搭建基于Hexo+Butterfly主题的个人博客站

# Main menu navigation (導航目錄)
# see https://butterfly.js.org/posts/4aa8abbe/#導航菜單
# --------------------------------------

menu:
  # Home: / || fas fa-home
  # Archives: /archives/ || fas fa-archive
  # Tags: /tags/ || fas fa-tags
  # Categories: /categories/ || fas fa-folder-open
  # List||fas fa-list:
  #   Music: /music/ || fas fa-music
  #   Movie: /movies/ || fas fa-video
  # Link: /link/ || fas fa-link
  # About: /about/ || fas fa-heart

# Code Blocks (代碼相關)
# --------------------------------------

highlight_theme: light #  darker / pale night / light / ocean / mac / mac light / false
highlight_copy: true # copy button
highlight_lang: true # show the code language
highlight_shrink: false # true: shrink the code blocks / false: expand the code blocks | none: expand code blocks and hide the button
highlight_height_limit: false # unit: px
code_word_wrap: false

# copy settings
# copyright: Add the copyright information after copied content (複製的內容後面加上版權信息)
copy:
  enable: true
  copyright:
    enable: false
    limit_count: 50

# social settings (社交圖標設置)
# formal:
#   icon: link || the description
social:
  # fab fa-github: https://github.com/xxxxx || Github
  # fas fa-envelope: mailto:xxxxxx@gmail.com || Email

# search (搜索)
# --------------------------------------

# Algolia search
algolia_search:
  enable: false
  hits:
    per_page: 6

# Local search
local_search:
  enable: false

# Math (數學)
# --------------------------------------
# About the per_page
# if you set it to true, it will load mathjax/katex script in each page (true 表示每一頁都加載js)
# if you set it to false, it will load mathjax/katex script according to your setting (add the 'mathjax: true' in page's front-matter)
# (false 需要時加載，須在使用的 Markdown Front-matter 加上 mathjax: true)

# MathJax
mathjax:
  enable: false
  per_page: false

# KaTeX
katex:
  enable: false
  per_page: false
  hide_scrollbar: true

# Image (圖片設置)
# --------------------------------------

# Favicon（網站圖標）
favicon: /img/favicon.png

# Avatar (頭像)
avatar:
  img: https://i.loli.net/2021/02/24/5O1day2nriDzjSu.png
  effect: false

# Disable all banner image
disable_top_img: false

# The banner image of home page
index_img:

# If the banner of page not setting, it will show the top_img
default_top_img:

# The banner image of archive page
archive_img:

# If the banner of tag page not setting, it will show the top_img
# note: tag page, not tags page (子標籤頁面的 top_img)
tag_img:

# The banner image of tag page
# format:
#  - tag name: xxxxx
tag_per_img:

# If the banner of category page not setting, it will show the top_img
# note: category page, not categories page (子分類頁面的 top_img)
category_img:

# The banner image of category page
# format:
#  - category name: xxxxx
category_per_img:

cover:
  # display the cover or not (是否顯示文章封面)
  index_enable: true
  aside_enable: true
  archives_enable: true
  # the position of cover in home page (封面顯示的位置)
  # left/right/both
  position: both
  # When cover is not set, the default cover is displayed (當沒有設置cover時，默認的封面顯示)
  default_cover:
    # - https://i.loli.net/2020/05/01/gkihqEjXxJ5UZ1C.jpg

# Replace Broken Images (替換無法顯示的圖片)
error_img:
  flink: /img/friend_404.gif
  post_page: /img/404.jpg

# A simple 404 page
error_404:
  enable: false
  subtitle: 'Page Not Found'
  background: https://i.loli.net/2020/05/19/aKOcLiyPl2JQdFD.png

post_meta:
  page: # Home Page
    date_type: created # created or updated or both 主頁文章日期是創建日或者更新日或都顯示
    date_format: date # date/relative 顯示日期還是相對日期
    categories: true # true or false 主頁是否顯示分類
    tags: false # true or false 主頁是否顯示標籤
    label: true # true or false 顯示描述性文字
  post:
    date_type: both # created or updated or both 文章頁日期是創建日或者更新日或都顯示
    date_format: date # date/relative 顯示日期還是相對日期
    categories: true # true or false 文章頁是否顯示分類
    tags: true # true or false 文章頁是否顯示標籤
    label: true # true or false 顯示描述性文字

