list.set(5,"newString");  

• Reference

You can replace the items at specific position using set method of ArrayList as below:

list.set( your_index, your_item );

But the element should be present at the index you are passing inside set() method else it will throw exception.

Also you can check oracle doc here

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:

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

al.set(index, new_value);

Easy ;)