Unexpected output in java when using ArrayList<ArrayList<String>>()

Question

I have the following code in java, which takes in input from the user. It is basically a simple database system.

ArrayList<String> commands = new ArrayList<String>();
ArrayList<ArrayList<String>> blocks = new ArrayList<ArrayList<String>>();
ArrayList<String> list = new ArrayList<String>();

System.out.println("Enter the transaction commands.\n");

Scanner scan = new Scanner(System.in);

while(!(line = scan.nextLine()).toLowerCase().equals("end"))
{
    commands.add(line);
}

for(String com : commands)
{
    String split[] = com.split(" ");
    if(!split[0].toLowerCase().equals("get") && !split[0].toLowerCase().equals("numequalto") && !split[0].toLowerCase().equals("rollback") && !split[0].toLowerCase().equals("commit"))
    {
        if(split[0].toLowerCase().equals("begin"))
        {
            if(!list.isEmpty())
            {
                blocks.add(list);
                System.out.println(blocks.get(0));
                list.clear();
            }
            else
            {
                continue;
            }
        }
        else
        {
            list.add(com);
            continue;
        }
    }
}
System.out.println(blocks.get(0));

The input I give for this program is:

set a 10
set b 20
begin
get a
get b
end

While the expected output is:

[set a 10, set b 20]
[set a 10, set b 20]

I get the output as:

[set a 10, set b 20]
[]

The problem seems to be that the value of the ArrayList> blocks, seems to be overwritten. The last print statement prints the value as an empty ArrayList. I cannot find the exact source of error. Any help in finding out the error will be greatly appreciated.

Have you tried stepping through your code with the debugger? — beny23
– beny23, Commented Nov 13, 2013 at 7:18

beny23 · Accepted Answer · 2013-11-13 08:21:50Z

3

I believe the following code is the culprit:

blocks.add(list);
System.out.println(blocks.get(0));
list.clear();

You add the list to blocks. Note, when you are adding your list object to blocks, you are not copying the list.

So when you clear the list, it clears the list object that is also referenced in the blocks list.

To avoid this you could just:

blocks.add(list);
System.out.println(blocks.get(0));
list = new ArrayList<String>();

This will create a new list object and leave the one in your blocks list untouched.

edited Nov 13, 2013 at 8:21

answered Nov 13, 2013 at 7:28

beny23

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Comments

Masudul · Accepted Answer · 2013-11-13 07:23:55Z

3

Your second output is got from last line of System.out.println(blocks.get(0));. Have a look at your code, you add blocks.add(list); after a while you clear the list. As List is mutable, so blocks List is empty. So, your second output print nothing.

 blocks.add(list); //list has added here with values
 System.out.println(blocks.get(0));
 list.clear();  // list here without values.

edited Nov 13, 2013 at 7:23

answered Nov 13, 2013 at 7:16

Masudul

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3 Comments

Maddy Over a year ago

Doesn't the line blocks.add(list); do that?

Masudul Over a year ago

Yes. After addition, you clear list as well as block list. Because, List is mutable.

Maddy Over a year ago

Thanks. I totally missed that one.! :)

Monicka Akilan · Accepted Answer · 2013-11-13 07:32:13Z

This giving correct answer as you expected.

remove that list.clear() statement

import java.util.ArrayList;
import java.util.Scanner;

public class Test {

    public static void main(String[] args) {
        ArrayList<String> commands = new ArrayList<String>();
        ArrayList<ArrayList<String>> blocks = new ArrayList<ArrayList<String>>();
        ArrayList<String> list = new ArrayList<String>();

        System.out.println("Enter the transaction commands.\n");
        String line;
        Scanner scan = new Scanner(System.in);

        while (!(line = scan.nextLine()).toLowerCase().equals("end")) {
            commands.add(line);
        }

        for (String com : commands) {
            String split[] = com.split(" ");
            if (!split[0].toLowerCase().equals("get")
                    && !split[0].toLowerCase().equals("numequalto")
                    && !split[0].toLowerCase().equals("rollback")
                    && !split[0].toLowerCase().equals("commit")) {
                if (split[0].toLowerCase().equals("begin")) {
                    if (!list.isEmpty()) {
                        blocks.add(list);
                        System.out.println("list :" + blocks.get(0));
                        // list.clear();
                    } else {
                        continue;
                    }
                } else {
                    list.add(com);
                    continue;
                }
            }
        }
        System.out.println("output :" + blocks.get(0));
    }

}

Answer i got is

list :[set a 10, set b 20]
output :[set a 10, set b 20]

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