JAVA Arraylist unexpected output

Question

Expected output : Cell-1 Cell-2 Cell-3 Cell-4

But the output that I am getting is : Cell-3 Cell-4 Cell-3 Cell-4

Why is this happening?

import java.util.ArrayList;

public class Test {
public static void main(String... a) {
    String[][] var1 = new String[] []{{"Cell-1","Cell-2"},{"Cell-3","Cell-4"}};
    ArrayList<String> r = new ArrayList<String>();
    ArrayList<ArrayList<String>> var = new ArrayList<ArrayList<String>>();

    //Insering data into the ArrayList.
    for(String[] row : var1){
        r.clear();
        for(String data : row){
            r.add(data);
        }
        var.add(r);
    }

    //Print data to console.
    for(ArrayList<String> r1 : var){
        for(String cell : r1)
            System.out.print(cell+" ");
        System.out.println();
    }
}

} `

This is not an answer. You should use List<List<String>> instead. — adarshr
– adarshr, Commented Apr 11, 2013 at 10:14
@adarshr : What is the advantage of using List<List<String>> over ArrayList<ArrayList<String>> ? — Sumit Nandi
– Sumit Nandi, Commented Apr 11, 2013 at 10:34

Sergey Kalinichenko · Accepted Answer · 2013-04-11 10:13:15Z

3

This is not unexpected at all: you add the same object, namely ArrayList r, to the ArrayList var twice. First, you add it when it has Cell-1 Cell-2, then you clear it, and then you add it with Cell-3 Cell-4. The problem is, when you call r.clear(), the array list that you added to var gets cleared as well.

You need to create a new ArrayList instead of clearing the existing one to fix this problem: replace r.clear() with r = new ArrayList<String>(), and the problem will go away.

answered Apr 11, 2013 at 10:13

Sergey Kalinichenko

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