# wordcount (字數統計)
wordcount:
  enable: false
  post_wordcount: true
  min2read: true
  total_wordcount: true

# Display the article introduction on homepage
# 1: description
# 2: both (if the description exists, it will show description, or show the auto_excerpt)
# 3: auto_excerpt (default)
# false: do not show the article introduction
index_post_content:
  method: 3
  length: 500 # if you set method to 2 or 3, the length need to config

# anchor
# when you scroll in post, the URL will update according to header id.
anchor: false

# Post
# --------------------------------------

# toc (目錄)
toc:
  post: true
  page: false
  number: true
  expand: false
  style_simple: false # for post

post_copyright:
  enable: true
  decode: false
  license: CC BY-NC-SA 4.0
  license_url: https://creativecommons.org/licenses/by-nc-sa/4.0/

# Sponsor/reward
reward:
  enable: false
  QR_code:
    # - img: /img/wechat.jpg
    #   link:
    #   text: wechat
    # - img: /img/alipay.jpg
    #   link:
    #   text: alipay

# Post edit
# Easily browse and edit blog source code online.
post_edit:
  enable: false
  # url: https://github.com/user-name/repo-name/edit/branch-name/subdirectory-name/
  # For example: https://github.com/jerryc127/butterfly.js.org/edit/main/source/
  url:

# Related Articles
related_post:
  enable: true
  limit: 6 # Number of posts displayed
  date_type: created # or created or updated 文章日期顯示創建日或者更新日

# figcaption (圖片描述文字)
photofigcaption: false

# post_pagination (分頁)
# value: 1 || 2 || false
# 1: The 'next post' will link to old post
# 2: The 'next post' will link to new post
# false: disable pagination
post_pagination: 1

# Displays outdated notice for a post (文章過期提醒)
noticeOutdate:
  enable: false
  style: flat # style: simple/flat
  limit_day: 500 # When will it be shown
  position: top # position: top/bottom
  message_prev: It has been
  message_next: days since the last update, the content of the article may be outdated.

# Share System (分享功能)
# --------------------------------------

# AddThis
# https://www.addthis.com/
addThis:
  enable: false
  pubid:

# Share.js
# https://github.com/overtrue/share.js
sharejs:
  enable: true
  sites: facebook,twitter,wechat,weibo,qq

# AddToAny
# https://www.addtoany.com/
addtoany:
  enable: false
  item: facebook,twitter,wechat,sina_weibo,facebook_messenger,email,copy_link

# Comments System
# --------------------------------------

comments:
  # Up to two comments system, the first will be shown as default
  # Choose: Disqus/Disqusjs/Livere/Gitalk/Valine/Waline/Utterances/Facebook Comments/Twikoo/Giscus
  use: # Valine,Disqus
  text: true # Display the comment name next to the button
  # lazyload: The comment system will be load when comment element enters the browser's viewport.
  # If you set it to true, the comment count will be invalid
  lazyload: false
  count: false # Display comment count in post's top_img
  card_post_count: false # Display comment count in Home Page

# disqus
# https://disqus.com/
disqus:
  shortname:
  apikey: # For newest comments widget

# Alternative Disqus - Render comments with Disqus API
# DisqusJS 評論系統，可以實現在網路審查地區載入 Disqus 評論列表，兼容原版
# https://github.com/SukkaW/DisqusJS
disqusjs:
  shortname:
  apikey:
  option:

# livere (來必力)
# https://www.livere.com/
livere:
  uid:

# gitalk
# https://github.com/gitalk/gitalk
gitalk:
  client_id:
  client_secret:
  repo:
  owner:
  admin:
  option:

# valine
# https://valine.js.org
valine:
  appId: # leancloud application app id
  appKey: # leancloud application app key
  avatar: monsterid # gravatar style https://valine.js.org/#/avatar
  serverURLs: # This configuration is suitable for domestic custom domain name users, overseas version will be automatically detected (no need to manually fill in)
  bg: # valine background
  visitor: false
  option:

# waline - A simple comment system with backend support fork from Valine
# https://waline.js.org/
waline:
  serverURL: # Waline server address url
  bg: # waline background
  visitor: false
  option:

