Stream

? ? ? ? 也叫Stream流，是Jdk8開始新增的一套API (java.util.stream.*)，可以用于操作集合或者數(shù)組的數(shù)據(jù)。

? ? ? ?Stream流大量的結(jié)合了Lambda的語(yǔ)法風(fēng)格來(lái)編程，提供了一種更加強(qiáng)大，更加簡(jiǎn)單的方式操作

public class Demo1 {public static void main(String[] args) {List<String> list = new ArrayList<>();list.add("張無(wú)忌");list.add("周芷若");list.add("趙敏");list.add("張強(qiáng)");list.add("張三豐");//需求:把集合中所有以"張"開頭，且是3個(gè)字的元素存儲(chǔ)到一個(gè)新的集合。//實(shí)現(xiàn)2: 使用Stream流方式實(shí)現(xiàn)List<String> list1 = list.stream().filter(name -> name.startsWith("張") ).filter(name -> name.length() == 3).collect(Collectors.toList());System.out.println(list1);}
}

Stream流的使用步驟?

1.? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?2.? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 3.

?? ?? ? ? ? ? ? ??

如何獲取Stream流?

如何獲取Stream流:Collection集合:單列集合都支持一個(gè)stream()方法,它可以直接獲取集合的Stream流數(shù)組:Arrays.stream(數(shù)組)零散數(shù)據(jù):Stream.of(T... values)Map雙列集合并沒有提供直接獲取Stream流的方法,他需要間接獲取

public class Demo2 {public static void main(String[] args) {//"玄奘", "悟空", "悟能", "悟凈"List<String> list = List.of("玄奘", "悟空", "悟能", "悟凈");//Collection集合: 單列集合都支持一個(gè)stream()方法,它可以直接獲取集合的Stream流list.stream().forEach(System.out::println);//數(shù)組:Arrays.stream(數(shù)組)System.out.println("-------------------------------------");String[] arr = {"玄奘", "悟空", "悟能", "悟凈"};Arrays.stream(arr).forEach(System.out::println);//零散數(shù)據(jù):Stream.of(T... values)System.out.println("-------------------------------------");Stream.of("玄奘", "悟空", "悟能", "悟凈").forEach(System.out::println);//Map:雙列集合并沒有提供直接獲取Stream流的方法,他需要間接獲取System.out.println("-------------------------------------");Map<String,String> map = new HashMap<>();map.put("001","玄奘");map.put("002","悟空");map.put("003","悟能");map.put("004","悟凈");//返回一個(gè)Set集合map.keySet().stream().forEach(System.out::println);map.values().stream().forEach(System.out::println);System.out.println("-------------------------------------");//這是一個(gè)整體EntrySetkeyvalue -> emap.entrySet().stream().forEach(e -> System.out.println(e.getValue() + e.getKey()));}
}

Stream流常見中間方法?

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public class Demo3 {public static void main(String[] args) {List<Integer> list = List.of(61, 57, 66, 77, 88, 44, 100, 89, 97, 47, 70);//需求1: 找出所有及格的分?jǐn)?shù),并打印System.out.println("=================");list.stream().filter(e -> e > 60).forEach(System.out::println);//需求2: 找出所有及格的分?jǐn)?shù), 正序排列, 打印輸出System.out.println("=================");list.stream().filter(e -> e > 60).sorted((o1, o2) -> o1 - o2).forEach(System.out::println);//需求3: 找出所有及格的分?jǐn)?shù), 倒序排列, 打印輸出System.out.println("=================");list.stream().filter(e -> e > 60).sorted((o1, o2) -> o2 - o1).forEach(System.out::println);//需求4: 找出所有及格的分?jǐn)?shù), 倒序排列, 取前3名, 打印輸出System.out.println("=================");list.stream().filter(e -> e > 60).sorted((o1, o2) -> o2 - o1).limit(3).forEach(System.out::println);//需求5: 找出所有及格的分?jǐn)?shù), 倒序排列, 取前4-6名, 打印輸出System.out.println("=================");list.stream().filter(e -> e > 60).sorted((o1, o2) -> o2 - o1).skip(3).limit(3).forEach(System.out::println);//需求6:  找出所有及格的分?jǐn)?shù), 倒序排列, 取前4-6名, 將每個(gè)人的分?jǐn)?shù)加10分, 打印輸出System.out.println("=================");list.stream().filter(e -> e > 60).sorted((o1, o2) -> o2 - o1).skip(3).limit(3).map(e -> e + 10).forEach(System.out::println);//需求7: 將下面兩個(gè)集合中的元素進(jìn)行合并去重System.out.println("=================");List<String> list1 = List.of("1","2","3","4");List<String> list2 = List.of("1","5","7","4");Stream.concat(list1.stream(),list2.stream()).distinct().forEach(System.out::println);}
}

