Beginner level snippet redo, from py to Java

[modern glen]

So, I made this small snippet, but can't get to to work on Java

# return true if a list is made of lists and integers only, it's recursive 

def checkIfValid(obj):
  for i in obj: 
    if type(i) == list: 
      return checkIfValid(i)
    elif type(i) == int : 
      pass 
    else: return False 
    return True 

a = [1, 2, 3, [1,2, 3, [2, 2, "a"]]] 
b = [1, 2, 3, [1,2, 3, [2, 2, []]]]

if (checkIfValid(a)): print("a")
if (checkIfValid(b)): print("b")

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[coarse fjord]

So what's the Java code you currently have?

Java is type-safe and you typically don't have lists containing both ints and strings or whatever

like you'd use List<Integer> and then it can't contain any String unless you are using unchecked operations somewhere

[modern glen]

package NetBeans4Tests;
 
import java.util.Scanner;
 
public class App {
    public static void main(String[] args) {
 
        Object[] a = {1, 2, 3};
        Object[] b = {1, 2, "3"};
        Object[] aa = {1, 2, a};
        Object[] bb = {1, 2, b};
 
        if(checkIfValid(a)){
            System.out.println("a");
        }
        if(checkIfValid(b)){
            System.out.println("a");
        }
        if(checkIfValid(aa)){
            System.out.println("a");
        }
        if(checkIfValid(bb)){
            System.out.println("a");
        }
    }
 
 
    public static boolean checkIfValid(Object[] obj){
        for(Object i: obj){
            if (i instanceof Object[]){
                return checkIfValid(obj);
            } else if (i instanceof Integer) {
                // pass
            } else {
                System.out.println("false because" + i);
            return false;
            }
        }
        return true;
 
    }
}

[coarse fjord]

oh you are using arrays and not lists?

ok

[modern glen]

In Java, yes, in python they're indeed lists

[coarse fjord]

in the return checkIfValid(obj);, you'd need to use (Object[]) i

since you'd essentially check the whole array again instead of the single object

[modern glen]

I don't really need it, I'm just curious about the Lang etc

[coarse fjord]

but you could also use instanceof patternmatching

    public static boolean checkIfValid(Object[] obj){
        for(Object i: obj){
            if (i instanceof Object[] arr){
                return checkIfValid(arr);
            } else if (i instanceof Integer) {
                // pass
            } else {
                System.out.println("false because" + i);
                return false;
            }
        }
        return true;
 
    }

[naive idol]

couldn't you just ignore the integer case enitrely and just !(i instanceof Integer)

in python as well

[modern glen]

Why do you add the extra arr? 🤔

[coarse fjord]

this is switch instanceof patternmatching

it does two things

at first, it does the instanceof check

and if it succeeds, it does a cast

[naive idol]

instanceof* pattern matching

[coarse fjord]

if (i instanceof Object[] arr){
  return checkIfValid(arr);
}

is equivalent to

if (i instanceof Object[]){
  Object[] arr = (Object[]) i;
  return checkIfValid(arr);
}

[modern glen]

I don't understand this,

I think in py at least Ive to narrow the types and it seemed like a good way, if I were to comment the check, it would be added into the else and thus return false

[coarse fjord]

he means

    public static boolean checkIfValid(Object[] obj){
        for(Object i: obj){
            if (i instanceof Object[] arr){
                return checkIfValid(arr);
            } else if (!(i instanceof Integer)) {
                System.out.println("false because" + i);
                return false;
            }
        }
        return true;
 
    }

[naive idol]

this would be switch pattern matching

switch (i) {
  case Object[] arr:
    return checkIfValid(arr);
  case Integer i:
    break;
  default:
    return false;
}

[modern glen]

wat

[coarse fjord]

yes I edited it

[naive idol]

yeah but it's also a valid solution on its own so.. why not show it

sidenote though; why return checkIfValid(arr) directly?
couldn't you have an array in the middle that is itself valid, but then have invalid elements after the array?