# utterances
# https://utteranc.es/
utterances:
  repo:
  # Issue Mapping: pathname/url/title/og:title
  issue_term: pathname
  # Theme: github-light/github-dark/github-dark-orange/icy-dark/dark-blue/photon-dark
  light_theme: github-light
  dark_theme: photon-dark

# Facebook Comments Plugin
# https://developers.facebook.com/docs/plugins/comments/
facebook_comments:
  app_id:
  user_id: # optional
  pageSize: 10 # The number of comments to show
  order_by: social # social/time/reverse_time
  lang: en_US # Language en_US/zh_CN/zh_TW and so on

# Twikoo
# https://github.com/imaegoo/twikoo
twikoo:
  envId:
  region:
  visitor: false
  option:

# Giscus
# https://giscus.app/
giscus:
  repo:
  repo_id:
  category_id:
  theme:
    light: light
    dark: dark
  option:

# Chat Services
# --------------------------------------

# Chat Button [recommend]
# It will create a button in the bottom right corner of website, and hide the origin button
chat_btn: false

# The origin chat button is displayed when scrolling up, and the button is hidden when scrolling down
chat_hide_show: false

# chatra
# https://chatra.io/
chatra:
  enable: false
  id:

# tidio
# https://www.tidio.com/
tidio:
  enable: false
  public_key:

# daovoice
# http://daovoice.io/
daovoice:
  enable: false
  app_id:

# gitter
# https://gitter.im/
gitter:
  enable: false
  room:

# crisp
# https://crisp.chat/en/
crisp:
  enable: false
  website_id:

# Footer Settings
# --------------------------------------
footer:
  owner:
    enable: true
    since: 2020
  custom_text:
  copyright: true # Copyright of theme and framework

# Analysis
# --------------------------------------

# Baidu Analytics
# https://tongji.baidu.com/web/welcome/login
baidu_analytics:

# Google Analytics
# https://analytics.google.com/analytics/web/
google_analytics:

# CNZZ Analytics
# https://www.umeng.com/
cnzz_analytics:

# Cloudflare Analytics
# https://www.cloudflare.com/zh-tw/web-analytics/
cloudflare_analytics:

# Microsoft Clarity
# https://clarity.microsoft.com/
microsoft_clarity:

# Advertisement
# --------------------------------------

# Google Adsense (谷歌廣告)
google_adsense:
  enable: false
  auto_ads: true
  js: https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js
  client:
  enable_page_level_ads: true

# Insert ads manually (手動插入廣告)
# ad:
#   index:
#   aside:
#   post:

# Verification (站長驗證)
# --------------------------------------

site_verification:
  # - name: google-site-verification
  #   content: xxxxxx
  # - name: baidu-site-verification
  #   content: xxxxxxx

# Beautify/Effect (美化/效果)
# --------------------------------------

# Theme color for customize
# Notice: color value must in double quotes like "#000" or may cause error!

# theme_color:
#   enable: true
#   main: "#49B1F5"
#   paginator: "#00c4b6"
#   button_hover: "#FF7242"
#   text_selection: "#00c4b6"
#   link_color: "#99a9bf"
#   meta_color: "#858585"
#   hr_color: "#A4D8FA"
#   code_foreground: "#F47466"
#   code_background: "rgba(27, 31, 35, .05)"
#   toc_color: "#00c4b6"
#   blockquote_padding_color: "#49b1f5"
#   blockquote_background_color: "#49b1f5"
#   scrollbar_color: "#49b1f5"

# The top_img settings of home page
# default: top img - full screen, site info - middle (默認top_img全屏，site_info在中間)
# The position of site info, eg: 300px/300em/300rem/10% (主頁標題距離頂部距離)
index_site_info_top:
# The height of top_img, eg: 300px/300em/300rem (主頁top_img高度)
index_top_img_height:

# The user interface setting of category and tag page (category和tag頁的UI設置)
# index - same as Homepage UI (index 值代表 UI將與首頁的UI一樣)
# default - same as archives UI 默認跟archives頁面UI一樣
category_ui: # 留空或 index
tag_ui: # 留空或 index

