Java 8 | Consumer Interface in Java with Examples

The Consumer Interface is a part of the java.util.function package which has been introduced since Java 8, to implement functional programming in Java. It represents a function which takes in one argument and produces a result. However these kind of functions don’t return any value.

Hence this functional interface which takes in one generic namely:-

  • T: denotes the type of the input argument to the operation

The lambda expression assigned to an object of Consumer type is used to define its accept() which eventually applies the given operation on its argument. Consumers are useful when it not needed to return any value as they are expected to operate via side-effects.



Functions in Consumer Interface

The Consumer interface consists of the following two functions:

1. accept()

This method accepts one value and performs the operation on the given argument

Syntax:

void accept(T t)

Parameters: This method takes in one parameter:

  • t– the input argument

Returns: This method does not return any value.

Below is the code to illustrate accept() method:

Program 1:

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java Program to demonstrate
// Consumer's accept() method
  
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.function.Consumer;
  
public class Main {
    public static void main(String args[])
    {
        // Consumer to display a number
        Consumer<Integer> display = a -> System.out.println(a);
  
        // Implement display using accept()
        display.accept(10);
  
        // Consumer to multiply 2 to every integer of a list
        Consumer<List<Integer> > modify = list ->
        {
            for (int i = 0; i < list.size(); i++)
                list.set(i, 2 * list.get(i));
        };
  
        // Consumer to display a list of numbers
        Consumer<List<Integer> >
            dispList = list -> list.stream().forEach(a -> System.out.print(a + " "));
  
        List<Integer> list = new ArrayList<Integer>();
        list.add(2);
        list.add(1);
        list.add(3);
  
        // Implement modify using accept()
        modify.accept(list);
  
        // Implement dispList using accept()
        dispList.accept(list);
    }
}

chevron_right


Output:

10
4 2 6
2. andThen()

It returns a composed Comsumer wherein the parameterized Consumer will be executed after the first one. If evaluation of either function throws an error, it is relayed to the caller of the composed operation.

Note: The function being passed as the argument should be of type Consumer.


Syntax:

default Consumer <T> 
        andThen(Consumer<? super T> after)

Parameters: This method accepts a parameter after which is the Consumer to be applied after the current one.

Return Value: This method returns a composed Consumer that first applies the current Consumer first and then the after operation.

Exception: This method throws NullPointerException if the after operation is null.

Below is the code to illustrate andThen() method:

Program 1:

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java Program to demonstrate
// Consumer's andThen() method
  
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.function.Consumer;
  
public class Main {
    public static void main(String args[])
    {
  
        // Consumer to multiply 2 to every integer of a list
        Consumer<List<Integer> > modify = list ->
        {
            for (int i = 0; i < list.size(); i++)
                list.set(i, 2 * list.get(i));
        };
  
        // Consumer to display a list of integers
        Consumer<List<Integer> >
            dispList = list -> list.stream().forEach(a -> System.out.print(a + " "));
  
        List<Integer> list = new ArrayList<Integer>();
        list.add(2);
        list.add(1);
        list.add(3);
  
        // using addThen()
        modify.andThen(dispList).accept(list);
        ;
    }
}

chevron_right


Output:

4 2 6

Program 2: To demonstrate when NullPointerException is returned.

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java Program to demonstrate
// Consumer's andThen() method
  
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.function.Consumer;
  
public class Main {
    public static void main(String args[])
    {
  
        // Consumer to multiply 2 to every integer of a list
        Consumer<List<Integer> > modify = list ->
        {
            for (int i = 0; i < list.size(); i++)
                list.set(i, 2 * list.get(i));
        };
  
        // Consumer to display a list of integers
        Consumer<List<Integer> >
            dispList = list -> list.stream().forEach(a -> System.out.print(a + " "));
  
        List<Integer> list = new ArrayList<Integer>();
        list.add(2);
        list.add(1);
        list.add(3);
        try {
            // using addThen()
            modify.andThen(null).accept(list);
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

chevron_right


Output:

Exception: java.lang.NullPointerException

Program 3: To demonstrate how an Exception in the after function is returned and handled.

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java Program to demonstrate
// Consumer's andThen() method
  
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.function.Consumer;
  
public class Main {
    public static void main(String args[])
    {
  
        // Consumer to multiply 2 to every integer of a list
        Consumer<List<Integer> > modify = list ->
        {
            for (int i = 0; i <= list.size(); i++)
                list.set(i, 2 * list.get(i));
        };
  
        // Consumer to display a list of integers
        Consumer<List<Integer> >
            dispList = list -> list.stream().forEach(a -> System.out.print(a + " "));
        System.out.println();
  
        List<Integer> list = new ArrayList<Integer>();
        list.add(2);
        list.add(1);
        list.add(3);
  
        // using addThen()
        try {
            dispList.andThen(modify).accept(list);
            ;
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

chevron_right


Output:

2 1 3 Exception: java.lang.IndexOutOfBoundsException: Index: 3, Size: 3


My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.