?Stream流常見終結(jié)方法

? ? ? ? ? ? ? ? ? ? ?調(diào)用完成后，不會(huì)返回新Stream了，沒法繼續(xù)使用流了。?

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public class Demo4 {public static void main(String[] args) {List<Student> list = List.of(new Student("玄奘", 60, 165.5),new Student("悟空", 50, 175.5),new Student("悟能", 55, 145.5),new Student("悟凈", 40, 185.5));//1. 打印出集合中所有元素list.stream().forEach(System.out::println);//2. 統(tǒng)計(jì)出身高不足170的人數(shù)Stream<Student> stream = list.stream().filter(e -> e.getHeight() < 170);long count = stream.count();System.out.println(count);//3. 請(qǐng)找出年齡最大的對(duì)象, 并輸出(了解)Student student = list.stream().max((o1, o2) -> o1.getAge() - o2.getAge()).get();System.out.println(student);//4. 請(qǐng)找出身高最低的對(duì)象, 并輸出(了解)Student student2 = list.stream().min((o1, o2) -> Double.compare(o1.getHeight(),o2.getHeight())).get();System.out.println(student2);}
}class Student {private String name;private int age;private double height;public Student() {}public Student(String name, int age, double height) {this.name = name;this.age = age;this.height = height;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public double getHeight() {return height;}public void setHeight(double height) {this.height = height;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +", height=" + height +'}';}
}

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Stream流轉(zhuǎn)數(shù)組/集合?

public class Demo5 {public static void main(String[] args) {List<Teacher> list = List.of(new Teacher("玄奘", 60, 165.5),new Teacher("悟空", 50, 175.5),new Teacher("悟空", 50, 175.5),new Teacher("悟能", 55, 145.5),new Teacher("悟凈", 40, 185.5));//1. 請(qǐng)找出身高超過170的教師, 并放到一個(gè)新數(shù)組中Object[] objects = list.stream().filter(e -> e.getHeight() > 170).toArray();System.out.println(Arrays.toString(objects));Teacher[] teacher =  list.stream().filter(e -> e.getHeight() > 170)//len代表元素中的個(gè)數(shù).toArray(len -> new Teacher[len]);System.out.println(Arrays.toString(teacher));//2. 請(qǐng)找出身高超過170的教師, 并放到一個(gè)新List集合中List<Teacher> list1 = list.stream().filter(e -> e.getHeight() > 170).collect(Collectors.toList());System.out.println(list1);//3. 請(qǐng)找出身高超過170的教師, 并放到一個(gè)新Set集合中Set<Teacher> list2 = list.stream().filter(e -> e.getHeight() > 170).collect(Collectors.toSet());System.out.println(list2);//4. 請(qǐng)找出所有的教師的姓名和身高, 放到一個(gè)新Map集合中Map<String,Double> map = list.stream().distinct().collect(Collectors.toMap(e -> e.getName(), e-> e.getHeight()));System.out.println(map);}
}class Teacher {private String name;private int age;private double height;public Teacher() {}public Teacher(String name, int age, double height) {this.name = name;this.age = age;this.height = height;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public double getHeight() {return height;}public void setHeight(double height) {this.height = height;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Teacher teacher = (Teacher) o;return age == teacher.age && Double.compare(teacher.height, height) == 0 && Objects.equals(name, teacher.name);}@Overridepublic int hashCode() {return Objects.hash(name, age, height);}@Overridepublic String toString() {return "Teacher{" +"name='" + name + '\'' +", age=" + age +", height=" + height +'}';}
}

