[JAVA] Now is the time to get started with the Stream API

I participated in JJUG CCC 2018 Fall. I personally summarized the contents of @ skrb's # ccc_g4 lecture.

What is Stream?

** Stream is a simple iterator **

• There are two types of iterators.
• External iterator
• Write your own repeat control
• for extension for iterator class
• Not only the loop part of the for statement, but also the get (x) part is the control part.
• complexity
• Not very descriptive.
• Internal iterator
• Leave the repetition to the library (Stream API)
• simple
• Highly descriptive.

Lambda-style standing position

Muhammad Ali dances like a butterfly and stabs like a bowl A lambda expression that is written like a function and behaves like a class

 Function <============ Lambda expression ============> class
 Processing: Described like a function, state + processing
 Behave like a class

Why don't you understand Stream?

Many people who tried it once but did not understand

If there is a new technology ...

• Would you like to try it first?
• Would you like to use it until you fully understand it?

Example: To make an omelet, the heat denaturation of the protein is 60 degrees, and there is a Maillard reaction ... Even if you know that, you can make it without worrying about that, right? ??

** Because it's a new concept, get used to it rather than learning it. You only have to understand the mechanism after you start calling. ** **

Stream = data flow

I think that it is processed and processed rather than a loop. I don't want to make a class to process it. → Lambda expression How to describe processing that determines only arguments and return values

 (Argument)-> {
	toDO
	return val;
}

What is Stream after all

** Reassemble what you want to do from the perspective of data processing. Simplify the process and divide it into 1 process-> simple 4 processes-> Finally boxed with toList () etc. It is good to start with for each and gradually try map-> filter etc. **

Commonly used lambda expression

1 argument with return value Function <T, R> 1 argument return value boolean Predicate 1 argument no return value Consumer No arguments, no return value, Supplier

Basic writing

 // Basic guy
(String s) -> { return s.length();
 // Type is optional
(s) -> { return s.length();
 // If it is one line, return can be omitted
(s) -> s.length();
 // () can be omitted if it is one argument
s -> s.length();

The IDE will do it for you even if you don't remember this.

Patterns that you should at least remember

• Intermediate processing to process data.
• Sort data * filter
• dropWhile * 1 Argument Return value boolean * takeWhile
• Process the data.
• map * 1 There is an argument return value
• flatmap * 1 Argument return value Stream ← Used when a nested stream appears. Convert to one dimension
• The final process of putting together the data.
• look for
• findFirst Return of single data
• findAny Return single data * No lambda expression
• anyMatch regular expression search
• allMatch regular expression search
• oneMatch Regular expression search * 1 Argument return value boolean
• Summarize
• collect * Collectors :: static method
• toList * No lambda expression
• groupingBy * 1 Argument with return value (map)
• Sequential processing
• for-each 1 argument No return value

Example: Extract words that start with a lowercase a from the List.

example.java

List<String> words = new ArrayList<>() {
    {
        add("apple");
        add("toast");
        add("about");
        add("sun");
    }
};
var results = words.stream()
    .map(word -> word.toLowerCase())
    .filter(word -> word.startsWith("a"))
    .collect(Collectors.toList());
System.out.println(results);

Stream is null-guaranteed, so you don't have to do a new ArrayList () for an array of outputs. There are some that you can't play with Stream, so don't overdo it.

Q&A Q1. I think the conversion processing section can refer to the method. A1. You can write. However, it is not very preferable because it is not friendly to the reader. Q2. The exception does not work. Is there a best practice A2. There is no correct answer. Use either

Supplement

The person himself has already posted the Presentation Material, so this article is for reference only.