別再寫一堆的 for 循環(huán)了！Java 8 中的 Stream 輕松遍歷樹(shù)形結(jié)構(gòu)，是真的牛逼！

可能平常會(huì)遇到一些需求，比如構(gòu)建菜單，構(gòu)建樹(shù)形結(jié)構(gòu)，數(shù)據(jù)庫(kù)一般就使用父id來(lái)表示，為了降低數(shù)據(jù)庫(kù)的查詢壓力，我們可以使用Java8中的Stream流一次性把數(shù)據(jù)查出來(lái)，然后通過(guò)流式處理。

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List

?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" style="color: rgb(58, 58, 58);" data-linktype="2">我們一起來(lái)看看，代碼實(shí)現(xiàn)為了實(shí)現(xiàn)簡(jiǎn)單，就模擬查看數(shù)據(jù)庫(kù)所有數(shù)據(jù)到List里面。

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List

?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" data-linktype="2">實(shí)體類：Menu.java

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" data-linktype="2">/**
?*?Menu
?*
?*?@author?lcry
?*?@date?2020/06/01?20:36
?*/
????@Data
????@Builder
????public?class?Menu?{
????/**
?????*?id
?????*/
?????public?Integer?id;
?????/**
?????*?名稱
?????*/
?????public?String?name;
?????/**
?????*?父id?，根節(jié)點(diǎn)為0
?????*/
?????public?Integer?parentId;
?????/**
?????*?子節(jié)點(diǎn)信息
?????*/
?????public?List?childList;


????public?Menu(Integer?id,?String?name,?Integer?parentId)?{
????????this.id?=?id;
????????this.name?=?name;
????????this.parentId?=?parentId;
????}
????
????public?Menu(Integer?id,?String?name,?Integer?parentId,?List?childList)?{
????????this.id?=?id;
????????this.name?=?name;
????????this.parentId?=?parentId;
????????this.childList?=?childList;
????}

}

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List

?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" data-linktype="2">遞歸組裝樹(shù)形結(jié)構(gòu)：

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" data-linktype="2">@Test
public?void?testtree(){
????//模擬從數(shù)據(jù)庫(kù)查詢出來(lái)
????List?menus?=?Arrays.asList(
????????????new?Menu(1,"根節(jié)點(diǎn)",0),
????????????new?Menu(2,"子節(jié)點(diǎn)1",1),
????????????new?Menu(3,"子節(jié)點(diǎn)1.1",2),
????????????new?Menu(4,"子節(jié)點(diǎn)1.2",2),
????????????new?Menu(5,"根節(jié)點(diǎn)1.3",2),
????????????new?Menu(6,"根節(jié)點(diǎn)2",1),
????????????new?Menu(7,"根節(jié)點(diǎn)2.1",6),
????????????new?Menu(8,"根節(jié)點(diǎn)2.2",6),
????????????new?Menu(9,"根節(jié)點(diǎn)2.2.1",7),
????????????new?Menu(10,"根節(jié)點(diǎn)2.2.2",7),
????????????new?Menu(11,"根節(jié)點(diǎn)3",1),
????????????new?Menu(12,"根節(jié)點(diǎn)3.1",11)
????);

????//獲取父節(jié)點(diǎn)
????List?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(
????????????(m)?->?{
????????????????m.setChildList(getChildrens(m,?menus));
????????????????return?m;
????????????}
????).collect(Collectors.toList());
????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");
????System.out.println(JSON.toJSON(collect));
}

/**
?*?遞歸查詢子節(jié)點(diǎn)
?*?@param?root??根節(jié)點(diǎn)
?*?@param?all???所有節(jié)點(diǎn)
?*?@return?根節(jié)點(diǎn)信息
?*/
private?List?getChildrens(Menu?root,?List?all)?{
????List?children?=?all.stream().filter(m?->?{
????????return?Objects.equals(m.getParentId(),?root.getId());
????}).map(
????????????(m)?->?{
????????????????m.setChildList(getChildrens(m,?all));
????????????????return?m;
????????????}
????).collect(Collectors.toList());
????return?children;
}

?collect?=?menus.stream().filter(m?->?m.getParentId()?==?0).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?menus));????????????????return?m;????????????}????).collect(Collectors.toList());????System.out.println("-------轉(zhuǎn)json輸出結(jié)果-------");????System.out.println(JSON.toJSON(collect));}/**?*?遞歸查詢子節(jié)點(diǎn)?*?@param?root??根節(jié)點(diǎn)?*?@param?all???所有節(jié)點(diǎn)?*?@return?根節(jié)點(diǎn)信息?*/private?List

?getChildrens(Menu?root,?List?all)?{????List?children?=?all.stream().filter(m?->?{????????return?Objects.equals(m.getParentId(),?root.getId());????}).map(????????????(m)?->?{????????????????m.setChildList(getChildrens(m,?all));????????????????return?m;????????????}????).collect(Collectors.toList());????return?children;}格式化打印結(jié)果：" linktype="text" imgurl="" imgdata="null" data-itemshowtype="0" tab="innerlink" style="color: rgb(58, 58, 58);" data-linktype="2">格式化打印結(jié)果：

原文鏈接：https://blog.csdn.net/qq_19244927/article/details/106481777/

版權(quán)聲明：本文為CSDN博主「Lcry」的原創(chuàng)文章，遵循CC 4.0 BY-SA版權(quán)協(xié)議，轉(zhuǎn)載請(qǐng)附上原文出處鏈接及本聲明。

Spring Cloud 爆高危漏洞，趕緊修復(fù)！

2021 年發(fā)生的 10 件技術(shù)大事??！

23 種設(shè)計(jì)模式實(shí)戰(zhàn)（很全）

換掉 Log4j2！tinylog 橫空出世

再見(jiàn)單身狗！Java 創(chuàng)建對(duì)象的 6 種方式

阿里為什么推薦使用 LongAdder？

重磅官宣：Redis 對(duì)象映射框架來(lái)了！！

別再寫爆爆爆炸類了，試試裝飾器模式！

程序員精通各種技術(shù)體系，45歲求職難！

Spring Boot 3.0 M1 發(fā)布，正式棄用 Java 8

Spring Boot 學(xué)習(xí)筆記，這個(gè)太全了！

關(guān)注Java技術(shù)?？锤喔韶?/strong>

獲取 Spring Boot 實(shí)戰(zhàn)筆記！