Retrieve array[] of values from ArrayList of objects

Question

Let's say I have an ArrayList of objects. For example: ArrayList<Person> personList , where each Person has 2 class variables String name and int age . These variables each have their own getter methods getName() and getAge() .

What is the simplest way to retrieve an array (or ArrayList) of int ages[] ?

Note this question is similar to the verbosely titled " Retrieve an array of values assigned to a particular class member from an array of objects in java ", though without the arbitrary restriction on for-loops, and using an ArrayList instead of an Array.

Answer 1

创建与列表大小相同的目标数组，然后遍历列表并将每个元素的age添加到目标数组。

Answer 2

Numerous ways to do this -- here is one.

First get the ages into a list (using a java8 stream), and then convert the list into an array.

public int[] getAges() {
    return personList.stream()
        .mapToInt(Person::getAge)
        .toArray();
}

Answer 3

Person P1 = new Person("Dev", 25);
Person P2 = new Person("Andy", 12);
Person P3 = new Person("Mark", 20);
Person P4 = new Person("Jon", 33);

ArrayList<Person> personList = new ArrayList<>(Arrays.asList(new Person[] { P1, P2, P3, P4 }));
int[] ages = getPersonAges(personList); // [25, 12, 20, 33]

---

private static int[] getPersonAges(ArrayList<Person> personList) {
    int[] ages = new int[personList.size()];
    int idx = 0;

    for (Person P : personList) {    // Iterate through the personList
        int age = P.getAge();
        ages[idx++] = age;
    }

    return ages;
}