Matrix into JTable

Question

I need to fill a JTable with a static 2D array. I\'ve created this model for the JTable:

 public class InsertMatToJTable extends AbstractTableModel{         


        

Answer 1

The problem is that you have written:

public void InsertMatToJTable()

but you should have written:

public InsertMatToJTable()

Notice that there is no void in the second snippet.

You have declared a method called InsertMatToJTable and not the constructor of the class with the same name. Therefore, when you invoke new InsertMatToJTable() you invoke the default no-args constructor and your code is never run, leaving your matrix remains uninitalized, hence the NullPointerException.

To avoid these kind of typo issues, add logs to your code and use a debugger to find problems.

Here is an example demo of a working code.

import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SwingUtilities;
import javax.swing.table.AbstractTableModel;

public class TestTables {

    private static final int ROWS = 270;
    private static final String titre[] = { "age real", "sex real", "chest real", "resting_blood_pressure real", "serum_cholestoral real",
            "fasting_blood_sugar real", "resting_electrocardiographic_results real", "maximum_heart_rate_achieved real",
            "exercise_induced_angina real", "oldpeak real", "slope real", "number_of_major_vessels real", "thal real", "class" };

    public static class InsertMatToJTable extends AbstractTableModel {

        private float[][] matrice_normalise;

        public InsertMatToJTable(float[][] matrice_normalise) {
            this.matrice_normalise = matrice_normalise;
        }

        @Override
        public int getRowCount() {
            return matrice_normalise.length;
        }

        @Override
        public int getColumnCount() {
            return titre.length;
        }

        @Override
        public Object getValueAt(int rowIndex, int columnIndex) {
            return matrice_normalise[rowIndex][columnIndex];
        }

        @Override
        public String getColumnName(int columnIndex) {
            return titre[columnIndex];

        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame(TestTables.class.getSimpleName());
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                float[][] matrix = new float[ROWS][titre.length];
                Random random = new Random();
                for (int i = 0; i < matrix.length; i++) {
                    for (int j = 0; j < matrix[i].length; j++) {
                        matrix[i][j] = random.nextFloat() * 100;
                    }
                }
                InsertMatToJTable model = new InsertMatToJTable(matrix);
                JTable table = new JTable(model);
                JScrollPane scroll = new JScrollPane(table);
                frame.add(scroll);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }
}