Java函数式编程Stream.collect()方法使用小结

steam():把一个源数据，可以是集合，数组，I/O channel， 产生器generator 等，转化成流。
Collectors(): 类实现了很多归约操作，例如将流转换成集合和聚合元素。通过stream().collect()方法可简单获得我们所需要的数据结构。
先创建一个prodList的集合数据

@Data
public class Product {
    private Long id;
    private Integer num;
    private BigDecimal price;
    private String name;
    private String category;

   public Product(Long id, Integer num, BigDecimal price, String name, String category) {
        this.id = id;
        this.num = num;
        this.price = price;
        this.name = name;
        this.category = category;
    }
}

		Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
        Product prod2 = new Product(2L, 2, new BigDecimal("20"), "饼干", "零食");
        Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月饼", "零食");
        Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
        Product prod5 = new Product(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
        List<Product> prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);

1. 将集合里的某个参数拿出来重新生成一个集合，如下将prodList 里的name参数重新生成一个集合

List<String> nameList = prodList.stream().map(item -> item.getName()).collect(Collectors.toList());

2. 对集合里的数据进行处理，如下将集合里的青岛啤酒名称改为哈尔滨啤酒

public class test {
    public static void main(String[] args) {
        Product prod1 = new Product(1L, 1, new BigDecimal("15.5"), "面包", "零食");
        Product prod2 = new Product(2L, 2, new BigDecimal("20"), "饼干", "零食");
        Product prod3 = new Product(3L, 3, new BigDecimal("30"), "月饼", "零食");
        Product prod4 = new Product(4L, 3, new BigDecimal("10"), "青岛啤酒", "啤酒");
        Product prod5 = new Product(5L, 10, new BigDecimal("15"), "百威啤酒", "啤酒");
        List<Product> prodList = Lists.newArrayList(prod1, prod2, prod3, prod4, prod5);
        prodList = prodList.stream().map(T -> setData(T)).collect(Collectors.toList());
        for (Product product : prodList) {
            System.out.println("product="+product);
        }
    }

    private static Product setData(Product t) {
        if (StringUtils.equals(t.getName(), "青岛啤酒")) {
            t.setName("哈尔滨啤酒");
        }
        return t;
    }
}

3. 将集合需要的参数数据转化成Map，如下将id与name转化成Map

		// 转为Map
        Map<Long, String> idMap = prodList.stream().collect(Collectors.toMap(Product::getId, Product::getName));
        System.out.println("idMap="+idMap);

4. 将集合通过某个参数进行分组，对组织人员进行部门分组的时候有奇效，如下将对category分组

		// 分组
        Map<String, List<Product>> groupListMap = prodList.stream().collect(Collectors.groupingBy(Product::getCategory));
        System.out.println("nameList="+groupListMap);

		//输出结果为：
		nameList={啤酒=[Product(id=4, num=3, price=10, name=青岛啤酒, category=啤酒), Product(id=5, num=10, price=15, name=百威啤酒, category=啤酒)], 
				  零食=[Product(id=1, num=1, price=15.5, name=面包, category=零食), Product(id=2, num=2, price=20, name=饼干, category=零食), Product(id=3, num=3, price=30, name=月饼, category=零食)]}

5. 将集合通过某个参数进行拼接，如下将name进行拼接

		// 拼接
        String nameJoin = prodList.stream().map(Product::getName).collect(Collectors.joining(","));
        System.out.println("nameJoin="+nameJoin);

6. 统计集合的总数

		//统计总数
        Long count = prodList.stream().collect(Collectors.counting());
        System.out.println("count="+count);

7.将集合通过某个参数进行拼接（忽略空值）StringUtils.isNotEmpty(s)).collect(Collectors.joining("|")

 public static void main(String[] args) {
        List<SysAccount> list = new ArrayList<>();
        SysAccount sys = new SysAccount();
        sys.setQyWechatUserid("xxxx");
        list.add(sys);
        SysAccount sys2 = new SysAccount();
        sys2.setQyWechatUserid("");
        list.add(sys2);
        SysAccount sys1 = new SysAccount();
        sys1.setQyWechatUserid("wwww");
        list.add(sys1);
        for (SysAccount account : list) {
            System.out.println("QyWechatUserid="+account.getQyWechatUserid());
        }
        String useridText = list.stream().map(SysAccount::getQyWechatUserid).filter(s -> StringUtils.isNotEmpty(s)).collect(Collectors.joining("|"));
        System.out.println("useridText = " + useridText);
    }

8.过滤对象中的空数据字段.filter(s -> s.getName() != null && StringUtils.isNotBlank(s.getName()))

 List<BeanMap> collect = sysResourceBeans.stream().filter(s ->  s.getName() != null && StringUtils.isNotBlank(s.getName())).map(BeanMap::create).collect(Collectors.toList());

以上就是我在开发中常用到的Stream.collect()方法。