如果我有一個Stream<Class>，我如何訪問類的固有資訊，例如變數和陣列？

我有一個名為 Person 的類，其結構如下：

public class Person {
   public final int num;
   public final String name;
   public final String gender;
   public final int age;

   public Person (int aNum, String aName, String aGender, int anAge){
      this.num = aNum;
      this.name = aName;
      this.gender = aGender;
      this.age = anAge;
   }

   public static Person lineValues(String line) {
      String array = line.split(",");
      int numA = array[0];
      String nameA = array[1];
      String genderA array[2];
      int ageA = array[3];
      return new Person(numA, nameA, genderA, ageA);
   }
}

單個 Person 的資料來自名為 people.csv 的 csv 檔案中的一行：

| Num | Name   | Gender  |Age  |
| --- | ----   | ------  | --- |
| 1   | Fred   | Male    | 41  |
| 2   | Wilma  | Female  | 36  |
| 3   | Barney | Male    | 38  |
| 4   | Betty  | Female  | 35  |

這是我的實際問題。我有一個名為 People 的介面，它有一個名為 getGenderCount 的函式。這個函式應該通過一個 Person 物件并檢索每個性別的計數圖。

static Function<Stream<Person>,Map<String,Long>> getGenderCount = null;

我的問題是我無法理解如何流式傳輸整個類物件的正確語法。我事先使用過原始流。執行諸如此類的拆分是.map(x -> x.split(","))行不通的，因為那將是 String 型別而不是 Person。

我提出的解決方案如下所示：

e -> e.map(x -> x.split(","))
.skip(1) // skips the title line
.filter(x -> x.length == 4)
.collect(Collectors.groupingBy(x -> x[2], Collectors.counting()));

但它應該映射一個 Person 物件的資訊。除了流操作和語法之外，我不想更改我目前擁有的任何東西。我想了解如何從 Person 中提取變數并在 Stream 中找到它們。

uj5u.com熱心網友回復：

這會做你想要的嗎？

Files.lines(Paths.get("people.csv"))
.skip(1)
.map(Person::lineValues) // this method must handle "|" divider chars
.collect(Collectors.groupingBy(Person::getGender, Collectors.counting()));

uj5u.com熱心網友回復：

您需要使用通常的方法對流進行操作map，filter或者foreach然后可以使用標準訪問操作。

Stream<Person> people;
people = ...; // initialize stream with some data

// iterate over stream and access each element
people.forEach(person -> { System.out.println(person.name); });

該函式getGenderCount獲取 Person 流作為輸入，并回傳包含性別及其頻率的地圖。

此外，在 Java 中，字串不能直接分配給 int。它需要轉換。

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Person {
    public final int num;
    public final String name;
    public final String gender;
    public final int age;

    public Person(int aNum, String aName, String aGender, int anAge) {
        this.num = aNum;
        this.name = aName;
        this.gender = aGender;
        this.age = anAge;
    }

    public static Person lineValues(String line) {
        String array[] = line.split(",");
        int numA = Integer.parseInt(array[0]);
        String nameA = array[1];
        String genderA = array[2];
        int ageA = Integer.parseInt(array[3]);
        return new Person(numA, nameA, genderA, ageA);
    }

    static Function<Stream<Person>, Map<String, Long>> getGenderCount = people -> {
        return people.collect(Collectors.groupingBy((person -> person.gender), Collectors.counting()));
    };

    public static void main(String args[]) {
        try {
            Stream<Person> people = Files.lines(Paths.get("people.csv"))
                    .map(line -> { System.out.println(line); return line;}) // just to show input
                    .skip(1) // remove first line
                    .map(line -> line.substring(line.indexOf("|")   1)) // remove first '|' and in front
                    .map(line -> line.substring(0, line.lastIndexOf("|"))) // remove last '|' and behind
                    .map(line -> line.replace(" ", "")) // remove spaces, can cause trouble with spaces within names
                    .map(line -> line.replace("|", ",")) // change pipe symbols into commas
                    .map(line -> Person.lineValues(line)); // transform lines into persons
            Map<String, Long> genderCount = Person.getGenderCount.apply(people);
            System.out.println(genderCount);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

生產

$ javac Person.java
$ java Person      
| Num | Name   | Gender | Age|
| 1   | Fred   | Male   | 41 |
| 2   | Wilma  | Female | 36 |
| 3   | Barney | Male   | 38 |
| 4   | Betty  | Female | 35 |
{Male=2, Female=2}
$ 

標籤：爪哇 CSV java流

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