CODEWITHSUNDEEP
javaarraylistsize
Ashley Stewart
Ashley Stewart

Reputation: 161

ArrayList.size() returning incorrect value

I have a strange error which I can not get my head around where the .size() method does not appear to return the correct value.

This first bit of code creates the ArrayList.

public void createResultList(String query) {
    ArrayList<ArrayList<String>> data = new ArrayList();

    try {
        ResultSet rs = st.executeQuery(query);
        ResultSetMetaData meta = rs.getMetaData();

        for(int i = 0; i < meta.getColumnCount(); i++) {
            data.add(i, new ArrayList<String>());
        }

        int x = 0;
        while(rs.next()) {
            for(int y = 0; y < meta.getColumnCount(); y++) {
                data.get(x).add(rs.getString(y + 1));
            }
            x++;
        }


    } catch(Exception e) {
        JOptionPane.showMessageDialog(null, e);
    }

    ResultTable result = new ResultTable(data);
    JTable table = new JTable(result);
    JScrollPane scrollpane = new JScrollPane(table);
    add(scrollpane);
    refresh();
}

This is my TableModel class which is used to create the table when it's passed to it.

public class ResultTable extends AbstractTableModel {

    private ArrayList<ArrayList<String>> data;

    public ResultTable(ArrayList<ArrayList<String>> data) {
        this.data = data;
    }

    public int getColumnCount() { 
        return data.get(0).size(); 
    }

    public int getRowCount() { 
        return data.size();
    }

    public Object getValueAt(int row, int col) { 
        return data.get(row).get(col);
    }
}

Now the the problem is in the ResultTable class, now for a select query returning one row with 12 columns, the first data.get(0).size() call correctly returns 12, but the 2nd data.size() call incorrectly returns 12 also instead of 1, this is causing out of bounds errors, can anyone please explain this seemingly paradoxical result?

Upvotes: 1

Views: 4689

Answers (2)

Micha&#235;l Benjamin Saerens
Micha&#235;l Benjamin Saerens

Reputation: 875

This is something you should've found easily when you debug your code...

public void createResultList(String query) {
    ArrayList<ArrayList<String>> data = new ArrayList();

data is an ArrayList of ArrayLists

    try {
        ResultSet rs = st.executeQuery(query);

A query returning 1 row of 12 columns

        ResultSetMetaData meta = rs.getMetaData();
        for(int i = 0; i < meta.getColumnCount(); i++) {

For each column in your recordset, being 12, you add an empty ArrayList in data

            data.add(i, new ArrayList<String>());
        }

resulting in data being an ArrayList of 12 empty Arraylists This already explains why data.size() == 12

        int x = 0;
        while(rs.next()) {

for each record in your recordset, being 1

            for(int y = 0; y < meta.getColumnCount(); y++) {

for each column in your recordset, being 12, you add a string to the ArrayList with the same index as the recordset

                data.get(x).add(rs.getString(y + 1));
            }

The first ArrayList in data (data.get(0)) will have 12 Strings All other ArrayLists in data (data.get(x) where x > 0) will remain empty

            x++;
        }

Resulting in data being an ArrayList of 12 ArrayLists of which only the first ArrayList has 12 Strings and the others are empty

    } catch(Exception e) {
        JOptionPane.showMessageDialog(null, e);
    }

    ResultTable result = new ResultTable(data);
    JTable table = new JTable(result);
    JScrollPane scrollpane = new JScrollPane(table);
    add(scrollpane);
    refresh();
}

Upvotes: 2

Joni
Joni

Reputation: 111219

When you create the data list, you create a sublist to hold the data for each column.

If that is what you want to do, you should add data from each column of the result set to the list that corresponds to it:

    while(rs.next()) {
        for(int y = 0; y < meta.getColumnCount(); y++) {
            data.get(y).add(rs.getString(y + 1));
        }
    }

Upvotes: 0

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