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How can i get rid of the blue highlight inside JCombobox?

The JCombobox's "content" window has this blue highlight that i don't know how to get rid of, please help.
Here is an example of the problem:
package example;
import java.awt.CardLayout;
import java.awt.Color;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Example {
public static void main(String[] args) {
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(1);
frame.setSize(500, 500);
frame.setLayout(new CardLayout());
frame.setVisible(true);
JPanel panel=new JPanel();
panel.setLayout(null);
frame.add(panel);
String[] model = {"pres.", "PPS.", "P. inp.", "P. mais q."};
JComboBox combox;
combox = new JComboBox(model);
combox.setBounds(100, 100, 145, 30);
combox.setBackground(new Color(215, 211, 165));
combox.setFocusable(false);
panel.add(combox);
panel.updateUI();
}
}

The combo box is a complex components that uses multiple components internally. The UI determines how the components interact with one another.
For example change your code to the following:
JComboBox combox;
combox = new JComboBox(model);
combox.setBorder( new LineBorder(Color.YELLOW) );
BasicComboBoxRenderer renderer = new BasicComboBoxRenderer();
renderer.setBorder( new LineBorder(Color.RED) );
combox.setRenderer(renderer);
And you will notice that the blue highlight is not a border of the combo box or its render, implying there is another internal container we don't have access to.
If you really want to solve the problem then you will need to customize the MetalComboBoxUI class, which is never an easy task because many of the painting methods will be private. But take a look at the code of the class to see if it can be done.
Other issues with your code:
Don't use updateUI(). The method is invoked internally in Swing when the LAF is changed. You are not changing the LAF
Components should be added to the frame BEFORE the frame is visible. This will eliminate the need for the updateUI().

Overlaying a JButton onto a JFrame with a JTabbedPan in its content pane?

The below code creates a frame with a JTabbedPane in the frame's contentPane and a "Help" button added directly to the frame's layered pane - at a higher index than the default, so it should always be painted in front of the tabbed pane. But as you can see if you run it, as soon as you click on one of the tabs (or, on Mac, as soon as you hover over a tab), the "Help" button gets painted over - i.e. disappears. You have to resize the frame to make the "Help" button re-appear.
Is this a Java bug or am I doing something wrong? If the latter, what needs to be done to fix the problem? I've consulted both https://docs.oracle.com/javase/tutorial/uiswing/components/rootpane.html and https://docs.oracle.com/javase/tutorial/uiswing/components/toplevel.html .
Please pardon the poor positioning of the "Help" button. I was just trying to write a quick test to include here. The idea is the to overlay a "Help" icon button in the unused space of the JTabbedPane.
import java.awt.BorderLayout;
import java.awt.Dimension;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLayeredPane;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;
import javax.swing.SwingUtilities;
public class TabTest {
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame frame = new JFrame();
JTabbedPane tp = new JTabbedPane();
tp.addTab("hello", new JPanel());
tp.addTab("there", new JPanel());
frame.getContentPane().add(tp, BorderLayout.CENTER);
JLayeredPane layeredPane = frame.getRootPane().getLayeredPane();
JButton helpButton = new JButton("Help");
helpButton.setBounds(800, 5, 50, 20);
layeredPane.add(helpButton, 400);
frame.setSize(new Dimension(900, 800));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
});
}
}

This is a common misunderstanding with JLayeredPane.
If you have a look at the JavaDocs for Container, you will note that there are (at least) two add methods
Container#add(Component, int)
Container#add(Component, Object)
The question you need to answer is, which one are you actually calling?
Both the JavaDocs for JLayeredPane and How to use JLayeredPane demonstrate that you should be calling the second one.
While the first can affect the z-ordering of the components, it's no guarantee that the component positions won't be changed.
Instead of:
layeredPane.add(helpButton, 400);
you should be using:
layeredPane.add(helpButton, new Integer(400));
which will pass the value as a constraint to the container, instead of the desired position within the container hierarchy - yeah, suitable I know
An alternative solution might be to use the glassPane instead, for example...
JFrame frame = new JFrame();
JTabbedPane tp = new JTabbedPane();
tp.addTab("hello", new JPanel());
tp.addTab("there", new JPanel());
frame.getContentPane().add(tp, BorderLayout.CENTER);
// Null layout used here for demonstration purposes only
JPanel glassPane = new JPanel(null);
glassPane.setOpaque(false);
frame.getRootPane().setGlassPane(glassPane);
// This is important, as setGlassPane makes it invisible
glassPane.setVisible(true);
JButton helpButton = new JButton("Help");
helpButton.setBounds(800, 5, 50, 20);
glassPane.add(helpButton);
frame.setSize(new Dimension(900, 800));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);