# Website Background (設置網站背景)
# can set it to color or image (可設置圖片 或者 顔色)
# The formal of image: url(http://xxxxxx.com/xxx.jpg)
background:

# Footer Background
footer_bg: false

# the position of bottom right button/default unit: px (右下角按鈕距離底部的距離/默認單位為px)
rightside-bottom:

# Enter transitions (開啓網頁進入效果)
enter_transitions: true

# Background effects (背景特效)
# --------------------------------------

# canvas_ribbon (靜止彩帶背景)
# See: https://github.com/hustcc/ribbon.js
canvas_ribbon:
  enable: false
  size: 150
  alpha: 0.6
  zIndex: -1
  click_to_change: false
  mobile: false

# Fluttering Ribbon (動態彩帶)
canvas_fluttering_ribbon:
  enable: false
  mobile: false

# canvas_nest
# https://github.com/hustcc/canvas-nest.js
canvas_nest:
  enable: false
  color: '0,0,255' #color of lines, default: '0,0,0'; RGB values: (R,G,B).(note: use ',' to separate.)
  opacity: 0.7 # the opacity of line (0~1), default: 0.5.
  zIndex: -1 # z-index property of the background, default: -1.
  count: 99 # the number of lines, default: 99.
  mobile: false

# Typewriter Effect (打字效果)
# https://github.com/disjukr/activate-power-mode
activate_power_mode:
  enable: false
  colorful: true # open particle animation (冒光特效)
  shake: true #  open shake (抖動特效)
  mobile: false

# Mouse click effects: fireworks (鼠標點擊效果: 煙火特效)
fireworks:
  enable: false
  zIndex: 9999 # -1 or 9999
  mobile: false

# Mouse click effects: Heart symbol (鼠標點擊效果: 愛心)
click_heart:
  enable: false
  mobile: false

# Mouse click effects: words (鼠標點擊效果: 文字)
ClickShowText:
  enable: false
  text:
    # - I
    # - LOVE
    # - YOU
  fontSize: 15px
  random: false
  mobile: false

# Default display mode (網站默認的顯示模式)
# light (default) / dark
display_mode: light

# Beautify (美化頁面顯示)
beautify:
  enable: false
  field: post # site/post
  title-prefix-icon: # '\f0c1'
  title-prefix-icon-color: # '#F47466'

# Global font settings
# Don't modify the following settings unless you know how they work (非必要不要修改)
font:
  global-font-size:
  code-font-size:
  font-family:
  code-font-family:

# Font settings for the site title and site subtitle
# 左上角網站名字 主頁居中網站名字
blog_title_font:
  font_link:
  font-family:

# The setting of divider icon (水平分隔線圖標設置)
hr_icon:
  enable: true
  icon: # the unicode value of Font Awesome icon, such as '\3423'
  icon-top:

# the subtitle on homepage (主頁subtitle)
subtitle:
  enable: false
  # Typewriter Effect (打字效果)
  effect: true
  # loop (循環打字)
  loop: true
  # source 調用第三方服務
  # source: false 關閉調用
  # source: 1  調用一言網的一句話（簡體） https://hitokoto.cn/
  # source: 2  調用一句網（簡體） http://yijuzhan.com/
  # source: 3  調用今日詩詞（簡體） https://www.jinrishici.com/
  # subtitle 會先顯示 source , 再顯示 sub 的內容
  source: false
  # 如果關閉打字效果，subtitle 只會顯示 sub 的第一行文字
  sub:

# Loading Animation (加載動畫)
preloader: false

# aside (側邊欄)
# --------------------------------------

aside:
  enable: true
  hide: false
  button: true
  mobile: true # display on mobile
  position: right # left or right
  card_author:
    enable: true
    description:
    button:
      enable: true
      icon: fab fa-github
      text: Follow Me
      link: https://github.com/xxxxxx
  card_announcement:
    enable: true
    content: This is my Blog
  card_recent_post:
    enable: true
    limit: 5 # if set 0 will show all
    sort: date # date or updated
    sort_order: # Don't modify the setting unless you know how it works
  card_categories:
    enable: true
    limit: 8 # if set 0 will show all
    expand: none # none/true/false
    sort_order: # Don't modify the setting unless you know how it works
  card_tags:
    enable: true
    limit: 40 # if set 0 will show all
    color: false
    sort_order: # Don't modify the setting unless you know how it works
  card_archives:
    enable: true
    type: monthly # yearly or monthly
    format: MMMM YYYY # eg: YYYY年MM月
    order: -1 # Sort of order. 1, asc for ascending; -1, desc for descending
    limit: 8 # if set 0 will show all
    sort_order: # Don't modify the setting unless you know how it works
  card_webinfo:
    enable: true
    post_count: true
    last_push_date: true
    sort_order: # Don't modify the setting unless you know how it works