[coarse fjord]

in your code, you used return checkIfValid(obj);.
This would check the whole array again in each iteration which would lead to infinite recursion

[modern glen]

This did fix it, wat the hek

[coarse fjord]

so you would need to use return checkIfValid(i); but the type checker doesn't allow this because it's an Object and not an Object[]

[naive idol]

should be something like

if (!checkIfValid(arr)) {
  return false;
}
``````py
if not check_if_valid(i):
  return False

[modern glen]

Yeah, I did have it as a i, but I did a lot of tests and this was the cleanest and quickest version to get, didn't review it completely, seemed fine

[naive idol]

to fix a false positive in the case [[0], "a"]

[coarse fjord]

(Object[]) i means "take i and convert it to an Object[] (and fail if that doesn't work)"

[modern glen]

Okay, interesting 🤔

So, this is different from class.getClass because getClass doesn't do type cast, right?

[coarse fjord]

if(i instanceof Object[] arr) means "take i and check whether it is an Object[]. If it is, create a new variable arr holding the Object[]"

[naive idol]

this is special syntax

instanceof checks behave differently to type equality checks

in python it'd be instanceof(obj, type) iirc

[modern glen]

Indeed!

[coarse fjord]

you can also use reflection (getClass() stuff) for checking the types

[naive idol]

@modern glen btw check this?

[modern glen]

I'm not understanding you and I'm already confused, please let's do one baby step at the time 🤣

Thank you, tho

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[naive idol]

@modern glen see this example

[modern glen]

Ohh

[naive idol]

your current algorithm (both py and java) return immediately upon encountering an array, not checking any remaining elements

it's like if you had a return true; in the if type(i) == int branch

[modern glen]

I'm reading all the msgs, just not understanding yours very much, although it does seem like an interesting idea

[naive idol]

it's not an idea.. your program has a bug

[coarse fjord]

With the python program, it's just messed up indentation I think

although not even that

[naive idol]

it isn't

[coarse fjord]

ah right that's what you mean

yeah

[modern glen]

Indeed, but not the problem I'm trying to solve

[naive idol]

i mean there is an indentation bug in the python code, but what im talking about is a bug in the algorithm

[coarse fjord]

so if we go back to the Python code:

def checkIfValid(obj):
  for i in obj: 
    if type(i) == list: 
      return checkIfValid(i)
    elif type(i) == int : 
      pass 
    else: return False 
    return True 

you have return checkIfValid(i).

Now, if you have a list like [[1],"Hi"]

[modern glen]

Formatting got lost in the way I've to copy and paste, please ignore my formatting kekw

[naive idol]

so... you acknowledge that you have a bug, but fixing the bug isn't logical because you aren't trying to do it?

[coarse fjord]

then you check the first list and immediately return true

but you are ignoring further elements

so [[1],"Hi"] would be true with your Python code

[naive idol]

dude just plagiarized my bug report

[coarse fjord]

and you also have that issue in your Java code

[modern glen]

Yes, because it's not what I'm trying to learn xdddd

[coarse fjord]

I am explaining what they didn't understand in your messages

[naive idol]

it's.. the same explanation though

[modern glen]

Kind of, yes xD

[naive idol]

what even is this logic

[modern glen]

Anyway, thank you both

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[coarse fjord]

I hoped they understand it when someone else explains it

[naive idol]

not even a postponement, just straight up, "bug not relevant ever"?

im so confused by your line of thinking

[coarse fjord]

oh and I didn't see some of your messages

[modern glen]

Not relevant in this case...

[naive idol]

no it absolutely is

[modern glen]

@coarse fjord thank you a lot for the instanceof casting thingy!

[naive idol]

i have no idea what you're talking about

[coarse fjord]

I think they mean it isn't relevant because they aren't really caring whether or not the code works

[naive idol]

translating code is an opportunity to restructure old code or fix bugs, and to introduce new ones

[modern glen]

It's not. At all. Not what I'm to learn at the moment.

The bug exists, yes, this isn't mean for production of anything like that.

[naive idol]

this isn't mean for production of anything like that.
not relevant