从映射流重用lambda

Question

还在学习Java8。我的代码看起来像：

List<String> list = userData.getEmails().stream()  
          .map(email -> codeThatGetsTheHost(email))  
          .peek((email, host) -> {//Only print if email is not null
                System.out.println("email=" + email + " host=" + host);})  
          .collect(Collectors.toList());  //should collect hosts.

上述代码不起作用，因为peek（）无法访问主流变量，即map（）中使用的电子邮件。我该如何解决这个问题？

Answer 1

保持一个流

.peek(email -> System.out.println("email=" + email + " host=" + email.getHost()))
.map(Email::getHost)

如果主机计算有些昂贵，您可以创建一个Stream

userData.getEmails()
    .stream()
      .filter(Objects::nonNull)
      .map(email -> new AbstractMap.SimpleEntry<>(email, codeThatGetsTheHost(email)))
      .peek(entry -> System.out.println("email=" + entry.getKey() + " host=" + entry.getValue()))
      .map(Map.Entry::getValue)
    .collect(Collectors.toList());

Answer 2

在将电子邮件映射到其主机之前，应该使用peek。这是您案例的示例代码：

import java.util.*;  
import java.util.stream.Collectors;


class Email{
    public String email;
    private String host;
    public Email(String email, String host){
        this.email = email;
        this.host = host;
    }
    public String getHost(){
        return host;
    }
}

class UserData{
    List<Email> emails = new LinkedList<>();
    public List<Email> getEmails(){
        return emails;
    }
}

public class MyClass {
    public static void main(String args[]) {
      UserData userData = new UserData();
      userData.emails.add(new Email("[e-mail]", "host1"));
      userData.emails.add(new Email("[e-mail]", "host2"));
      userData.emails.add(new Email("[e-mail]", "host3"));
      List<String> list = userData.getEmails().stream()
          .peek(email -> System.out.println("email=" + email + " host=" + email.getHost()))  
          .map(email -> email.getHost())  
          .collect(Collectors.toList());
      System.out.println(list);
    }
}

email=Email@7bfcd12c host=host1
email=Email@1c2c22f3 host=host2
email=Email@18e8568 host=host3
[host1, host2, host3]

Answer 3

或者将peek放在map之前，以访问所需的所有数据：

List<String> list = userData.getEmails().stream()  
          .peek(email -> System.out.println("email=" + email + " host=" + email.getHost())) 
          .map(email -> email.getHost())   
          .collect(Collectors.toList());

或者在map方法中执行这两个动作（完全合法），以避免重复getter调用：

List<String> list = userData.getEmails().stream()  
          .map(email -> {
               String host = email.getHost();
               System.out.println("email=" + email + " host=" + host));
               return host;
          })
          .collect(Collectors.toList());