Java - iterate over ArrayList rows' (columns)?

Question

What is the simplest way to just iterate over ArrayList rows values?

For example I have two ArrayList:

ArrayList<Object> main = new ArrayList<Object>();
ArrayList<Object> row = new ArrayList<Object>();

I use two fors to add values into main. First I add row, then when row iterations end, I add it to main, and then repeat for another row etc. For example:

for(int i=0; i < somearray.length; i++){
  for(int j=0; j < somearray2.length; j++){
    if(somearray2[j] == true)
      row.add(somearray2[j]);
    }
  main.add(row);

}

So now I get ArrayList filled with rows like I need. But later I need to iterate over rows values, not just rows themselves from main ArrayList. How can I do that? As I only see the method:

main.get(index), which let's me get a row, but nothing more.

JB Nizet · Accepted Answer · 2013-11-10 13:44:52Z

7

So, based on your description, main is not a list of Objects, but a list of lists. So it should be declared as

List<List<Object>> main = new ArrayList<>();

The iteration now becomes obvious:

for (List<Object> row : main) {
    for (Object element : row) {
        // do what you want here
    }
}

This has the additional advantage of making your code type-safe: you won't be able to add, inadvertently, anything other than a List<Object> inside the main list.

answered Nov 10, 2013 at 13:44

JB Nizet

694k94 gold badges1.3k silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Sage · Accepted Answer · 2013-11-10 14:47:18Z

0

Yes then access them as: ((ArrayList<Object>)main.get(index)).get(index2);. But if you want all of your row element to be added to main array, you can make use of:

Collections.addAll(main, row.toArray());

edited Nov 10, 2013 at 14:47

answered Nov 10, 2013 at 13:45

Sage

15.5k3 gold badges36 silver badges40 bronze badges

Collectives™ on Stack Overflow

Java - iterate over ArrayList rows' (columns)?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related