Agregar Scrollbar a JPanel

[GSecurity]

- A mí si que me funciona. No muestras tu main ni tampoco initComponents así que el problema está ahí. En mi caso el initComponents me da problemas por lo que lo he comentado:

Código (java) [Seleccionar] Expandir

import java.awt.*;
import javax.swing.*;

public class frmTest extends JFrame {

  public frmTest() {
    //initComponents();

    JPanel pnlcenter = new JPanel();
    for (int i = 0; i < 50; i++) {
      JButton btnAgrega = new JButton("Hola");
      pnlcenter.add(btnAgrega);
    }
    pnlcenter.setLayout(new FlowLayout());
    pnlcenter.setBorder(BorderFactory.createLineBorder(Color.black));
    pnlcenter.setBounds(0, 0, 200, 200);
    pnlcenter.setPreferredSize(new Dimension(2000, 2000));
    pnlcenter.setVisible(true);

    final JScrollPane scroller = new JScrollPane(pnlcenter, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
    scroller.setViewportView(pnlcenter);
    this.setSize(new Dimension(400, 400));
    this.add(scroller);
    this.pack();
    this.setLocationRelativeTo(null);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setVisible(true);
  }

  public static void main(String[] args) {
    frmTest frm = new frmTest();
  }
}

- El código anterior funciona incluso sin usar NetBeans, desde un block de notas y compilando a mano con javac frmTest.java && java frmTest

- No te manda ningún error o advertencia? has hecho uno Click derecho > Clean a tu proyecto?

Hola, a mi tambien me funciona cuando comento el initComponent.

Mi initComponent:

Código (java) [Seleccionar] Expandir

  private void initComponents() {

        jPanel1 = new javax.swing.JPanel();

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 100, Short.MAX_VALUE)
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 100, Short.MAX_VALUE)
        );

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 479, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 336, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>                       


mi main:

Código (java) [Seleccionar] Expandir

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new frmTest().setVisible(true);
            }
        });
    }

Cual sera el problema?

[GSecurity]

Hola, a mi tambien me funciona cuando comento el initComponent.

Mi initComponent:

Código (java) [Seleccionar] Expandir

  private void initComponents() {

        jPanel1 = new javax.swing.JPanel();

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 100, Short.MAX_VALUE)
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 100, Short.MAX_VALUE)
        );

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 479, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 336, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>                       


mi main:

Código (java) [Seleccionar] Expandir

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(frmTest.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new frmTest().setVisible(true);
            }
        });
    }

Cual sera el problema?

Acabo de probar y era por  el Layout, estaba en Free Design, acabo de cambiarlo a Border Layout y funciona. Seguire probando y si tengo algun problema les comento por aqui.

Muchas gracias !!!