ArrayList add() function overwrites all the elements (processing/java)

Question

So I am trying to shuffle the same ArrayList (knots) 50 times in a for-loop and adding the shuffled list to another ArrayList (gen0). But every time I add a new ArrayList it overwrites all existing ArrayList-elements to the same ArrayList I just added, can someone tell me why?

ArrayList<ArrayList> seed(ArrayList<PVector> knots) {

  ArrayList<ArrayList> gen0 = new ArrayList<ArrayList>();
  for(int i=1; i<=50; i++) {

    Collections.shuffle(knots);

    gen0.add(knots);    
  }

  return gen0;
}```

You're only adding a single object, the same ArrayList, to gen0. You need to create a new array list -- `gen0.add(new ArrayList(knots));` for this to work. — Hovercraft Full Of Eels, May 12 '19 at 21:12

score 0 · Answer 1 · answered May 12 '19 at 21:13

0

You are shuffling the reference to the same ArrayList every time. Instead, consider copying the ArrayList on each iteration

ArrayList<PVector> newList = new ArrayList<>(knots); // creates a copy
Collections.shuffle(newList);
gen0.add(newList);

answered May 12 '19 at 21:13

Earthcomputer

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ArrayList add() function overwrites all the elements (processing/java)

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