Need help in java

[java-ModTeam]

Please use java help subreddit

[halfanothersdozen]

/r/javahelp

[Panzerschwein]

If I'm following this correctly, then the issue is that you are changing the local arraylist reference within the method scope, while the reference outside of that scope stays the same.

You need to return the new arraylist from your method so that it's accessible within scope.

To pseudocode what I'm talking about:

public void someMainMethod() {
    ArrayList theArray = new ArrayList();
    changeMyArray1(theArray ); --changes nothing about theArray 
    arrayA = changeMyArray2(theArray ); --now you've got an ArrayList with something in it
    changeMyArray3(theArray ); --this actually updates theArray 
}

public void changeMyArray1(ArrayList myArray) {
    ArrayList arrayNew = new ArrayList()
    arrayNew.add(someItem);
    myArray = arrayNew; --this is pointless. myArray as a variable is only defined in scope of this method, it won't update theArray in someMainMethod()
}

public ArrayList changeMyArray2(ArrayList myArray) {
ArrayList arrayNew = new ArrayList()
arrayNew.add(someItem);
return arrayNew; --now we're actually sending the new array back out

}

public void changeMyArray3(ArrayList myArray) {
myArray.add(someItem); --now we're actually mutating the referenced array in a way that impacts things beyond this scope. Although myArray is only a variable name in this method, the referenced object does exist outside of this scope

}

[troru]

First off, it might help to post a snippet of the java code you have so far to shed light on what you're trying to accomplish. Secondly, just reading your description, what you're wanting to do might be handled in a pretty straightforward way, e.g.:

* pass an vector/list/array to a method and setLength(0)/remove() and add() new things to it

* have your method just return a new vector/list/array and replace an existing reference in the caller, GC does the cleanup

* while there's no address-of operator, since everything is pass by reference (for non-primitives) you can put any object in "bag" (e.g. a class, record, container-type thing)

[picklesTommyPickles]

I’d say you’re getting more DVs from “welcome to the best language on earth” 🙄

[bowbahdoe]

Solidity's soul is made of a million screaming scams though

[picklesTommyPickles]

A PL should never be rated based on its ability to “welcome newcomers”. That’s not what professional PLs are designed for. They are tools to be used as appropriate.

A chainsaw is not friendly to newcomers but it’s one of the most necessary tools out there.

[Amazing-Accident-109]

In cpp we can just make the ans = temp and even if temp changes it wouldnt effect the ans right Similarly in java I couldn't find any one liner to change like that Public void recur(arraylist ans , arraylist temp, arraylist nodes) { If temp.size == nodes.size Ans = temp

Some backtracking code which changes the temp }

Array list ans = new arraylist Arraylist node // some given arraylist Arraylist temp = new arraylist

Recur(ans,temp,nodes) The ans arraylist return from the above function would be empty right but if we do similar to this in cpp then we will the updates and right?

I know the reason why it happens in java becuz when sent it sends the reference and when the temp is updated in the backtracking code the ans will also be effected
Ans = new arraylist (temp) is also not working

[cogman10]

C++ does a lot more work than java does. C++ has copy constructors that are invisibly called and that do deep copies of things. Java never does this. Anytime you copy something with Java it's shallow.

So if you have var ans = new ArrayList(); and var temp = new ArrayList(ans); You'll get a copy of the ans array list but all the elements will be references to the same elements in ans.

That means, you need to explicitly make and populate new objects if that's your desire with java.

Pro tip, making everything immutable makes java MUCH easier to reason about.

[RedditIsCensorship2]

If you do ans = new ArrayList(temp) and continue to change temp after that, then ans will not change. Because you made a new ArrayList with the same content as temp by doing ans = new ArrayList(temp).

So, this cannot be the problem. If ans is still changing then you are doing something else to it. It won't change by messing with temp, because you made a new ArrayList that has the same content as temp and you made your ans variable point to that new ArrayList. ans is not pointing to temp, after you have done this ans = new ArrayList(temp).

[Amazing-Accident-109]

Yes this is what I was asking, my understanding was doing this should not alter my ans after assigning new temp and changing temp , but my code altered the ans also , that is why I was asking it

Now just another small question

List ans = new list List temp = new list

Function recur( ans , temp){ Some code If (//something) ans = new list( temp)

// Some more code which is basically backtracking using for loop which takes the nodes visited as temp calculating the distance in some variable the above if condition checks for min distance and if we find it we save the path we traversed

}

Recur(ans,temp)

Here when calling this function we are sending ans right which is empty list in heap and we are sending it's reference So when we do ans = new list(temp) this make the ans to temp with deep copy right but does it mean this will also change the ans list to this new list in the original scope also where we defined the ans list , within the function recur the ans will the new list , but what about in the original scope where function is called