How to change value of a specific element in an ArrayList?

Question

So here I have an array of numbers and I want to change the value of a specific index only.

ArrayList <Integer> numbers = new ArrayList <> ();
numbers.add(5);
numbers.add(10);
numbers.add(20);

I was trying to do something like userNumbers.get(1 (* 3)); where 1 should be the index and I'd multiply the value in that index by 3 therefore will result as 30 since 10 is the 1st index. Tried searching up but no luck for me!

You need to do a get and a set, numbers.set(1, numbers.get(1) * 3) — MadProgrammer
– MadProgrammer, Commented Jun 20, 2018 at 2:16

kingkupps · Accepted Answer · 2018-06-20 02:11:38Z

6

Refer to the Java documentation whenever possible. This in particular.

numbers.set(1, numbers.get(1) * 3);

answered Jun 20, 2018 at 2:11

kingkupps

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Comments

epicGeek · Accepted Answer · 2018-06-21 01:59:05Z

Personally, I think it is because "Integer" is an immutable class, which means you cannot change the value of an immutable object with its member method.

If you put Integer, Long , Float ,Double, Boolean, Short, Byte, Character, String and other immutable classes in the list, you cannot change the value instantly.

But if you put customized objects in the list , you can change the value.

Demo code:

public class RRR {
    public static void main(String[] args) {
        ArrayList <Hi> hiList = new ArrayList <> ();
        Hi hi1 = new Hi("one");
        Hi hi2 = new Hi("two");
        Hi hi3 = new Hi("three");
        hiList.add(hi1);
        hiList.add(hi2);
        hiList.add(hi3);
        Hi hix = hiList.get(0);
        hix.setName("haha");
        System.out.println(hiList.get(0).getName());    // changed from "one" to "haha"
    }


}

class Hi {
    public Hi(String name) {
        this.name = name;
    }
    private String name;
    public void setName(String name) {
        this.name = name;
    }
    public String getName() {
        return this.name;
    }
}

See? The class Hi is not Immutable , you can change this value with "setName"

Back to the question, change this Integer object , you can:

Copy the new value and origin values to a new list, if your list is not too large(not good).
Delete the old element, then set the new value to the right index.(should consider thread safe problem)

alexrnov · Accepted Answer · 2020-05-01 00:53:16Z

I wrote a static method to make the changes, you could possibly add this method to the ArrayList class as well. This will multiply the element by 3 as you said you wanted in your post:

public static void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<>();
    numbers.add(5);
    numbers.add(10);
    numbers.add(20);

    System.out.println(numbers.get(2)); //before

    changeList(numbers, 2);

    System.out.println(numbers.get(2)); //After!
}

//This method will take in the list, and the element number
static void changeList(ArrayList<Integer> arrayList, int elementNum) {
    int temp = arrayList.get(elementNum);
    arrayList.remove(elementNum); //remove it

    int newValue = temp * 3; //do anything you want here to the data
    arrayList.add(elementNum, newValue); //place the value back in the arrayList
}

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