# busuanzi count for PV / UV in site
# 訪問人數
busuanzi:
  site_uv: true
  site_pv: true
  page_pv: true

# Time difference between publish date and now (網頁運行時間)
# Formal: Month/Day/Year Time or Year/Month/Day Time
runtimeshow:
  enable: false
  publish_date:

# Aside widget - Newest Comments
newest_comments:
  enable: false
  sort_order: # Don't modify the setting unless you know how it works
  limit: 6
  storage: 10 # unit: mins, save data to localStorage
  avatar: true

# Bottom right button (右下角按鈕)
# --------------------------------------

# Conversion between Traditional and Simplified Chinese (簡繁轉換)
translate:
  enable: false
  # The text of a button
  default: 繁
  # the language of website (1 - Traditional Chinese/ 2 - Simplified Chinese）
  defaultEncoding: 2
  # Time delay
  translateDelay: 0
  # The text of the button when the language is Simplified Chinese
  msgToTraditionalChinese: '繁'
  # The text of the button when the language is Traditional Chinese
  msgToSimplifiedChinese: '簡'

# Read Mode (閲讀模式)
readmode: true

# dark mode
darkmode:
  enable: true
  # Toggle Button to switch dark/light mode
  button: true
  # Switch dark/light mode automatically (自動切換 dark mode和 light mode)
  # autoChangeMode: 1  Following System Settings, if the system doesn't support dark mode, it will switch dark mode between 6 pm to 6 am
  # autoChangeMode: 2  Switch dark mode between 6 pm to 6 am
  # autoChangeMode: false
  autoChangeMode: false

# Don't modify the following settings unless you know how they work (非必要請不要修改 )
# Choose: readmode,translate,darkmode,hideAside,toc,chat,comment
# Don't repeat 不要重複
rightside_item_order:
  enable: false
  hide: # readmode,translate,darkmode,hideAside
  show: # toc,chat,comment

# Lightbox (圖片大圖查看模式)
# --------------------------------------
# You can only choose one, or neither (只能選擇一個 或者 兩個都不選)

# medium-zoom
# https://github.com/francoischalifour/medium-zoom
medium_zoom: false

# fancybox
# http://fancyapps.com/fancybox/3/
fancybox: true

# Tag Plugins settings (標籤外掛)
# --------------------------------------

# mermaid
# see https://github.com/mermaid-js/mermaid
mermaid:
  enable: false
  # built-in themes: default/forest/dark/neutral
  theme:
    light: default
    dark: dark

# Note (Bootstrap Callout)
note:
  # Note tag style values:
  #  - simple    bs-callout old alert style. Default.
  #  - modern    bs-callout new (v2-v3) alert style.
  #  - flat      flat callout style with background, like on Mozilla or StackOverflow.
  #  - disabled  disable all CSS styles import of note tag.
  style: flat
  icons: true
  border_radius: 3
  # Offset lighter of background in % for modern and flat styles (modern: -12 | 12; flat: -18 | 6).
  # Offset also applied to label tag variables. This option can work with disabled note tag.
  light_bg_offset: 0

# other
# --------------------------------------

# Pjax
# It may contain bugs and unstable, give feedback when you find the bugs.
# https://github.com/MoOx/pjax
pjax:
  enable: false
  exclude:
    # - xxxx
    # - xxxx

# Inject the css and script (aplayer/meting)
aplayerInject:
  enable: false
  per_page: true

# Snackbar (Toast Notification 彈窗)
# https://github.com/polonel/SnackBar
# position 彈窗位置
# 可選 top-left / top-center / top-right / bottom-left / bottom-center / bottom-right
snackbar:
  enable: false
  position: bottom-left
  bg_light: '#49b1f5' # The background color of Toast Notification in light mode
  bg_dark: '#1f1f1f' # The background color of Toast Notification in dark mode

