How to set value of HashMap into a custom java object using java 8 stream?

问题

I have a HashMap which I want to convert to custom object Response. I am not sure how to set the values (80, 190, 900, 95) from HashMap to the custom object? How to write a separate function in Response object which sets price fields or set two parameters (key and value) to fromString function.

class Converter{

   public static void main(String[] args) {

      Map<String, Long> map = new HashMap<String, Long>();
      map.put("111", 80) // first 
      map.put("1A9-ppp", 190) // second
      map.put("98U-6765", 900) // third
      map.put("999-aa", 95) // fourth

     List<Response> list = 
         map.keySet().stream().map(ent-> Response.fromString(ent.getKey(), ent.getValue())).collect(Collectors.toList());
     // how to set price field from value of the key
   }
}

class Response{}{
  String name;
  Long code;
  String city;
  Long price;

  public static Response fromString(String key, Long value){
    Response res = new Response();
    String[] keys = key.split("//-");
    // some logic to set name and city 
    res.name = keys[0];
    if(keys.length == 2) {
      if(keys[1].matches("-?\\d+(\\.\\d+)?") // check if string is numeric
           { 
                  res.code = keys[1]
           }
       else{
                  res.city = keys[1]
       }
    }
    res.price = value;
    return res;
  }
}

回答1:

I had to fix some things in your code that were syntactically incorrect.

1. Longs need to be suffixed with an L
2. You needed semicolons after your put statements
3. I removed the extra braces from your class

I added

1. A constructor
2. A utility routine to extract the name and city or provide a default city name.
3. And a toString method to let you see the contents of the Response class.

    public class Converter {

       public static void main(String[] args) {

          Map<String, Long> map = new HashMap<>();
          map.put("111", 80L); // first
          map.put("1A9-ppp", 190L); // second
          map.put("98U-6765", 900L); // third
          map.put("999-aa", 95L); // fourth

          List<Response> list = map.entrySet().stream().map(
                ent -> Response.instanceOf(ent.getKey(), ent.getValue())).collect(
                      Collectors.toList());

          System.out.println(list);
       }
    }

    class Response {
       String name;
       String city;
       long   price;

       public Response(String name, String city, long price) {
          this.name = name;
          this.city = city;
          this.price = price;
       }

       public static Response instanceOf(String str, long price) {
          if (str.indexOf("-") == -1) {
             str += "-NoName";
          }
          String[] items = str.split("-");
          return new Response(items[0], items[1], price);
       }

       public String toString() {
          return "name = " + name + ", city = " + city + ", " + "price = " + price;
       }
    }

回答2:

Create a constructor in Response class that accepts two parameters and initialize the corresponding properties

class Response {

    String name;
    String city;
    Long price;

  public Response(String key, Long price) {
     this.price=price;
     String[] keys = key.split("-");
     //check conditions and set values to properties
     this.name = keys[0];
     this.city = keys[1];
  }
}

Now use stream and map to convert into Response object

List<Response> list = map.entrySet().stream().map(entry -> new Response(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());

回答3:

I would just use the Method Map#entrySet() instead of keySet().

class Converter{
    public static void main(String[] args) {
        Map<String, Long> map = new HashMap<String, Long>();
        map.put("111", 80); // first
        map.put("1A9-ppp", 190); // second
        map.put("98U-6765", 900); // third.
        map.put("999-aa", 95) // fourth
        List<Response> list = map.entrySet().stream().map(Response::fromEntry).collect(Collectors.toList());
    }
}
class Response{}{
    String name;
    String city;
    Long price;
    public static Response fromEntry(Map.Entry<String,Long> entry){
        String key=entry.getKey();
        Long price=entry.getValue();
        Response res = new Response();
        String[] keys = key.split("//-");
        // some logic to set name and city
        res.name = keys[0];
        if(keys.length > 1) {
            res.city = keys[1];
        }
        //set price
        price=value;
        return res;
    }
}

来源：https://stackoverflow.com/questions/59021628/how-to-set-value-of-hashmap-into-a-custom-java-object-using-java-8-stream