Java How to know if arraylist contains value in property of object [duplicate]

Question

I want to find if the property numb in the ArrayList grids contains a certain value or not. Then I want to get the index of the entry that contains that value.

public class Grid {
  public int numb;

  public GridSeg(int numb) {
    this.numb = numb;
  }
}

public ArrayList<Grid> grids = new ArrayList<Grid>();

for (int i = 0; i < 6; i++) {
  Grid grid = new Grid(i);
  grids.add(grid);
}

/pseudo code since I don't know how it is done
if (grids.numb.contains(12)) {
  if (grid.numb == 12) //get grid index
}

What exactly are you struggling with? Why not just use an index based for-loop, check the numb property of the current element and if it matches return the current index (and of course return -1 or something similar if no such element has been found)? — Thomas
– Thomas, Commented Feb 26, 2020 at 15:26
@Thomas I thought there would be a better way. Like you have .contains if the ArrayList is an array of primitives. — Hasen
– Hasen, Commented Feb 26, 2020 at 15:31
Well, list.contains() tells you whether an element is contained in the list (as defined by equals()) but it doesn't tell you where. — Thomas
– Thomas, Commented Feb 26, 2020 at 15:44
@Thomas Yes, so I thought there must be some way to check only the deeper elements. Wishful thinking I guess. — Hasen
– Hasen, Commented Feb 26, 2020 at 15:47
Well something like for(int i; i < list.size(); i++) { if(list.get(i).numb == 12) return i; } return -1; isn't that much code (or something like int index = 0; for( Grid g : list) { if( g.numb == 12) return index; index++; } return -1; if you want to use a foreach). — Thomas
– Thomas, Commented Feb 26, 2020 at 15:57

zfChaos · Accepted Answer · 2020-02-26 15:37:32Z

2

You can achieve it with streams

    ArrayList<Grid> grids = new ArrayList<>();
    Grid grid = grids.stream().filter(l -> l.numb == 12).findFirst().orElse(null);
    int index = grids.indexOf(grid);

answered Feb 26, 2020 at 15:37

zfChaos

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2 Comments

Ezequiel Over a year ago

This solution iterates twice the list, first for filtering, then for getting the index of the object.

Hasen Over a year ago

Interesting answer.

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