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In this tutorial, we will learn about Java 8 stream peek method.

peek() method :-

This method takes one Consumer as an argument, this consumer accepts one parameter T as an input argument and return one stream of parameter T as a return value. The main purpose of this method is to support debugging, When you want to see ( peek ) the elements as they flow through the certain point in a stream pipeline. You can achieve this by using a Consumer Interface with System.out.println in it.

Syntax :-  Stream<T> peek ( Consumer<? super T> action )

Reference :-  peek() method JavaDocs

Problem Statement :-

You have given a list of integers first, find all the integers which are greater than 5 and then all the integers which are smaller than 9. Print all the values on the system console during first filter operation for debugging purpose.

Java 7 :- Implementation in Java 7.

  public static void main(String[] args){
 
    List<Integer> filterList = new ArrayList<Integer>();
    List<Integer> numList = Arrays.asList(1,3,4,5,2,7,9);
    for(Integer num:numList){
      if(num >= 5){
        filterList.add(num);
        System.out.println("Filtered Value : "+num);
      }
    }

    List<Integer> finalList = new ArrayList<Integer>();
    for(Integer num:filterList){
      if(num < 9){
        finalList.add(num);
      }
    }
    System.out.println(finalList);
 
  }

Java 8 Stream peek :- Implementation using stream API.

  public static void main(String[] args) {
 
    List<Integer> numList = Arrays.asList(1,3,4,5,2,7,9);
    List<Integer> filterList = numList.stream().filter(e -> e >= 5)
                                 .peek(e -> System.out.println("Filtered value: " + e))
                                 .filter(e -> e < 9)
                                 .collect(Collectors.toList());
    System.out.println(filterList);

  }

Result :- 

Filtered Value : 5
Filtered Value : 7
Filtered Value : 9
[5, 7] 

That’s all for Java 8 Stream peek method. If you liked it, please share your thoughts in comments section and share it with others too.