CODEWITHSUNDEEP
java
TheUser
TheUser

Reputation: 129

Reference of List doesn't get reset

I have created a List and added 2 elements to it. I pass this List to a method and initialized the list to new ArrayList().

When the method returns I print the size of list but it still shows 2.

public class ValueChange {
    public void fun(List<Integer> l1){
        l1=new ArrayList<Integer>();
    }

    public static void main(String[] args){
        List<Integer> l1=new ArrayList<Integer>();
        l1.add(5);
        l1.add(6);
        System.out.println("Before = "+l1.size());
        ValueChange vc=new ValueChange();
        vc.fun(l1);
        System.out.println("After = "+l1.size());
    }
}

Upvotes: 2

Views: 422

Answers (4)

Dan Temple
Dan Temple

Reputation: 2764

That won't work because of the scope of the parameter.

You want to clear the list:

public void fun(List<Integer> l1){
    l1.clear();
}

As other answers have pointed out. Java's pass-by-value is what's causing your issue here. See THIS PAGE for explanation of this.

Upvotes: 1

Adam Arold
Adam Arold

Reputation: 30528

Java is pass by value but it passes the value of the reference. This basically means that if you reassign a method parameter it does not get reflected in the original object.

Java makes a copy of the original reference when you pass it to a method so if you assign a new value to that reference you won't see the changes outside.

Both references are pointing to the same objet though.

Upvotes: 1

Freaky Thommi
Freaky Thommi

Reputation: 746

Java uses pass by value of reference. Its like you are passing the reference as value. What you have done in the invoked method is you have changed the value of the formal parameter by assigning a new memory to it, It wont reflect in the actual parameter

Upvotes: 3

benjamin.d
benjamin.d

Reputation: 2871

The line:

l1=new ArrayList<Integer>();

in your fun method will not actually change the list outside of this method scope. You can change the values inside them, but not reassign a new list to your arguments.

This actually a frequent error in java. If you want to know how the parameters are passed in a method, please read this post.

Upvotes: 7

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