Java button hovering

I'm new to Java programming and I'd like to know how to mess around with buttons, I have created a "Home Screen" and some buttons as well as text, however it's not as advanced as I'd like it to be, I want to know how to create things like image effects, so let's say I hover over a button, I want it to display a glowing animation behind it, or since I don't have that animation, if there's no easy way to create it, just displaying an image behind it is alright, also I don't have anything happening when pressing the button bcs IDK how to do that yet so if you could help with that it'd be awesome!
Here's the code I currently have:
package Menu;
import java.awt.FlowLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
public class BackgroundPanel extends JFrame {
private BufferedImage image;
public BackgroundPanel() {
ImageIcon icon = new ImageIcon("image_path.png");
JButton btn = new JButton(icon);
btn.setOpaque(false);
btn.setContentAreaFilled(false);
btn.setBorderPainted(false);
btn.setFocusPainted(false);
try {
image = ImageIO.read(new File("image_path.jpg"));
// Set your Image Here.
this.setContentPane(new JLabel(new ImageIcon(image)));
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
JPanel panel = new JPanel();
panel.add(new JLabel("Username:"));
panel.add(new JTextField("",20));
panel.add(new JLabel("Password:"));
panel.add(new JTextField("",20));
panel.add(btn);
//Adding components to Panel and JFrame
this.add(panel);
// JFrame Properties
this.setSize(1280,720);
this.setLayout(new FlowLayout());
this.setResizable(true);
this.setLocationRelativeTo(null);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setTitle("Panel");
this.setVisible(true);
}
public static void main(String[] args) {
new BackgroundPanel();
}
}

[...] also I don't have anything happening when pressing the button bcs IDK how to do that yet so if you could help with that it'd be awesome [...]
You need to add an ActionListener to your button. There are various other ways to detect if the button was pressed, but this one is the (in my opinion) easiest. This is how you do it:
btn.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent event) {
// code you write in here will be executed if the button was pressed
}
});
[...] let's say I hover over a button, I want it to display a glowing animation behind it, or since I don't have that animation, if there's no easy way to create it, just displaying an image behind it is alright [...]
For this, you'll have to deal with JLayeredPanes and MouseListeners. Here is an example that I created "on the run"; the layouting is very dirty and has to be improved. Anyhow, you'll notice that once you hover over the button, a black-bordered box containing LA- -LL! will appear behind the button. That's a JLabel and you can use it to display images and such by using the JLabel.setIcon method.

let's say I hover over a button, I want it to display a glowing animation behind it, or since I don't have that animation, if there's no easy way to create it, just displaying an image behind it is alright
This is not that easy, it requires jumping through a whole bunch of hoops, especially when the button isn't rectangle.
A while I ago a did a prototype of a "validation highlighter" which highlighted invalid fields, it makes use of JXLayer library, but which should be convertible to use the included JLayer library on the core libraries
See How can I change the highlight color of a focused JComboBox for more details
The other possibility we'd be to create a custom JPanel and override its paintComponent method and paint your effect there. You would place your button it and with a combination of a a layout manager and borders you should be able to get the button positioned where you need it.
The problem with this, is it will effect the over layout of your form, as the effect will be considered while the primary form is laid out
I really don't have anything happening when pressing the button bcs IDK how to do that yet so if you could help with that it'd be awesome
I suggest you have a look at How to use buttons and How to write ActionListeners

How to set a specific dimension of a Button object in Java Swing?

