Create a new list with values from fields from existing list

collections, field, java, list

Solution

You can using Java 8 with lambda expressions :

List<String> listNames = people.stream().map(u -> u.getName()).collect(Collectors.toList());

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Test {
  public static void main(String args[]){
    List<Person> people = Arrays.asList(new Person("Bob",25,"Geneva"),new Person("Alice",27,"Paris"));
    List<String> listNames = people.stream().map(u -> u.getName()).collect(Collectors.toList());
    System.out.println(listNames);
  }
}
class Person
{
   private String name;
   private int age;
   private String location;

  public Person(String name, int age, String location){
    this.name = name;
    this.age = age;
    this.location = location;
  }

  public String getName(){
    return this.name;
  }

}

Output :

[Bob, Alice]

Demo here.

Alternatively, you can define a method that will take your list as parameter and the function you want to apply for each element of this list :

public static <X, Y> List<Y> processElements(Iterable<X> source, Function <X, Y> mapper) {
    List<Y> l = new ArrayList<>();
    for (X p : source) 
        l.add(mapper.apply(p));
    return l;
}

Then just do :

List<String> lNames = processElements(people, p -> p.getName()); //for the names
List<Integer> lAges = processElements(people, p -> p.getAge()); //for the ages
//etc.

If you want to group people by age, the `Collectors` class provide nice utilities (example):

Map<Integer, List<Person>> byAge = people.stream()
                                         .collect(Collectors.groupingBy(Person::getAge));

Problem

Let's say there is a class: ``` class Person { String name; int age; City location; } ``` Is there some library that will let me create a list of Strings containing each name from the list of persons in one line instead of creating a new list and looping through the other list? Something like: ``` List<Person> people = getAllOfThePeople(); List<String> names = CoolLibrary.createList("name", people); ``` Rather than: ``` List<Person> people = getAllOfThePeople(); List<String> names = new LinkedList<String>(); for(Person person : people) { names.add(person.getName()); } ```

Original source

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