# https://instant.page/
# prefetch (預加載)
instantpage: false

# https://github.com/vinta/pangu.js
# Insert a space between Chinese character and English character (中英文之間添加空格)
pangu:
  enable: false
  field: site # site/post

# Lazyload (圖片懶加載)
# https://github.com/verlok/vanilla-lazyload
lazyload:
  enable: false
  field: site # site/post
  placeholder:
  blur: false

# PWA
# See https://github.com/JLHwung/hexo-offline
# ---------------
# pwa:
#   enable: false
#   manifest: /pwa/manifest.json
#   apple_touch_icon: /pwa/apple-touch-icon.png
#   favicon_32_32: /pwa/32.png
#   favicon_16_16: /pwa/16.png
#   mask_icon: /pwa/safari-pinned-tab.svg

# Open graph meta tags
# https://developers.facebook.com/docs/sharing/webmasters/
Open_Graph_meta: true

# Add the vendor prefixes to ensure compatibility
css_prefix: true

# Inject
# Insert the code to head (before '' tag) and the bottom (before '' tag)
# 插入代码到头部  之前 和 底部  之前
inject:
  head:
    # - 
  bottom:
    # - <script src="xxxx"></script>

# CDN
# Don't modify the following settings unless you know how they work
# 非必要請不要修改
CDN:
  # main
  main_css:
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---

title: java8函数式编程
date: 2022-02-19 14:17:52
cover: /res/java8函数式编程/cover.jpg
tags: [java,java8,函数式编程]
categories:
    - java
---
**什么是函数式编程?**

百度百科： 函数式编程，是一种编程范式，它将电脑运算视为函数运算，并且避免使用程序状态以及易变对象。其中，λ演算为该语言最重要的基础。而且，λ演算的函数可以接受函数作为输入参数和输出返回值。

讲人话： 函数式编程一种编程范式,允许使用一种表达式（lambda表达式）来表示一个函数

# 函数式接口

> Java语言中函数式编程，通过Java8版本提供的`函数式接口规范`来实现,它指的是有且只有一个未实现的方法的接口，一般通过`FunctionalInterface`这个注解来表明某个接口是一个函数式接口(非必须，如果标注了该注解，编译器检查接口是否符合函数式接口规范，如出现两个及以上数量的普通方法，编译器会报错)。函数式接口是Java支持函数式编程的基础。java8及以上版本才支持。



## 入门

使用Consumer作为示例，它是一个函数式接口，包含一个抽象方法accept，这个方法只有输入而无输出。
现在我们要定义一个Consumer对象，传统的方式是这样定义的：

```java
Consumer c = new Consumer() {
    @Override
    public void accept(Object o) {
        System.out.println(o);
    }
};
```

而在Java8中，针对函数式编程接口，可以这样定义：

```java
Consumer c = (o) -> {
    System.out.println(o);
};  
```



上面已说明，函数式编程接口都只有一个抽象方法，因此在采用这种写法时，编译器会将这段函数编译后当作该抽象方法的实现。
如果接口有多个抽象方法，编译器就不知道这段函数应该是实现哪个方法的了。
因此，=后面的函数体我们就可以看成是accept函数的实现。

- 输入：->前面的部分，即被()包围的部分。此处只有一个输入参数，实际上输入是可以有多个的，如两个参数时写法：(a, b);当然也可以没有输入，此时直接就可以是()
- 函数体：->后面的部分，即被{}包围的部分；可以是一段代码。
- 输出：函数式编程可以没有返回值，也可以有返回值。如果有返回值时，需要代码段的最后一句通过return的方式返回对应的值

当函数体中只有一个语句时，可以去掉{}进一步简化：

```java
Consumer c = (o) -> System.out.println(o);
```

然而这还不是最简的，由于此处只是进行打印，调用了System.out中的println静态方法对输入参数直接进行打印，因此可以简化成以下写法：

```java
Consumer c = System.out::println;
```