I have this simple Java Swing test application that show an upper label and under this label a button:
package com.techub.exeute;
import java.awt.BorderLayout;
import java.awt.Button;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingConstants;
import org.jdesktop.application.SingleFrameApplication;
public class Main extends SingleFrameApplication{
public static void main(String[] args) {
Main a = new Main();
a.startup();
}
#Override
protected void startup() {
JFrame frame = new JFrame("FrameDemo");
frame.setMinimumSize(new Dimension(800, 400));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel myLabel = new JLabel("Hello World !!!", SwingConstants.CENTER);
myLabel.setFont(new Font("Serif", Font.BOLD, 22));
myLabel.setBackground(Color.RED);
myLabel.setOpaque(true);
myLabel.setPreferredSize(new Dimension(100, 80));
frame.getContentPane().add(myLabel, BorderLayout.NORTH);
Button myButton = new Button("Click Me !!!");
myButton.setMaximumSize(new Dimension(50, 25));
frame.getContentPane().add(myButton, BorderLayout.CENTER);
//Display the window.
frame.pack();
frame.setVisible(true);
}
}
The button is in the BorderLayout.CENTER position. The problem is that this button occupies all available space in the CENTER position also if I have set the Maximum Size property with a Dimension object.
What am I wrong? What can I do to create a button having specific dimension?

You're currently telling Java to place the button in the center of your BorderLayout. Java then thinks that you want the entire area filled with this button. If you want to place a normal sized button in the center of your BorderLayout, add the button to a new JPanel and place the JPanel inside BorderLayout.CENTER.
Doing this, you're telling Java to fill out BorderLayout.CENTER with your JPanel. The elements that you place inside this JPanel will appear normal, because these elements are not getting "stretched" because of your BorderLayout - the JPanel is.

You fail to realize that it's not only the max/min/preferred sizes of the components that determine the outcome. The LayoutManager has a lot to say in this, and as you noticed, BorderLayout will fill the whole area with the component.
What you can do is create a JPanel, make it use for example FlowLayout and put that JPanel to BorderLayout.CENTER and the JButton to the JPanel.

You are using BorderLayout, North region has JLabel. JButton is put on center. JButton will occupy all the space left as per BorderLayout.
Please find more information about Layout on link : http://docs.oracle.com/javase/tutorial/uiswing/layout/using.html

U are using BorderLayout, button occupy all the left space of the frame.
use setBounds() function .
myButton.setBounds(10,200,100,30);

How to draw a colored rectangle on a JButton?

I'm going to enhance an application with a Swing UI, to allow the user to pick colors so they're not stuck with the default color choices.
It is common for other applications to have shaded rectangles drawn on each button that activates a color selector, with the rectangle's color changing accordingly when a new color is selected. I am trying to achieve the same effect by placing a small JPanel with the selected color on the button, but that results in a tiny square in the middle of the button, instead of filling most of the surface of the button.
I figure another way would be to dynamically generate rectangular icons with the colors and then add the appropriate icon to each button, but surely there must be a simpler way?

put there JButton.setIcon with expected Rectangle, for example
EDIT
I am trying to achieve the same effect by placing a small JPanel with the selected color on the button, but that results in a tiny square in the middle of the button, instead of filling most of the surface of the button.
only JFrame (BorderLayout) and JPanel (FlowLayout) have got pre-implemented LayoutManager, for rest of JComponents (add one JComponent to the another JComponent) you have to define LayoutManager, please read this thread

Here is an example using setBackground that works for me:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JColorChooser;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Test {
public static void main(String... args) {
JFrame frame = new JFrame();
JPanel panel = new JPanel();
final JButton button = new JButton("Hello");
button.setOpaque(true);
panel.add(button);
button.setBackground(Color.RED);
button.setOpaque(true);
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
Color c = JColorChooser.showDialog(button, "Choose a color", button.getBackground());
if (c != null) {
button.setBackground(c);
}
}
});
frame.setContentPane(panel);
frame.setPreferredSize(new Dimension(800, 600));
frame.pack();
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setVisible(true);
}
}

Every swing component is drawn using the JComponent.drawComponent(...) method, you can override the default behavior if you want. For example, to make a panel like you suggest. It is worth noting though, that you can change a jpanels background color to achieve the exact same thing.
JColorChooser chooser = new JColorChooser(Color.BLACK);
chooser.setVisible(true);
Color color = chooser.getColor();
if (color!=null) {
colorPanel.setBackground(color);
}
Where colorPanel would be your JPanel indicating your color.