Map集合

1.Map集合稱為雙列集合，一次需要存一對(duì)數(shù)據(jù)做為一個(gè)元素, 格式：{key1=value1 ,? ? ? ? key2=value2 , key3=value3 , ...}

2.Map集合的所有鍵是不允許重復(fù)的，但值可以重復(fù)，鍵和值是一一對(duì)應(yīng)的，每一個(gè)鍵只能找到自己對(duì)應(yīng)的值

Map集合體系

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Map集合的實(shí)現(xiàn)類有哪些？各自的特點(diǎn)是？?

? ?1.HashMap: 無(wú)序，不重復(fù)

? ?2.LinkedHashMap: 有序，不重復(fù)

? ?3.TreeMap：排序，不重復(fù)?

?Map接口常用方法

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Map集合三種遍歷方式?

public class Demo2 {public static void main(String[] args) {//1. 創(chuàng)建mapHashMap<String, String> map = new HashMap<>();map.put("001", "玄奘");map.put("002", "悟空");map.put("003", "悟能");map.put("004", "悟凈");//2. 各種方式進(jìn)行遍歷test1(map);System.out.println("=================");test2(map);System.out.println("=================");test3(map);}//遍歷方式1: 先獲取Map集合全部的鍵，再通過遍歷鍵來(lái)找值private static void test1(HashMap<String, String> map) {Set<String> set = map.keySet();for (String s : set) {System.out.println(s + map.get(s));}}//遍歷方式2: 將map中的所有鍵值對(duì)放入一個(gè)set集合中, 然后遍歷set集合拿到每個(gè)鍵值對(duì), 再取里面的鍵值private static void test2(HashMap<String, String> map) {Set<Map.Entry<String, String>> entries = map.entrySet();for (Map.Entry<String, String> entry : entries) {System.out.println(entry.getKey() + entry.getValue());}}//遍歷方式3: Lambda, 使用foreach(BiConsumer bc)private static void test3(HashMap<String, String> map) {map.forEach((k,v) -> System.out.println(k+v));}
}

Map集合案例?

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現(xiàn)有字符串?dāng)?shù)組如下:String[] bookArr = {"《紅樓夢(mèng)》-曹雪芹","《西游記》-吳承恩","《三國(guó)演義》-羅貫中","《水滸傳》-施耐庵"};
需求:請(qǐng)將字符串中的書名提取為Map集合的鍵，將作者提取為Map集合的值并使用三種不同方式，遍歷Map集合打印鍵值對(duì)元素內(nèi)容

public class Demo3 {public static void main(String[] args) {Map<String,String> map = new HashMap<>();String[] bookArr = {"《紅樓夢(mèng)》-曹雪芹","《西游記》-吳承恩","《三國(guó)演義》-羅貫中","《水滸傳》-施耐庵"};for (String book : bookArr) {String[] split = book.split("-");map.put(split[0],split[1] );}System.out.println(map);Set<String> set = map.keySet();for (String key : set) {System.out.println(key + map.get(key));}System.out.println("----------------------------");Set<Map.Entry<String, String>> entries = map.entrySet();for (Map.Entry<String, String> entry : entries) {System.out.println(entry.getKey()+entry.getValue());}System.out.println("----------------------------");map.forEach((key , value) -> System.out.println(key+value));}
}

?HashMap底層原理

? ? ? 和HashSet一樣?:Day06List接口&Set接口&樹-CSDN博客https://blog.csdn.net/m0_60388241/article/details/133930070?spm=1001.2014.3001.5501

LinkedHashMap

底層數(shù)據(jù)結(jié)構(gòu)依然是基于哈希表實(shí)現(xiàn)的，只是每個(gè)鍵值對(duì)元素又額外的多了一個(gè)雙鏈表的機(jī)制記錄元素順序(保證有序)。?

