java - 使用 lambda 表达式将列表转为 Hashmap

Question

你能帮我用 ArrayList 中的对象构造 Hashmap 吗？

public class Store {

    private int storeId;
    private String storeName;
    private int deliveryDays;
    private List<String> servingZipCodes;
    private StoreAddress storeAddress;


    public Store() {
    }

    public Store(int storeId,String storeName,int deliveryDays,List<String> servingZipCodes,StoreAddress storeAddress){
        this.storeId = storeId;
        this.storeName=storeName;
        this.deliveryDays=deliveryDays;
        this.servingZipCodes=servingZipCodes;
        this.storeAddress = storeAddress;
    }

//getters and setters.
}

以及 StoreAddress 类

public class StoreAddress {

    private String streetAddress1;
    private String streetAddress2;
    private String streetAddress3;
    private String city;
    private String state;
    private String postalCode;
    private String country;
    private String phone;

    public StoreAddress() {
    }

    public StoreAddress(String streetAddress1, String streetAddress2, String streetAddress3, String city, String state, String postalCode, String country, String phone) {
        this.streetAddress1 = streetAddress1;
        this.streetAddress2 = streetAddress2;
        this.streetAddress3 = streetAddress3;
        this.city = city;
        this.state = state;
        this.postalCode = postalCode;
        this.country = country;
        this.phone = phone;
    }

这是用于测试的 Test 类。

public class Test {

    public static void main(String args[]){

        List<Store> storeList=new ArrayList();
        StoreAddress storeAddress1 = new StoreAddress("1500 Polaris Pkwy",null,null,"Columbus","OH","43240","US","9165452345");
        StoreAddress storeAddress2 = new StoreAddress("160 Easton Town Center",null,null,"Columbus","OH","43240","US","9165452345");
        storeList.add(new Store(1,"Store 1",7,new ArrayList<String>(Arrays.asList("43240","43241","43242")),storeAddress1));
        storeList.add(new Store(2,"Store 2",7,new ArrayList<String>(Arrays.asList("43240","43082","43081")),storeAddress2));
        Map<String,List<Store>> zipCodeStoreList = null;
        storeList.forEach(store -> {
            List<String> servingZipCodes = store.getServingZipCodes();
            servingZipCodes.stream().filter(x -> x.matches(store.getStoreAddress().getPostalCode().substring(0,5))).map(x ->new HashMap<String, Object>(){{
                put(x, store);
            }});
        });

虽然它在 Java 7 中是可能的，但在 Java 8 中寻找解决方案。

Key: 43240, value: Store1 , Store2
Key: 43241, value: null
Key: 43242, value: null
Key: 43082, value: null
Key: 43081, value: null

Answer 1

正如我所说，你应该首先考虑做什么而不是如何，这样问题就很容易解决。然后就可以获得自描述代码，例如:

//             v--- union all zip codes from stores
Stream<String> union = storeList.stream().map(Store::getServingZipCodes)
                                         .flatMap(List::stream)
                                         .distinct();

//             v--- find out intersections between zip codes
Stream<String> intersection = union.filter(zip ->
        storeList.stream().map(Store::getServingZipCodes)
                          .allMatch(it -> it.contains(zip))
);

//                       v--- create a Map simply from intersections 
Map<String, List<Store>> result = intersection.collect(toMap(
        Function.identity(),
        unused -> storeList
));

输出

assert result.get("43240") == [store1, store2];
assert result.get("others") == null;