
Hello Nguyen
Hello Pham

Below is a list of the methods in the java.util package using Consumer:

Modifier and Type	Method and Description
void	ArrayList.forEach(Consumer<? super E> action)
void	Vector.forEach(Consumer<? super E> action)
default void	PrimitiveIterator.OfDouble.forEachRemaining(Consumer<? super Double> action)
default void	Spliterator.OfDouble.forEachRemaining(Consumer<? super Double> action)
default void	Iterator.forEachRemaining(Consumer<? super E> action)
default void	PrimitiveIterator.OfInt.forEachRemaining(Consumer<? super Integer> action)
default void	Spliterator.OfInt.forEachRemaining(Consumer<? super Integer> action)
default void	PrimitiveIterator.OfLong.forEachRemaining(Consumer<? super Long> action)
default void	Spliterator.OfLong.forEachRemaining(Consumer<? super Long> action)
default void	Spliterator.forEachRemaining(Consumer<? super T> action)
void	Optional.ifPresent(Consumer<? super T> consumer)
default boolean	Spliterator.OfDouble.tryAdvance(Consumer<? super Double> action)
default boolean	Spliterator.OfInt.tryAdvance(Consumer<? super Integer> action)
default boolean	Spliterator.OfLong.tryAdvance(Consumer<? super Long> action)
boolean	Spliterator.tryAdvance(Consumer<? super T> action)

https://docs.oracle.com/javase/8/docs/api/java/util/function/class-use/Consumer.html

See also: BiConsumer is a class similar to Consumer, the difference is that it accepts 2 parameters:
Java BiConsumer

2. Consumer + Method reference

No ADS

If a method has a single parameter and returns nothing, its reference can be considered a Consumer.

Example: System.out.println(param) method takes one parameter and returns nothing, then its reference System.out::println will be considered as a Consumer.

ConsumerEx5.java

package org.o7planning.ex;

import java.util.function.Consumer;

public class ConsumerEx5 {

    public static void main(String[] args) {

        myPrintln(System.out::println, "Hello World!");
    }
    
    public static <T> void myPrintln(Consumer<T> c, T t) {
         c.accept(t);
    }
}

Output:

Hello World!

Example:

ConsumerEx6.java

package org.o7planning.ex;

import java.util.stream.Stream;

public class ConsumerEx6 {

    public static void main(String[] args) {
        
        Stream<String> names = Stream.of("Tran", "Nguyen", "Pham");
        
        // Call method: Stream.forEach(Consumer)
        names.forEach(System.out::println);
    }
    
}

Output:

Tran
Nguyen
Pham

Java Method Reference
Java Stream

3. Consumer.andThen

No ADS

andThen(after) method returns an associated Consumer. First, the current Consumer will be called then after will be called. If an error occurs in one of the two steps above, the error will be passed to the caller. If an error occurs at the current Consumer then after is ignored.

Consumer

default Consumer<T> andThen(Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> {
       accept(t);
       after.accept(t);
    };
}

Example:

ConsumerEx3.java

package org.o7planning.ex;

import java.util.function.Consumer;

public class ConsumerEx3 {
    
    public static void main(String[] args)  {
        
        Consumer<String> c = text -> System.out.println(text.toLowerCase());   
         
        c.andThen(c).accept("Hello");
    }
}

The result is that the word "hello" was printed out twice.

hello
hello

Example:

ConsumerEx4.java

package org.o7planning.ex;

import java.util.function.Consumer;

public class ConsumerEx4 {

    public static void main(String[] args) {

        Consumer<UserAccount> c1 = test -> test.auth();

        Consumer<UserAccount> c2 = test -> test.welcome();

        UserAccount user = new UserAccount("tran", "123");

        try {
            c1.andThen(c2).accept(user);
        } catch (Exception e) {

        }
    }

    public static class UserAccount {
        private String userName;
        private String password;

        public UserAccount(String userName, String password) {
            this.userName = userName;
            this.password = password;
        }

        public void auth() {
            if ("123".equals(password)) {
                System.out.println("Valid Account!");
            } else {
                throw new RuntimeException("Invalid Password");
            }
        }

        public void welcome() {
            System.out.println("Welcome! " + this.userName);
        }
    }
}

Output:

Valid Account!
Welcome! tran

No ADS

Java Consumer Tutorial with Examples

1. Consumer

2. Consumer + Method reference

3. Consumer.andThen

Java Basic

Java Basic

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