Why doesn't my second button show in Java

Try using JButton instead of Button, if that doesn't work, jbg ga. Also try extending class with JFrame -> extends JFrame, if you want the subclass to inherit everything from jframe class. Also u have to add it using frame.add(button)

The second button is not added to frame. You need to call:

frame.add(b2);

Regarding button size, use the method .setSize for each button to declare what size you want them to have.

The other answers are correct that you don't add the second button at all, however, you shouldn't add components to frames directly. What you want to do is

frame.getContentPane().add(button1);
frame.getContentPane().add(button2);

You should also probably be setting a layout manager to the pane.

Code :

package hw10;

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class HW10 {
    JButton b1;
    JButton b2;
//    L1 l1;
/*
    class L1 implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
//            int tmp = Integer.parseInt(b1.getLabel());
            int tmp = Integer.parseInt(b1.getText());
            tmp++;
//            b1.setLabel(""+tmp);
            b1.setText(Integer.toString(tmp));
        }

    }
*/

    public HW10() {
        JFrame frame = new JFrame("Homework 15");
//        l1 = new L1();
        b1 = new JButton("0");
//        b1.setSize(100, 40);
        b2 = new JButton("KURAC");
//        b2.setSize(100, 40);
        /*
            We can use lambda expressions
         */
        b1.addActionListener((ActionEvent e) -> {
            int tmp = Integer.parseInt(b1.getText());
            tmp++;
            b1.setText(Integer.toString(tmp));
        });
        //Use GridBagLayout as your layout manager
        GridBagLayout layout = new GridBagLayout();
        JPanel panel = new JPanel();
        panel.setLayout(layout);
        GridBagConstraints c = new GridBagConstraints();
        c.insets = new Insets(2, 2, 2, 2);
        //0th row
        c.gridx = 0;
        //0th column
        c.gridy = 0;
        //width
        c.ipadx = 100;
        //height
        c.ipady = 30;
        //Adding first button
        panel.add(b1, c);
        //1st column we
        c.gridx = 1;
        //Adding second button
        panel.add(b2, c);
        frame.add(panel);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    public static void launch() {
        SwingUtilities.invokeLater(() -> {
            new HW10();
        });
    }

    public static void main(String... $) {
        launch();
    }

}

Output :

Resource :

geeksforgeeks

As others pointed out, you missed adding the second button to your frame. A quick fix would be to add both the buttons as below:

frame.getContentPane().add(b1, BorderLayout.NORTH);
frame.getContentPane().add(b2, BorderLayout.SOUTH);