For each loop

The Java forEach is a utility method. It helps iterate over a collection such as (list, set, or map) & stream and perform a certain action on each element of it.

  • A new method forEach() is introduced in Java 8 to iterate over elements.
  • It is a default method available in the Iterable interface.
  • We can use forEach to iterate over a collection and perform a certain action on each element. 
  • A class that implements the Consumer interface contains the action and it passes the action as an argument to forEach. 
  • The Consumer interface is a functional interface with a single abstract method.
  • This method accepts a single parameter which is a functional interface. So, one can pass a lambda expression as an argument.
  • The method accepts an input and returns no result.
  • Maps are not Iterable, however, maps do provide their own variant of forEach that accepts a BiConsumer. 
  • Iterable’s forEach introduces A BiConsumer instead of a Consumer. As a result, an action can be performed on both the key and value of a Map simultaneously.
  • The forEach loop just needs to know what is to be done and all the work of iterating will be taken care of internally. However, in a traditional loop, we need to tell how to perform the iterations.

List –

List<String> names = Arrays.asList("A", "B", "C", "D");
names.forEach(name -> System.out.println(name));

ArrayList<String> namesList = new ArrayList<>(names);
Consumer<String> lambdaExp = x -> System.out.println(x.toLowerCase());

namesList.forEach(name -> System.out.println(name.toLowerCase()));

Map –

HashMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

Consumer<Map.Entry<String, Integer>> action = System.out::println;

Consumer<String> actionOnKeys = System.out::println;

Consumer<Integer> actionOnValues = System.out::println;

map.forEach((key, value) -> System.out.println(key + " " + value));

Stream –

List<Integer> numberList = Arrays.asList(1,2,3,4,5);
Consumer<Integer> action = System.out::println;
	.filter(n -> n%2 == 0)