文章目录

• • 一、Stream概述
  • 二、Stream的创建
  • 三、Stream的操作符
  • • 中间操作符
    • • • 1.filter
        • 2.distinct
        • 3.limit
        • 4.skip
        • 5.map
        • 6.flatmap
        • 7.sorted
    • 终止操作符
    • • • 1.anyMatch
        • 2.allMatch
        • 3.noneMatch
        • 4.findAny
        • 5.findFirst
        • 6.forEach
        • 7.collect
        • 8.reduce
        • 9.count
• `重要`

一、Stream概述

Java 8 是一个非常成功的版本，这个版本新增的Stream，配合同版本出现的 Lambda ，给我们操作集合（Collection）提供了极大的便利。

那么什么是Stream？

Stream将要处理的元素集合看作一种流，在流的过程中，借助Stream API对流中的元素进行操作，比如：筛选、排序、聚合等。

二、Stream的创建

list.stream()   //转换成流进行操作

三、Stream的操作符

stream的操作符可以分为两种：中间操作符和终止操作符

中间操作符

1.filter

 /     * 用于过滤不满足条件的元素     */    @Test    public void filter(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg"); List<String> filtered = list.stream().filter(str -> str.contains("a")).collect(Collectors.toList()); System.out.println(filtered);    }

2.distinct

/     * 用于去掉重复的元素     */    @Test    public void distinct(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<String> distincted = list.stream().distinct().collect(Collectors.toList()); System.out.println(distincted);    }

3.limit

/     * 用于获取前n个元素     */    @Test    public void limit(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<String> limited = list.stream().limit(3).collect(Collectors.toList()); System.out.println(limited);    }

4.skip

 /     * 用于获取除前n个元素以外的所有元素     */    @Test    public void skip(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<String> skiped = list.stream().skip(3).collect(Collectors.toList()); System.out.println(skiped);    }

5.map

/     * 用于对所有元素进行集体的操作     */    @Test    public void map(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<String> mapped = list.stream().map(str->"han Shen_"+str).collect(Collectors.toList()); System.out.println(mapped);    }

6.flatmap

/     * 用于对所有元素进行扁平化的操作     */    @Test    public void flatMap(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<Character> flatMaped = list.stream().flatMap(str -> javaStream.getCharacter(str)).collect(Collectors.toList()); System.out.println(flatMaped);    }    //创建了一个字符流转换的方法    public static Stream<Character> getCharacter(String str){ ArrayList<Character> list = new ArrayList<>(); for (Character character : str.toCharArray()) {  list.add(character); } return list.stream();    }

map和flatmap的区别：

7.sorted

/     * 用于对所有元素进行以a到z顺序进行排序     */    @Test    public void sorted(){ //---------------------------字母排序------------------------------------- List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); List<String> sorted = list.stream().sorted().collect(Collectors.toList());//正序 List<String> collect1 = list.stream().sorted(Collections.reverseOrder(Collator.getInstance(Locale.US))).collect(Collectors.toList()); System.out.println(sorted); System.out.println(collect1); //---------------------------汉字排序------------------------------------- List<String> list3 = Arrays.asList("吕布","刘备","阿斗","赵云","张辽","马超","法正"); List<String> sorted1 = list3.stream().sorted(Collator.getInstance(Locale.CHINA)).collect(Collectors.toList());//正序 List<String> collect2 = list3.stream().sorted(Collections.reverseOrder(Collator.getInstance(Locale.CHINA))).collect(Collectors.toList()); System.out.println(sorted1); System.out.println(collect2); //---------------------------数字排序------------------------------------- List<Integer> list1 = Arrays.asList(1,5,9,54,33,-52,54); List<Integer> collect = list1.stream().sorted().collect(Collectors.toList());//正序 Stream<Integer> sorted2 = list1.stream().sorted(Collections.reverseOrder());//反序 System.out.println(collect); sorted2.forEach(s-> System.out.print(s+","));    }

终止操作符

1.anyMatch

/     * 用于判断集合中是否有一个元素满足条件     */    @Test    public void anyMatch(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); boolean a = list.stream().anyMatch(s -> s.contains("a")); System.out.println(a);    }

2.allMatch

/     * 用于判断集合中是否所有元素都满足条件     */    @Test    public void allMatch(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); boolean a = list.stream().allMatch(s -> s.contains("a")); System.out.println(a);    }

3.noneMatch

/     * 用于判断集合中是否所有元素都不满足条件     */    @Test    public void noneMatch(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); boolean a = list.stream().noneMatch(s -> s.length()>4); System.out.println(a);    }

4.findAny

/     * 用于返回集合中的任意元素     */    @Test    public void findAny(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); for (int i=0;i<1000;i++){     Optional<String> any = list.parallelStream().findAny();     if (any.isPresent()) System.out.println(any.get()); }    }

5.findFirst

/     * 用于返回集合中的第一个元素     */    @Test    public void findFirst(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); Optional<String> first = list.stream().findFirst(); if (first.isPresent()) System.out.println(first.get());    }

6.forEach

/     * 用于循环     */    @Test    public void forEach(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); list.stream().forEach(s -> System.out.println(s));    }

7.collect

/     * 用于将流转换成其他类型的数据：list，map，set     */    @Test    public void collect(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); Set<String> collect = list.stream().collect(Collectors.toSet()); System.out.println(collect); //------------------------------------------------------------------- Map<String, Object> collect1 = list.stream().collect(Collectors.toMap(v -> "han_" + v, v -> v,(oldvalue, newvalue)->newvalue)); System.out.println(collect1);    }

8.reduce

/     * 将流中的元素反复的结合起来得到一个结果     */    @Test    public void reduce(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); Optional<String> reduce = list.stream().reduce((acc,item) -> {return (acc+item).replace("s","h");}); if(reduce.isPresent()) System.out.println(reduce.get());    }

9.count

/     * 获取集合中元素的数量     */    @Test    public void count(){ List<String> list = Arrays.asList("abc","sd","as","few","wth","dfg","as"); long count = list.stream().count(); System.out.println(count);    }

重要

isPresent();
为了判断查询的类对象是否存在，采用此方法
.isPresent()一般与.get()方法合用，当optional中存在course时，optional.isPresent()值为true，通过get（）方法返回对象。

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