JScrollPane sp = new JScrollPane(pt);
fr.getContentPane().add(sp);

TextArea textArea = new JTextArea(40, 60);
JScrollPane scrollableTextArea = new JScrollPane(textArea);
  scrollableTextArea.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package letc;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JPasswordField;
import javax.swing.JOptionPane;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.JButton;
import javax.swing.JComboBox;

public class Letc {

    public static void main(String[] args) {

        JFrame fr = new JFrame();
        JPanel p = new JPanel();
        p.setBackground(Color.blue);

        JTextField tf = new JTextField(8);
        JTextArea pt = new JTextArea();
        pt.setLineWrap(true);
        pt.setWrapStyleWord(true);

        tf.setBounds(20,80,100,40);
        pt.setBounds(200,20,400,400);

        JScrollPane sp = new JScrollPane(pt);
        fr.getContentPane().add(sp);
        // I used the above code snippet to add scrollbar in the textarea but it's not working :(

        JButton b= new JButton("click");
        b.setBounds(20,20,100,50);

        b.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String st= tf.getText(),ck="";
                if(st.equals("")) {
                   JOptionPane.showMessageDialog(null,"enter the length:");
                }
                else {
                    int val=Integer.valueOf(st);
                    String z="";
                    for(int i=0;i<10000;i++) {
                        z+='a';
                    }
                    pt.setText(z);
                }
            }
        });

        fr.add(sp);
        fr.add(tf);
        fr.add(pt);
        fr.add(b);
        fr.add(p);
        fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        fr.setSize(600, 600);
        fr.setVisible(true);
    }
}

p.add(sp);
    p.add(tf);
    p.add(pt);
    p.add(b);
    p.add(p);
    fr.add(p);
   fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    fr.setSize(600, 600);
   fr.setVisible(true);