The Consumer Interface is a functional interface, which takes in one argument but does not return any value.
It's part of the java.util.function package which has been introduced since Java 8, to implement functional programming in Java.
This functional interface takes in one generic argument 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 is not needed to return any value as they are expected to operate via side-effects.
Methods in Consumer Interface
The Consumer interface consists of the following two methods:
1. accept()
This method accepts one value and performs the operation on the given argument
Syntax:
void accept(T t);
Returns: This method does not return any value.
Below is the code to illustrate accept() method:
Program 1:
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);
}
}
10
4 2 6
2. andThen()
It returns a composed Consumer 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:
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);
}
}
Output:
4 2 6
Program 2: To demonstrate when NullPointerException is returned.
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);
}
}
}
Output:
Exception: java.lang.NullPointerException
Program 3: To demonstrate how an Exception in the after function is returned and handled.
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);
}
}
}
Output:
2 1 3 Exception: java.lang.IndexOutOfBoundsException: Index: 3, Size: 3