last modified February 24, 2024
In this article we show how to work with the Consumer functional interface in Java.
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.
The following example creates a simple consumer.
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 represents an operation that accepts a single
int-valued argument and returns no result. This is the primitive type
specialization of Consumer for int.
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
The forEach method accepts a Consumer as a parameter.
The consumer can be simplified with a lambda expression or a method reference.
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
The andThen method returns a composed Consumer that
performs, in sequence, this operation followed by the next operation.
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.
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}
The following example creates a custom, generic forEach method.
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
Java Consumer - language reference
In this article we have worked with Java Consumer interface.
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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