使用Java过滤对象的ArrayList，以返回字符串类型的ArrayList

Question

所以我找不到确切的解决办法。我有一个Crowd类，它由一个Crowd对象组成，它是一个类型为People的数组列表。People是一个具有String name, Double bankBalance, Integer numberOfCarsOwned属性的类。

在我的人群类中，我有下面的方法，用字母P开头的名字进行过滤，并返回String类型的数组列表

  public ArrayList<String> filterByLetterP(){
       ArrayList<String> filteredNames =  this.crowd.stream()
               .filter(name -> name.getName().contains("P"));
               return filteredNames;
    }

我的错误是:必需的类型ArrayList<String>提供了Stream<People>

注意:我的解决方案必须使用流。如何纠正我的解决方案以使其工作？

下面是参考信息。

People类定义：

public class People {

    private String name;
    private Double bankBalance;
    private Integer numberOfCarsOwned;

    public People(String name, Double bankBalance, Integer numberOfCarsOwned) {
        this.name = name;
        this.bankBalance = bankBalance;
        this.numberOfCarsOwned = numberOfCarsOwned;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Double getBankBalance() {
        return bankBalance;
    }

    public void setBankBalance(Double bankBalance) {
        this.bankBalance = bankBalance;
    }

    public Integer getNumberOfCarsOwned() {
        return numberOfCarsOwned;
    }

    public void setNumberOfCarsOwned(Integer numberOfCarsOwned) {
        this.numberOfCarsOwned = numberOfCarsOwned;
    }
}

Crowd类定义：

public class Crowd {

    private ArrayList<People> crowd;

    public Crowd() {
        this.crowd = new ArrayList<>();
    }

    public ArrayList<People> getCrowd() {
        return crowd;
    }

    public void setCrowd(ArrayList<People> crowd) {
        this.crowd = crowd;
    }

    public void addPeopleToCrowd(People people){
        this.crowd.add(people);
    }

    public ArrayList<String> filterByLetterP(){
       ArrayList<String> filteredNames =  this.crowd.stream()
               .filter(name -> name.getName().contains("P"));
               return filteredNames;
    }
}

Answer 1

有几件事需要解决：

您应该使用的是String#contains.

• You，而不是String#startsWith，必须将Stream映射到People#name.

• You，错过了收集Stream.

如下所示：

public List<String> filterByLetterP() {
    List<String> filteredNames = this.crowd.stream()
            .map(p -> p.getName())
            .filter(s -> s.startsWith("P"))             
            .collect(Collectors.toList());
    return filteredNames;
}

Answer 2

您应该更改您的筛选器，因为它不检查开头字母‘P’的名称：

public List<String> filterByLetterP(){
    return this.crowd.stream()
            .map(People::getName)
            .filter(name -> name.charAt(0) == 'P')
            .collect(Collectors.toList());
}