它表示的意思就是针对输入的参数将其调用System.out中的静态方法println进行打印。
到这一步就可以感受到函数式编程的强大能力。
通过最后一段代码，我们可以简单的理解函数式编程，Consumer接口直接就可以当成一个函数了，这个函数接收一个输入参数，然后针对这个输入进行处理；当然其本质上仍旧是一个对象，但我们已经省去了诸如老方式中的对象定义过程，直接使用一段代码来给函数式接口对象赋值。
而且最为关键的是，这个函数式对象因为本质上仍旧是一个对象，因此可以做为其它方法的参数或者返回值，可以与原有的代码实现无缝集成！

## Java预设函数式接口

> 下面对Java中的几个预先定义的函数式接口及其经常使用的类进行分析学习。在`java.util.function`包下提供了很多函数式接口，这里挑几个常用的进行讲解。

### Consumer

Consumer是一个函数式编程接口； 顾名思义，Consumer的意思就是消费，即针对某个东西我们来使用它，因此它包含有一个有输入而无输出的accept接口方法；
除accept方法，它还包含有andThen这个方法；
其定义如下：

```java
default Consumer andThen(Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> { accept(t); after.accept(t); };
}
```

可见这个方法就是指定在调用当前Consumer后是否还要调用其它的Consumer；
使用示例：

```java
public static void consumerTest() {
    Consumer f = System.out::println;
    Consumer f2 = n -> System.out.println(n + "-F2");

    //执行完F后再执行F2的Accept方法
    f.andThen(f2).accept("test");

    //连续执行F的Accept方法
    f.andThen(f).andThen(f).andThen(f).accept("test1");
}
```



### Function

Function也是一个函数式编程接口；它代表的含义是“函数”，而函数经常是有输入输出的，因此它含有一个apply方法，包含一个输入与一个输出；
除apply方法外，它还有compose与andThen及indentity三个方法，其使用见下述示例；

```java
/**
 * Function测试
 */
public static void functionTest() {
    Function f = s -> s++;
    Function g = s -> s * 2;

    /**
     * 下面表示在执行F时，先执行G，并且执行F时使用G的输出当作输入。
     * 相当于以下代码：
     * Integer a = g.apply(1);
     * System.out.println(f.apply(a));
     */
    System.out.println(f.compose(g).apply(1));

    /**
     * 表示执行F的Apply后使用其返回的值当作输入再执行G的Apply；
     * 相当于以下代码
     * Integer a = f.apply(1);
     * System.out.println(g.apply(a));
     */
    System.out.println(f.andThen(g).apply(1));

    /**
     * identity方法会返回一个不进行任何处理的Function，即输出与输入值相等； 
     */
    System.out.println(Function.identity().apply("a"));
}
```



### Predicate

Predicate为函数式接口，predicate的中文意思是“断定”，即判断的意思，判断某个东西是否满足某种条件； 因此它包含test方法，根据输入值来做逻辑判断，其结果为True或者False。
它的使用方法示例如下：

```
/**
 * Predicate测试
 */
private static void predicateTest() {
    Predicate p = o -> o.equals("test");
    Predicate g = o -> o.startsWith("t");

    /**
     * negate: 用于对原来的Predicate做取反处理；
     * 如当调用p.test("test")为True时，调用p.negate().test("test")就会是False；
     */
    Assert.assertFalse(p.negate().test("test"));

    /**
     * and: 针对同一输入值，多个Predicate均返回True时返回True，否则返回False；
     */
    Assert.assertTrue(p.and(g).test("test"));

    /**
     * or: 针对同一输入值，多个Predicate只要有一个返回True则返回True，否则返回False
     */
    Assert.assertTrue(p.or(g).test("ta"));
}
```

# Stream

> Stream可以对多个元素进行一系列的操作，也可以支持对某些操作进行并发处理。Java8中对集合、数组进行了stream的实现，方便开发人员通过stream对集合或数组进行数据操作。



