链接
    public class ListGroupDemo {

    1. static class Product {<br />        private long id; // 商品Id<br />        private String name;// 商品名称<br />        private byte type;// 商品类型
    3.     public Product(long id, String name, byte type) {<br />            this.id = id;<br />            this.name = name;<br />            this.type = type;<br />        }<br />       <br />        ...<br />       <br />        @Override<br />        public String toString() {<br />            return "Product{" +<br />                    "id=" + id +<br />                    ", name='" + name + '\'' +<br />                    ", type=" + type +<br />                    '}';<br />        }<br />    }
    5. public static void main(String[] args) {<br />        // 元数据<br />        Product product1 = new Product(1001L, "Java", (byte) 10);<br />        Product product2 = new Product(1002L, "Python", (byte) 20);<br />        Product product3 = new Product(1002L, "Python", (byte) 20);<br />        Product product4 = new Product(1003L, "Netty", (byte) 30);<br />        Product product5 = new Product(1003L, "Netty", (byte) 30);<br />        Product product6 = new Product(1003L, "Netty", (byte) 30);<br />        Product product7 = new Product(1002L, "Python", (byte) 20);<br />        Product product8 = new Product(1004L, "Tomcat", (byte) 40);<br />        // 存入一个List<br />        List<Product> products = Arrays.asList(new Product[]{product1, product2, product3, product4, product5, product6, product7, product8});
    8.     // 以id进行分组<br />        Map<Long, List<Product>> map = new ConcurrentHashMap<>();<br />        for (int i = 0; i < products.size(); i++) {<br />            List<Product> productsTemp = map.get(products.get(i).getId());<br />            if (productsTemp == null) {<br />                productsTemp = new ArrayList<>();<br />                productsTemp.add(products.get(i));<br />                map.put(products.get(i).getId(), productsTemp);<br />            } else {<br />                productsTemp.add(products.get(i));<br />            }<br />        }<br />        // 遍历输出<br />        for (Long id : map.keySet()) {<br />            System.out.println(map.get(id));<br />        }<br />    }<br />}