.forEach()

The .forEach() method performs a specified action on each element in an ArrayList. This method traverses each element in the ArrayList until all the elements have been processed or an exception is raised from the action. It only iterates through the elements without modifying the ArrayList and does not return any value.

Syntax

arrayListInstance.forEach(Consumer<E> action);

• arrayListInstance: The name of the ArrayList to be iterated.
• Consumer: A functional interface representing an operation to be performed. It can be used as the assignment target for a lambda expression or method reference.
• action: The operation that accepts an element of type E in the ArrayList as its only argument and does not return any value.

Example

The following example uses the .forEach() method with a lambda expression to print out the elements in an ArrayList called students:

// Import the ArrayList class from the java.util package
import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    // Create an ArrayList of strings called 'students'
    ArrayList<String> students = new ArrayList<>();

    students.add("Anna");
    students.add("Bailey");
    students.add("Chris");
    students.add("Donna");

    students.forEach((s) -> System.out.println(s));
  }
}

The above example produces the following output:

Anna
Bailey
Chris
Donna

Apart from using a lambda expression, each student in the ArrayList can also be printed out using a method reference:

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> students = new ArrayList<>();

    students.add("Anna");
    students.add("Bailey");
    students.add("Chris");
    students.add("Donna");

    students.forEach(System.out::println);
  }
}

The output matches with the previous one:

Anna
Bailey
Chris
Donna

The operation passed into .forEach() is meant to be performed on each element of the ArrayList, not the ArrayList itself. Therefore, if an operation to be performed on the ArrayList is passed into .forEach(), an error occurs.

In the following example, the .get() operation passed into .forEach() is to be performed on students to retrieve a student name from the ArrayList:

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> students = new ArrayList<>();

    students.add("Anna");
    students.add("Bailey");
    students.add("Chris");
    students.add("Donna");

    students.forEach(System.out.println(students.get(0)));
  }
}

The above code will throw an error:

Main.java:12: error: 'void' type not allowed here
      students.forEach(System.out.println(students.get(0)));
                                         ^
1 error