CODEWITHSUNDEEP
javaarraylistmethods
user8491933
user8491933

Reputation:

How can I change an ArrayList that is returned to a method? - Java

That is the code:

import java.util.Arrays;
import java.util.List;

public class Test {
    public static void main(String[]args) {
        changeList();

        System.out.println(Arrays.toString(example().toArray()));
    }

    public static void changeList() {
        example().set(2, "Hi!");
    }

    public static List<String> example () {

        String str = new String("word1|word2|word3|word4|word5");

        return Arrays.asList(str.split("\\|"));
    }
}

I want the ArrayList to be like this: [word1, word2, Hi!, word4, word5]
But it doesn't change... It prints[word1, word2, word3, word4, word5]

Is it even possible to change it that way? Thanks in advance!

Upvotes: 0

Views: 90

Answers (4)

Code-Apprentice
Code-Apprentice

Reputation: 83527

Let's step through your code one line at a time to understand what it is doing:

 public static void main(String[]args) {
    changeList();

This calls the changeList() method:

public static void changeList() {
    example().set(2, "Hi!");

First call example():

public static List<String> example () {
    String str = new String("word1|word2|word3|word4|word5");
    return Arrays.asList(str.split("\\|"));
}

This creates a new List and returns it back to changeList():

example().set(2, "Hi!");

Now we modify the returned List by setting the element at index 2 to "Hi!". However, the modified list is immediately discarded because you have no reference to it any more.

Then we return back to main:

    System.out.println(Arrays.toString(example().toArray()));

Now we call example again:

public static List<String> example () {
    String str = new String("word1|word2|word3|word4|word5");
    return Arrays.asList(str.split("\\|"));
}

This creates a new List. This new list is in no way related to the list that was created when we called example() the previous time.

If you want to keep the same list, you need to assign it to a reference in changeList() and then return it:

public static void changeList() {
    List<String> theList = example();
    theList.set(2, "Hi!");
    return theList;
}

Now main() needs to save a reference to the list returned by changeList():

public static void main(String[]args) {
    List<String> mainList = changeList();

    System.out.println(mainList);
}

(Note: You can print the List directly. Your original version is much more complicated than necessary.)

Upvotes: 0

Chai T. Rex
Chai T. Rex

Reputation: 3018

The problem is that example gives a brand new List each time it's called. When you create the list in changeList, it's a different List than the one you create in main, and changes made to one List won't affect another.

One way to fix this is to:

  1. Have the changeList method do what it says: take in a List (rather than producing one from scratch) and change that List.
  2. Have the main method pass in the List to changeList.
public static void main(String[]args) {
    List<String> list = example();
    changeList(list);

    System.out.println(Arrays.toString(list.toArray()));
}

public static void changeList(List<String> list) {
    list.set(2, "Hi!");
}

Upvotes: 1

ajc
ajc

Reputation: 1725

    public static void main(String[]args) {
        System.out.println(Arrays.toString(changeList().toArray()));
    }

    public static List<String>  changeList() {
        List<String> list = example();
        list.set(2, "Hi!");
        return list;
    }

    public static List<String> example () {

        String str = new String("word1|word2|word3|word4|word5");

        return Arrays.asList(str.split("\\|"));
    }

Upvotes: 0

ΦXocę 웃 Пepe&#250;pa ツ
ΦXocę 웃 Пepe&#250;pa ツ

Reputation: 48268

But it doesn't change... Is it even possible to change it that way?

yes, you change the list but you lost the ref of the returned collection since the method public E set(int index, E element) returns E and not the list itself.. see the doc

as I said, move the return of example() into a variable that you can manipulate...

public static void main (String[] args) {
    List<String> retVal = example();
    System.out.println(retVal);
    retVal.set(2, "Hi!");
    System.out.println(retVal);
}

public static List<String> example () {
    String str = new String("word1|word2|word3|word4|word5");
    return Arrays.asList(str.split("\\|"));
}

Upvotes: 0

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