Java Consumer

last modified February 24, 2024

In this article we show how to work with the Consumer functional interface in Java.

Consumer

Consumer is a Java functional interface which represents an operation that accepts a single input argument and returns no result. Unlike most other functional interfaces, Consumer is expected to operate via side-effects.

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

The Consumer’s functional method is accept(Object). It can be used as the assignment target for a lambda expression or method reference.

Java Consumer example

The following example creates a simple consumer.

Main.java

import java.util.function.Consumer;

void main() {

Consumer<String> showThreeTimes = value -> {

    System.out.println(value);
    System.out.println(value);
    System.out.println(value);
};

showThreeTimes.accept("blue sky");
showThreeTimes.accept("old falcon");

}

The showThreeTimes consumer prints the input three times.

$ java Main.java blue sky blue sky blue sky old falcon old falcon old falcon

IntConsumer

IntConsumer represents an operation that accepts a single int-valued argument and returns no result. This is the primitive type specialization of Consumer for int.

Main.java

import java.util.function.Consumer; import java.util.function.IntConsumer;

void main() {

Consumer<Integer> printMultiplyBy100 = (val) -> System.out.println(val * 100);

printMultiplyBy100.accept(3);
printMultiplyBy100.accept(4);
printMultiplyBy100.accept(5);

IntConsumer printMultiplyBy500 = a -> System.out.println(a * 50);
printMultiplyBy500.accept(1);
printMultiplyBy500.accept(2);
printMultiplyBy500.accept(3);

}

In the example, the consumers multiply the input value.

$ java Main.java 300 400 500 50 100 150

Consumer forEach

The forEach method accepts a Consumer as a parameter. The consumer can be simplified with a lambda expression or a method reference.

Main.java

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

void main() {

var words = List.of("falcon", "wood", "rock", "forest",
        "river", "water");

words.forEach(new Consumer<String>() {
    @Override
    public void accept(String s) {

        System.out.println(s);
    }
});

}

In the example, we go over the elements of a list with forEach. The consumer simply prints each of the elements.

$ java Main.java falcon wood rock forest river water

Consumer andThen

The andThen method returns a composed Consumer that performs, in sequence, this operation followed by the next operation.

Main.java

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

void main() {

var vals = new ArrayList<Integer>();
vals.add(2);
vals.add(4);
vals.add(6);
vals.add(8);

Consumer<List<Integer>> addTwo = list -> {

    for (int i = 0; i < list.size(); i++) {

        list.set(i, 2 + list.get(i));
    }
};

Consumer<List<Integer>> showList = list -> 
    list.forEach(System.out::println);

addTwo.andThen(showList).accept(vals);

}

In the example, we add value 2 to each of the elements in the list and then we print all the elements.

$ java Main.java 4 6 8 10

In the following example, we work with a list of products.

Main.java

import java.math.BigDecimal; import java.math.RoundingMode; import java.util.ArrayList; import java.util.List; import java.util.function.Consumer;

final RoundingMode ROUNDING_MODE = RoundingMode.HALF_EVEN; final int DECIMALS = 2;

void main() {

List<Product> products = new ArrayList<>();
products.add(new Product("A", new BigDecimal("2.54")));
products.add(new Product("B", new BigDecimal("3.89")));
products.add(new Product("C", new BigDecimal("5.99")));
products.add(new Product("D", new BigDecimal("9.99")));

Consumer<Product> incPrice = p -> {
    p.setPrice(rounded(p.getPrice().multiply(new BigDecimal("1.1"))));
};

process(products, incPrice.andThen(System.out::println));

}

BigDecimal rounded(BigDecimal number){ return number.setScale(DECIMALS, ROUNDING_MODE); }

void process(List<Product> data, Consumer<Product> cons) {

for (var e : data) {

    cons.accept(e);
}

}

class Product {

private String name;
private BigDecimal price;

public Product(String name, BigDecimal price) {
    this.name = name;
    this.price = price;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public BigDecimal getPrice() {
    return price;
}

public void setPrice(BigDecimal price) {
    this.price = price;
}

@Override
public String toString() {

    var sb = new StringBuilder("Product{");
    sb.append("name='").append(name).append('\'');
    sb.append(", price=").append(price);
    sb.append('}');
    return sb.toString();
}

}

This example increases the prices of products by 10%.

Consumer<Product> incPrice = p -> { p.setPrice(rounded(p.getPrice().multiply(new BigDecimal(“1.1”)))); };

The consumer increases the product price by 10% and rounds the value.

process(products, incPrice.andThen(System.out::println));

We increase the prices and then print the modified products.

void process(List<Product> data, Consumer<Product> cons) {

for (var e : data) {

    cons.accept(e);
}

}

The consumer is applied on each product of the list.

$ java Main.java Product{name=‘A’, price=2.79} Product{name=‘B’, price=4.28} Product{name=‘C’, price=6.59} Product{name=‘D’, price=10.99}

Consumer with a custom forEach

The following example creates a custom, generic forEach method.

Main.java

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

void main() {

var data = List.of(1, 2, 3, 4, 5, 6, 7);

// Consumer<Integer> consumer = (Integer x) -> System.out.println(x);
Consumer<Integer> consumer = System.out::println;
forEach(data, consumer);

System.out.println("--------------------------");

forEach(data, System.out::println);

}

<T> void forEach(List<T> data, Consumer<T> consumer) {

for (T t : data) {

    consumer.accept(t);
}

}

We have a list of integers. We print all the elements of the list with a custom forEach method.

$ java Main.java 1 2 3 4 5 6 7

1 2 3 4 5 6 7

Source

Java Consumer - language reference

In this article we have worked with Java Consumer interface.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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