How to center a component using null layout?

Question

my knob I want to center is 100 pixels wide. Jframe is 350 wide. how to center?

/*
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 * and open the template in the editor.
 */
package ComboBoxLUL;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

/**
 *
 * @author Igor
 */
public class PainelPrincipal extends JPanel {

    private JComboBox jcbGenero;

    private JComboBox jcbCasado;

    private JButton btnSalva;

    private JLabel lblNome;

    private JLabel lblIdade;

    private JLabel lblCpf;

    private JTextField txtCpf;

    private JTextField txtNome;

    private JTextField txtIdade;

    private final String[] genero = {"Masculino ", "Feminino"};

    private final String[] estCivil = {"Casado", "Solteiro"};

    // private Integer[] genero = {1,2,3};// essa forma funciona. int nao. mas o resultado é o msm.

    PainelPrincipal() {
        this.setLayout(null);

        txtNome = new JTextField();
        txtNome.setBounds(45, 90, 120, 25);
        this.add(txtNome);
        ///////////
        txtCpf = new JTextField();
        txtCpf.setBounds(45, 50, 120, 25);
        this.add(txtCpf);
        ///////////
        txtIdade = new JTextField();
        txtIdade.setBounds(45, 130, 120, 25);
        this.add(txtIdade);

        //////////
        lblCpf = new JLabel("CPF");
        lblCpf.setBounds(10, 50, 120, 25);
        this.add(lblCpf);
        //////////

        lblIdade = new JLabel("Idade");
        lblIdade.setBounds(10,90,120,25);
        this.add(lblIdade);

        lblNome = new JLabel("Nome");
        lblNome.setBounds(10, 130, 120, 25);
        this.add(lblNome);

        jcbGenero = new JComboBox(genero);
        jcbGenero.setBounds(170, 50, 120, 25);
        this.add(jcbGenero);

        jcbCasado = new JComboBox(estCivil);
        jcbCasado.setBounds(170, 90, 120, 25);
        this.add(jcbCasado);

        btnSalva = new JButton("Salvar");
        btnSalva.setBounds(175, 175, 100, 25);
        this.add(btnSalva);

        btnSalva.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent ae) {
              //@@@@@@@@@@@@@@@@@@@@@@
            }

        });
        this.add(btnSalva);

    }

}

Answer 1

Just use a little math: if the screen is 350px full width and the component is 100, for it to occupy a central portion, its horizontal position on the screen is:

(outflow/2) - (outflow/2)

Applying this calculation with the informed data results:

350/2 - 100/2 = 125

This 125 is the horizontal position from where your button should be positioned on setBounds. Behold:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JFrame;
import javax.swing.JTextField;
import java.awt.*;

/**
 *
 * @author Igor
 */
public class PainelPrincipal extends JPanel {

    private JComboBox jcbGenero;

    private JComboBox jcbCasado;

    private JButton btnSalva;

    private JLabel lblNome;

    private JLabel lblIdade;

    private JLabel lblCpf;

    private JTextField txtCpf;

    private JTextField txtNome;

    private JTextField txtIdade;

    private final String[] genero = {"Masculino ", "Feminino"};

    private final String[] estCivil = {"Casado", "Solteiro"};

    // private Integer[] genero = {1,2,3};// essa forma funciona. int nao. mas o resultado é o msm.
    PainelPrincipal() {
        this.setLayout(null);

        txtNome = new JTextField();
        txtNome.setBounds(45, 90, 120, 25);
        this.add(txtNome);
        ///////////
        txtCpf = new JTextField();
        txtCpf.setBounds(45, 50, 120, 25);
        this.add(txtCpf);
        ///////////
        txtIdade = new JTextField();
        txtIdade.setBounds(45, 130, 120, 25);
        this.add(txtIdade);

        //////////
        lblCpf = new JLabel("CPF");
        lblCpf.setBounds(10, 50, 120, 25);
        this.add(lblCpf);
        //////////

        lblIdade = new JLabel("Idade");
        lblIdade.setBounds(10, 90, 120, 25);
        this.add(lblIdade);

        lblNome = new JLabel("Nome");
        lblNome.setBounds(10, 130, 120, 25);
        this.add(lblNome);

        jcbGenero = new JComboBox(genero);
        jcbGenero.setBounds(170, 50, 120, 25);
        this.add(jcbGenero);

        jcbCasado = new JComboBox(estCivil);
        jcbCasado.setBounds(170, 90, 120, 25);
        this.add(jcbCasado);

        btnSalva = new JButton("Salvar");
        // btnSalva.setPreferredSize(new Dimension(100, 25));
        // JPanel btnPan = new JPanel(new FlowLayout(FlowLayout.CENTER));
        // btnPan.add(btnSalva);
        // this.add(btnPan);
        btnSalva.setBounds(125, 175, 100, 25);
        this.add(btnSalva);

        btnSalva.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent ae) {
                //@@@@@@@@@@@@@@@@@@@@@@
            }

        });
        this.add(btnSalva);

    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                JFrame f = new JFrame();
                f.getContentPane().add(new PainelPrincipal(), BorderLayout.CENTER);
                f.setPreferredSize(new Dimension(350, 350));
                f.pack();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setVisible(true);
            }
        });
    }

}

---

But as a suggestion, avoid always using absolute layout. It makes it difficult to maintain the interface, because any future changes you will have to leave by repositioning all components manually, imagine a screen with dozens of these? Not to mention that depending on the resolution and size of the monitor, your interface may look different than you programmed. Read on Layoutmanager, there are several native and other very good community options, and the best is that you can merge all of them.

Here are some links about Layoutmanager:

• More Swing: layout managers, more components and details

• Getting to know Java GUI Layout Managers