The original link: segmentfault.com/a/119000002…

1. Introduction

I explained the difference between the Map and flatMap operations in the Java8 Stream API in the Detailed Guide to using the Java8 Stream API. Then a friend told me that the peek operation can also achieve element processing. But do you know the difference between Map and Peek? Map was covered at the beginning of this article, so you can look at it in more detail. This article focuses on the Peek operation.

2. peek

The peek operation receives a Consumer

function. As the name implies, the peek operation consumes each element in the flow using the logic provided by the Consumer

function, possibly changing some attributes inside the element. Here we will mention the word Consumer

to understand what consumption is.


2.1 What is Consumer?

package java.util.function;

import java.util.Objects;

 
@FunctionalInterface
public interface Consumer<T> {
 
    void accept(T t);

     Accept (accept); after. Accpet (accept
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return(T t) -> { accept(t); after.accept(t); }; }}Copy the code

Consumer

is a functional interface. An abstract method void Accept (T T) means to take a parameter of type T and consume it. In fact, consumption gives me the feeling of “use up”, and the natural return is void. There are usually two ways to “use up” T:

  • Void method of T itselfThe typical one issetter 。
  • Pass T to the void method of another interface (class) for processingFor example, we often print an objectSystem.out.println(T)

2.2 Peek Operation demonstration

 Stream<String> stream = Stream.of("hello"."felord.cn");
   stream.peek(System.out::println);
Copy the code

If you test the code shown above you will find that it does not run logically at all. Why? This is because the life cycle of a flow has three phases:

  • Initial generation phase.
  • Intermediate operations fetch and process elements one by one. Dispensable. All intermediate operations are inert, so no action has any effect until the flow flows through the pipe.
  • Terminal operation. Usually divided intoFinal consumption （foreachAnd so on) andinductive （collectTwo classes). It is also important that terminal operations initiate the flow through the pipe.

So it should be the following:

 Stream<String> stream = Stream.of("hello"."felord.cn");
  List<String> strs= stream.peek(System.out::println).collect(Collectors.toLIst());
Copy the code

For example, in the image below, we add a box to the sphere:

3. peek VS map

The peek operation is generally used when you do not want to change the type of the element itself in the stream or just the internal state of the element. A map, on the other hand, is used to change the type of the element itself in the stream, that is, to derive another type from the element. That’s the big difference between them. So what scenarios do we use Peek in practice? For example, peek is suitable for batch processing of some attributes of T in Collection

. Map is best used if we want to retrieve a Collection of attributes of T from Collection

.

4. To summarize

Today we looked at the Stream peek operation and reviewed the Stream life cycle. The Consumer

function is also explained in passing. It also compares map with map and explains the usage scenarios of each. I’m sure you’ll understand them better after reading this article.

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