TreeMap?

public class Demo6 {private String put;public static void main(String[] args) {//創(chuàng)建集合Map<Teacher, String> map = new TreeMap<>((o1, o2) -> o1.getAge() - o2.getAge());map.put(new Teacher("張三", 21), "河北");map.put(new Teacher("李四", 20), "山東");map.put(new Teacher("王五", 19), "山西");map.put(new Teacher("趙六", 21), "河南");map.forEach((k, v) -> {System.out.println(k + "-----" + v);});}
}class Teacher {private String name;private int age;public Teacher() {}public Teacher(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Teacher{" +"name='" + name + '\'' +", age=" + age +'}';}
}

?集合的嵌套

集合嵌套要求在程序中記住如下省份和其對(duì)應(yīng)的城市信息，記錄成功后，要求可以查詢出湖北省的城市信息。數(shù)據(jù)江蘇省 = "南京市","揚(yáng)州市","蘇州市","無(wú)錫市","常州市"湖北省 = "武漢市","孝感市","十堰市","宜昌市","鄂州市"河北省 = "石家莊市","唐山市","邢臺(tái)市","保定市","張家口市"分析:定義一個(gè)Map集合，鍵用表示省份名稱，值表示城市名稱，注意：城市會(huì)有多個(gè)。 Map<String,List<String>>根據(jù)“湖北省”這個(gè)鍵獲取對(duì)應(yīng)的值展示即可。

public class Demo7 {public static void main(String[] args) {Map<String, List<String>> map = new HashMap<>();List<String> list = List.of("南京市","揚(yáng)州市","蘇州市","無(wú)錫市","常州市");List<String> list2 = List.of("石家莊市","唐山市","邢臺(tái)市","保定市","張家口市");map.put("江蘇省",list);map.put("河北省",list2);System.out.println(map);}
}

?Collections

? 可變參數(shù)?

可變參數(shù)就是一種特殊形參，定義在方法、構(gòu)造器的形參列表里，格式是：數(shù)據(jù)類型... 參數(shù)名稱優(yōu)點(diǎn)特點(diǎn)：可以不傳數(shù)據(jù)給它；可以傳一個(gè)或者同時(shí)傳多個(gè)數(shù)據(jù)給它；也可以傳一個(gè)數(shù)組給它。好處：常常用來(lái)靈活的接收數(shù)據(jù)。注意事項(xiàng)1. 可變參數(shù)在方法內(nèi)部就是一個(gè)數(shù)組2. 一個(gè)形參列表中可變參數(shù)只能有一個(gè)3. 可變參數(shù)必須放在形參列表的最后面

public class Demo {public static void main(String[] args) {sum(1546);}//計(jì)算2個(gè)整數(shù)的和//計(jì)算3個(gè)整數(shù)的和//計(jì)算4個(gè)整數(shù)的和//計(jì)算n個(gè)整數(shù)的和public static void sum(int... a){for (int i : a) {System.out.println(i);}}
}

?Collections工具類

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public class Demo {public static void main(String[] args) {List<Integer> list = new ArrayList<>();//static <T> boolean addAll(單列集合，可變參數(shù))   批量添加元素Collections.addAll(list,1,2,100,3,45);System.out.println(list);//static void shuffle(List集合)    打亂List集合元素順序，每次調(diào)用都會(huì)打亂Collections.shuffle(list);System.out.println(list);//static <T> void  sort(List集合)   List集合進(jìn)行自然排序Collections.sort(list);System.out.println(list);//排自定義類對(duì)象，需要指定排序規(guī)則List<Student> stuList = new ArrayList<>();stuList.add(new Student("zhangsan", 18));stuList.add(new Student("wangwu", 22));stuList.add(new Student("zhaoliu", 21));stuList.add(new Student("lisi", 19));stuList.add(new Student("qianqi", 20));//static<T> void sort(List集合，比較器);List集合進(jìn)行比較器排序Collections.sort(stuList,(o1, o2) -> o1.getAge() - o2.getAge());System.out.println(stuList);}
}class Student {private String name;private int age;public Student() {}public Student(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +'}';}
}

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