How can Java ArrayList change value while passing as argument?

Question

Good Day.

I am pretty sure that this question has been asked before but I couldn't find it.

Basically, how does the below code work:

import java.util.ArrayList;

public class Main{
    public static void main(String[] args) {
        ArrayList<Integer> a = new ArrayList<Integer>();
        
        a.add(10);
        
        change(a);
        
        System.out.println(a.get(0));
    }
    
    static void change(ArrayList v){
        if(v.size() > 0){
            v.set(0, "Hello");
        }
    }
}

It compiles and also runs successfully, printing "Hello". How could Java replace Integer with String?

I also tried adding in Main:

        a.set(0, 1002);
        
        for(Integer c: a){
            System.out.print(c + " ");
        }
        System.out.println("");

Once again, very surprising. I thought that maybe somwhow a gets changed to ArrayList<String> but the set method works normally?

Can anyone please explain this to me? Is this undefined behaviour?

Thanks.

You could do it you you used Object as your list type (e.g. new ArrayList<Object> But lists of mixed types is not not a good idea and can lead to problems. — WJS
– WJS, Commented Apr 21, 2022 at 18:16
When you compiled this code, you would have got a warning. That warning was telling you this could happen. — Louis Wasserman
– Louis Wasserman, Commented Apr 21, 2022 at 18:17
Java doesn't replace Integer with String. System.out.println(a.get(0));Is using println(Object) So it doesn't need to cast the String to an Integer. If you tired Integer b = v.get(0); you'd get different results. — matt
– matt, Commented Apr 21, 2022 at 18:57

Michel K · Accepted Answer · 2022-04-21 18:16:27Z

2

Generic types like the <Integer> in your ArrayList are a compile-time concept, this information is lost at runtime. This is known as "type erasure".

You just use ArrayList (without type parameter) in the change method, which means that the compiler will not check for a type, so it's possible to add a String to an Integer List. If you used ArrayList<Integer>, the compiler would complain.

Internally, ArrayList uses a simple array of objects that can hold any type.

answered Apr 21, 2022 at 18:16

Michel K

4212 silver badges7 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How can Java ArrayList change value while passing as argument?

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related