Java8中的Stream用好

川长思鸟来 2024-03-30 15:27 58阅读 0赞

stream作为java8的新特性，基于lambda表达式，是对集合对象功能的增强，它专注于对集合对象进行各种高效、便利的聚合操作或者大批量的数据操作，提高了编程效率和代码的可读性, stream的原理把处理的元素看做一种流，流在管道中传输，并且可以在管道的节点上处理，包括过滤筛选、去重、排序、聚合等。

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下来测试列举下常用的一些API：

定义一个实体类

@DatapublicclassOrder{
        private Integer goodsId;
        private String name;
        private BigDecimal price;
        private Integer num;
        public Order(Integer goodsId, String name, BigDecimal price, Integer num) {
            this.goodsId = goodsId;
            this.name = name;
            this.price = price;
            this.num = num;
        }
    }

初始化数据

publicclassTest {
        static List<Order> orderList = new ArrayList<Order>();//存放order对象集合static {
            Order order1 = new Order(1, "笔记本电脑", new BigDecimal("5345.25"), 2);
            Order order2 = new Order(2, "平板电脑", new BigDecimal("1999.35"), 4);
            Order order3 = new Order(3, "电视", new BigDecimal("2244.89"), 3);
            Order order4 = new Order(4, "手机", new BigDecimal("3699.99"), 6);
            orderList.add(order1);
            orderList.add(order2);
            orderList.add(order3);
            orderList.add(order4);
        }

**1.提取集合中的某个属性的值放到一个新的集合中**

@org.junit.Test
    publicvoidtest1(){
        List<String> orders = orderList.stream().map(s ->"商品名称:"+s.getName()).collect(Collectors.toList());
        System.out.println(orders);
    }

运行结果:

![edf37bb41d910d7975c7b65afc9bf023.png][]

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**2.distinct去重**

初始化数据加上一条重复的数据

![1b2de1605c1db619ad7bf1c6e36f8a9e.png][]

@org.junit.Test
    publicvoidtest2(){
        List<Order> orders = orderList.stream().distinct().collect(Collectors.toList());
        System.out.println(orders);
    }

去重后的结果：

\[Order(goodsId=1, name=笔记本电脑, price=5345.25, num=2), Order(goodsId=2, name=平板电脑, price=1999.35, num=4), Order(goodsId=3, name=电视, price=2244.89, num=3), Order(goodsId=4, name=手机, price=3699.99, num=6)\]

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**3.soterd排序**

@org.junit.Test
    public void test3(){
        orderList.stream()
                .sorted((order1,order2) ->Long.compare(order2.getGoodsId(), order1.getGoodsId()))
                .sorted((order1,order2) -> Integer.compare(order2.getNum(),order1.getNum()))
                .forEach(System.out::println);
    }

上面指定排序规则，先按照订单的id进行降序排序，再按照数量进行降序排序，运行结果：

![17399b4e3e4e4326f8d5a44a1e37ff60.jpeg][]

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**4.min(求最小值)**

@org.junit.Testpublic void test4(){
        Order minNum = orderList.stream().min(Comparator.comparingInt(Order::getNum)).get();
        System.out.println(minNum.toString());
    }

上面是求所有订单中中数量最小的一个，max同理，求最大值。

![5eb5895b488927fe8d77a8826c9186b2.png][]

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**5.anyMatch/allMatch/noneMatch（匹配）**

@org.junit.Test
    publicvoidtest5(){
        boolean anyMatch = orderList.stream().anyMatch(s ->"手机".equals(s.getName()));
        if (anyMatch) {
            System.out.println("订单中有手机");
        }
        boolean allMatch = orderList.stream().allMatch(s -> s.getNum()>=2);
        if (allMatch) {
            System.out.println("所有订单数量都大于等于2");
        }
        boolean noneMatch = orderList.stream().noneMatch(s -> "沙发".equals(s.getName()));
        if (noneMatch) {
            System.out.println("订单中没有沙发");
        }
    }

运行结果：

![b783e8e3794d1c5ba8b21d3291f00215.png][]

anyMatch：Stream 中任意一个元素符合传入的 predicate，返回 true

allMatch：Stream 中全部元素符合传入的 predicate，返回 true

noneMatch：Stream 中没有一个元素符合传入的 predicate，返回 true

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**6.limit（限制返回个数）**

@org.junit.Test
    public void test6(){
        List<String> list = Arrays.asList("4","3","2","1");
        list.stream().limit(2).forEach(System.out::println);
    }

![9e3fbc5c68db2bc7809546399f6708fc.png][]

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**7.skip(删除元素)**

@org.junit.Test
    public void test7(){
        List<String> list =  Arrays.asList("4","3","2","1");
        list.stream().skip(2).forEach(System.out::println);
    }

![5a62e08167a5b1fd656443d6d32439b9.png][]

--------------------

**8.reduce(聚合)**

@org.junit.Test
    publicvoidtest8(){
        List<String> list = Arrays.asList("繁","荣","昌","盛");
        String appendStr = list.stream().reduce("祝福祖国",(a,b) -> a+b);
        System.out.println(appendStr);
    }

![6270478f0636fde5437c824f006ef464.png][]

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[6270478f0636fde5437c824f006ef464.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/4573b453e8884b3ab33b0177112194a9.png