Calling setVisible(true) on an already visible frame
awt, java, jpanel, repaint, swing
Solution
Calling `setVisible(true)` on a `JFrame` which is already visible works for you because this ends up calling `validate()` internally, which in turn revalidates all subcomponents in the frame.
To see why, refer to the implementation of `Component.setVisible(boolean b)`:
public void setVisible(boolean b) {
show(b);
}
public void show(boolean b) {
if (b) {
show();
} else {
hide();
}
}
But the `show()` method is overriden in `Window` (of which `JFrame` is a subclass). So this ends up calling `Window.show()`:
public void show() {
if (peer == null) {
addNotify();
}
validate();
[...]
Hope this explains the behaviour you are seeing.
Problem
Question: What does calling setVisible(true) on a JFrame which is already visible do? I was digging through the source code of JFrame, and ultimately, it boils down to this function in Component which does nothing to a frame if it is already visible. Why does it act like `revalidate(); repaint();`? (See SSCCE below) Motivation: I am working on a java app, for which I wrote a class `JImagePanel` which extends `JPanel` and allows the user to set an image as the background (see SSCCE). I have found that after editing the background of the panel, I was having issues repainting the background to the correct size. After scouring the internet, I found that the following works: ``` if(frame.isVisible()) frame.setVisible(true); ``` Ultimately, I solved the issue using ``` panel.revalidate(); panel.repaint(); ``` , which I think is the better solution, but it got me to thinking what `setVisible(true)` actually does on an already visible frame. From my viewpoint, it shouldn't work - but in fact it does. SSCCE Here is an example that illustrates my issue. If nothing else, hopefully you find this class extremely useful in the future. NOTE: Updated source of this file can be found on the project homepage on GitHub of the project this was created for. Enjoy! ``` package com.dberm22.utils; import java.awt.Color; import java.awt.Dimension; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.Image; import java.awt.RenderingHints; import java.awt.Toolkit; import java.awt.image.BufferedImage; import javax.swing.ImageIcon; import javax.swing.JFrame; import javax.swing.JPanel; public class JImagePanel extends JPanel { private static final long serialVersionUID = 6841876236948317038L; private Image img = null; private Position position = Position.CENTER; public enum Position{ STRETCH, CENTER, FIT, FILL, NONE; } public JImagePanel() { } public JImagePanel(String img) { this(new ImageIcon(img).getImage()); } public JImagePanel(Image img) { setBackgroundImage(img); } @Override public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g; g2.setColor(getBackground()); g2.fillRect(0, 0, getWidth(), getHeight()); if (this.position.equals(Position.STRETCH)) { if(this.img != null) g2.drawImage(img, 0, 0, getWidth(), getHeight(), null); } else if (this.position.equals(Position.FILL) || this.position.equals(Position.FIT)) { if(this.img != null) { double scaleFactor = getScaleFactor(new Dimension(img.getWidth(null), img.getHeight(null)), getSize()); int scaleWidth = (int) Math.round(img.getWidth(null) * scaleFactor); int scaleHeight = (int) Math.round(img.getHeight(null) * scaleFactor); //Image img_scaled = img.getScaledInstance(scaleWidth, scaleHeight, Image.SCALE_SMOOTH); g2.drawImage(scaleImage(img, scaleWidth, scaleHeight, getBackground()), (getWidth() - scaleWidth)/2, (getHeight() - scaleHeight)/2, scaleWidth, scaleHeight, null); } } else if (this.position.equals(Position.CENTER)) { if(this.img != null) g2.drawImage(img, (getWidth() - img.getWidth(null))/2, (getHeight() - img.getHeight(null))/2, null); } } public void setBackgroundImage(String img) { setBackgroundImage(new ImageIcon(img).getImage()); } public void setBackgroundImage(Image img) { this.img = img; Dimension size = new Dimension(img.getWidth(null), img.getHeight(null)); setPreferredSize(size); setMinimumSize(size); setMaximumSize(size); setSize(size); repaint(); } public static double getScaleFactor(int iMasterSize, int iTargetSize) { double dScale = 1; if (iMasterSize > iTargetSize) { dScale = (double) iTargetSize / (double) iMasterSize; } else { dScale = (double) iTargetSize / (double) iMasterSize; } return dScale; } public double getScaleFactor(Dimension original, Dimension targetSize) { double dScale = 1d; if (original != null && targetSize != null) { double dScaleWidth = getScaleFactor(original.width, targetSize.width); double dScaleHeight = getScaleFactor(original.height, targetSize.height); if (this.position.equals(Position.FIT)) dScale = Math.min(dScaleHeight, dScaleWidth); else if(this.position.equals(Position.FILL)) dScale = Math.max(dScaleHeight, dScaleWidth); } return dScale; } public BufferedImage scaleImage(Image img, int width, int height, Color background) { BufferedImage newImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB); Graphics2D g = newImage.createGraphics(); try { g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC); g.setBackground(background); g.clearRect(0, 0, width, height); g.drawImage(img, 0, 0, width, height, null); } finally { g.dispose(); } return newImage; } public void setBackgroundImagePosition(String pos) { if("Stretch".equals(pos)) setBackgroundImagePosition(Position.STRETCH); else if("Center".equals(pos)) setBackgroundImagePosition(Position.CENTER); else if("Fit".equals(pos)) setBackgroundImagePosition(Position.FIT); else if("Fill".equals(pos)) setBackgroundImagePosition(Position.FILL); else if("None".equals(pos)) setBackgroundImagePosition(Position.NONE); } public void setBackgroundImagePosition(Position pos) { this.position = pos; repaint(); } public static void main(String[] args) { JFrame frame = new JFrame("JImagePanel Test"); frame.setSize( Toolkit.getDefaultToolkit().getScreenSize()); frame.setPreferredSize( Toolkit.getDefaultToolkit().getScreenSize()); frame.setExtendedState(JFrame.MAXIMIZED_BOTH); //sets appropriate size for frame JImagePanel panel = new JImagePanel(); frame.add(panel); frame.setVisible(true); try {Thread.sleep(2000);} catch (InterruptedException e) {} panel.setBackgroundImage("C:\\Users\\David\\Pictures\\Wood.jpg"); panel.setBackgroundImagePosition(JImagePanel.Position.STRETCH); panel.revalidate(); // need to revalidate() panel.repaint(); //doesnt work by itself try {Thread.sleep(2000);} catch (InterruptedException e) {} panel.setBackgroundImage("C:\\Users\\David\\Pictures\\Wood.jpg"); panel.setBackgroundImagePosition(JImagePanel.Position.FIT); frame.setVisible(true); //also works --why? } } ```