Java Consumer

last modified July 6, 2020

Java Consumer tutorial shows how to work with the Consumer functional interface in Java.

Consumer

Java Consumer is a 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.

package com.zetcode;

import java.util.function.Consumer;

public class ConsumerEx {

    public static void main(String[] args) {

        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.

blue sky
blue sky
blue sky
old falcon
old falcon
old falcon

This is the output.

Java 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.

package com.zetcode;

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

public class ConsumerEx2 {

    public static void main(String[] args) {

        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.

300
400
500
50
100
150

This is the output.

Java Consumer forEach

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

package com.zetcode;

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

public class ConsumerEx3 {

    public static void main(String[] args) {

        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.

falcon
wood
rock
forest
river
water

This is the output.

Java Consumer andThen

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

package com.zetcode;

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

public class ConsumerEx4 {

    public static void main(String[] args) {

        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.

4
6
8
10

This is the output.

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

package com.zetcode;

import java.math.BigDecimal;

public 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 is the Product class.

package com.zetcode;

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

public class ConsumerEx5 {

    private static RoundingMode ROUNDING_MODE = RoundingMode.HALF_EVEN;
    private static int DECIMALS = 2;

    public static void main(String[] args) {

        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));
    }

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

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

        for (var e : data) {

            cons.accept(e);
        }
    }
}

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.

private static 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.

Product{name='A', price=2.79}
Product{name='B', price=4.28}
Product{name='C', price=6.59}
Product{name='D', price=10.99}

This is the output.

Java Consumer custom forEach

The following example creates a custom, generic forEach method.

package com.zetcode;

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

public class ConsumerEx5 {

    public static void main(String[] args) {

        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);
    }

    static <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.

1
2
3
4
5
6
7
--------------------------
1
2
3
4
5
6
7

This is the output.

In this tutorial, we have worked with Java Consumer interface.

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