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Java 8 Lambda创建一个映射,其中Value是列表的包装对象

java-8 lambda java

jjones · 技术社区 · 8 年前

我有这个地址类作为列表,我想创建一个以街道名称为键的地图,并将所有地址对象的列表作为值,街道名称是其中的一部分。假设我得到的地址是一个列表,所以我有一个地址列表。

public class Address{
    private String streetName;
    private String city;
    private String country;
    private String zipCode;
    public String getStreetName() {
        return streetName;
    }
    public void setStreetName(String streetName) {
        this.streetName = streetName;
    }
    public String getCity() {
        return city;
    }
    public void setCity(String city) {
        this.city = city;
    }
    public String getCountry() {
        return country;
    }
    public void setCountry(String country) {
        this.country = country;
    }
    public String getZipCode() {
        return zipCode;
    }
    public void setZipCode(String zipCode) {
        this.zipCode = zipCode;
    }

}

I was able to do this using Lambda with the below Code

Map<String, List<Address>> resultMap =
addressList.stream().collect(Collectors.groupingBy(address-> address.getStreetName())); // This creates Map with street name as key and list of addresses as values

我需要在地址的顶部创建一个包装,这样看起来

public class AddressWrapper {
     private String streetName;
    private List<Address> addressWrapperList;
    public String getStreetName() {
        return streetName;
    }
    public void setStreetName(String streetName) {
        this.streetName = streetName;
    }
    public List<Address> getAddressWrapperList() {
        return addressWrapperList;
    }
    public void setAddressWrapperList(List<Address> addressWrapperList) {
        this.addressWrapperList= addressWrapperList;
    }
}

现在有了地址列表,我如何创建一个映射,以便我想创建一个地图,将街道名称作为键,将属于该街道名称的地址列表作为值,而不是将街道名称作为键,将值作为addressWrapperList(这是一个地址列表)

1 回复 | 直到 8 年前

Ousmane D. 8 年前

一种方法是根据 groupingBy 操作并收集到 Map 将值映射器函数指定为 AddressWrapper 。

Map<String, AddressWrapper> result = addressList.stream()
                .collect(Collectors.groupingBy(Address::getStreetName))
                .entrySet()
                .stream()
                .collect(Collectors.toMap(Map.Entry::getKey, e -> {
                    AddressWrapper addressWrapper = new AddressWrapper();
                    addressWrapper.setStreetName(e.getKey());
                    addressWrapper.setAddressWrapperList(e.getValue());
                    return addressWrapper;
                }));

通过创建带有签名的构造函数,可以简化值映射器逻辑 public AddressWrapper(String streetName, List<Address> addressWrapperList) {...} 。

然后代码变为:

Map<String, AddressWrapper> result = addressList.stream()
                .collect(Collectors.groupingBy(Address::getStreetName))
                .entrySet()
                .stream()
                .collect(Collectors.toMap(Map.Entry::getKey, 
                              e -> new AddressWrapper(e.getKey(), e.getValue())));

此外,您还可以使用 toMap 收集器如下:

Map<String, AddressWrapper> resultSet = addressList.stream()
      .collect(Collectors.toMap(Address::getStreetName,
              e -> new AddressWrapper(e.getStreetName(),
                      new ArrayList<>(Collections.singletonList(e))),
            (left, right) -> {
                left.getAddressWrapperList().addAll(right.getAddressWrapperList());
                return left;
            }));