##  Stream对象的创建

Stream对象的创建途径有以下几种

a. 创建空的Stream对象

```java
Stream stream = Stream.empty();
```

b. 通过集合类中的stream或者parallelStream方法创建；

```java
Stream stream = Arrays.stream(new String[]{"1", "2", "3"});
List list = Arrays.asList("a", "b", "c", "d");
Stream listStream = list.stream();                   //获取串行的Stream对象
Stream parallelListStream = list.parallelStream();   //获取并行的Stream对象  
```

c. 通过Stream中的of方法创建：

```
Stream s = Stream.of("test");
Stream s1 = Stream.of("a", "b", "c", "d");
```

d. 通过Stream中的iterate方法创建：
iterate方法有两个不同参数的方法：

```java
public static Stream iterate(final T seed, final UnaryOperator f);  
public static Stream iterate(T seed, Predicate<? super T> hasNext, UnaryOperator next)
```

其中第一个方法将会返回一个无限有序值的Stream对象：它的第一个元素是seed，第二个元素是f.apply(seed); 第N个元素是f.apply(n-1个元素的值)；生成无限值的方法实际上与Stream的中间方法类似，在遇到中止方法前一般是不真正的执行的。因此无限值的这个方法一般与limit等方法一起使用，来获取前多少个元素。
当然获取前多少个元素也可以使用第二个方法。
第二个方法与第一个方法生成元素的方式类似，不同的是它返回的是一个有限值的Stream；中止条件是由hasNext来断定的。

第二种方法的使用示例如下：

```
/**
 * 本示例表示从1开始组装一个序列，第一个是1，第二个是1+1即2，第三个是2+1即3..，直接10时中止；
 * 也可简化成以下形式：
 *        Stream.iterate(1,
 *        n -> n <= 10,
 *        n -> n+1).forEach(System.out::println);
 * 写成以下方式是为简化理解
 */
Stream.iterate(1,
        new Predicate() {
            @Override
            public boolean test(Integer integer) {
                return integer <= 10;
            }
        },
    new UnaryOperator() {
        @Override
        public Integer apply(Integer integer) {
            return integer+1;
        }
}).forEach(System.out::println);
```

e. 通过Stream中的generate方法创建
与iterate中创建无限元素的Stream类似，不过它的每个元素与前一元素无关，且生成的是一个无序的队列。也就是说每一个元素都可以随机生成。因此一般用来创建常量的Stream以及随机的Stream等。
示例如下：

```
/**
 * 随机生成10个Double元素的Stream并将其打印
 */
Stream.generate(new Supplier() {
    @Override
    public Double get() {
        return Math.random();
    }
}).limit(10).forEach(System.out::println);

//上述写法可以简化成以下写法：
Stream.generate(() -> Math.random()).limit(10).forEach(System.out::println);
```

## Stream对象的使用

Stream对象提供多个非常有用的方法，这些方法可以分成两类：
中间操作：将原始的Stream转换成另外一个Stream；如filter返回的是过滤后的Stream。
终端操作：产生的是一个结果或者其它的复合操作；如count或者forEach操作。

下面就几个比较常用的方法举例说明其用法：

### filter

用于对Stream中的元素进行过滤，返回一个过滤后的Stream
其方法定义如下



```java
Stream filter(Predicate<? super T> predicate);
```

使用示例：

```
Stream s = Stream.of("test", "t1", "t2", "teeeee", "aaaa");
//查找所有包含t的元素并进行打印
s.filter(n -> n.contains("t")).forEach(System.out::println);
```

###  map

元素一对一转换。
它接收一个Funcation参数，用其对Stream中的所有元素进行处理，返回的Stream对象中的元素为Function对原元素处理后的结果
其方法定义如下：

```
 Stream map(Function<? super T, ? extends R> mapper);
```

示例，假设我们要将一个String类型的Stream对象中的每个元素添加相同的后缀.txt，如a变成a.txt，其写法如下：

```
Stream s = Stream.of("test", "t1", "t2", "teeeee", "aaaa");
s.map(n -> n.concat(".txt")).forEach(System.out::println);
```

### flatMap

元素一对多转换：对原Stream中的所有元素使用传入的Function进行处理，每个元素经过处理后生成一个多个元素的Stream对象，然后将返回的所有Stream对象中的所有元素组合成一个统一的Stream并返回；
方法定义如下：

```
 Stream flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);
```

示例，假设要对一个String类型的Stream进行处理，将每一个元素的拆分成单个字母，并打印：

```java
Stream s = Stream.of("test", "t1", "t2", "teeeee", "aaaa");
s.flatMap(n -> Stream.of(n.split(""))).forEach(System.out::println);
```