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center align the title in JFrame

I want to know how to center the JFrame title.I referred following link
How to center align the title in a JFrame?
but I need to center title without putting spacing.

Consider leaving the title left-justified...but...this will get you near the center. For resizable frames, you need to rewrite the title on resize.
JFrame t = new JFrame();
t.setSize(600,300);
t.setFont(new Font("System", Font.PLAIN, 14));
Font f = t.getFont();
FontMetrics fm = t.getFontMetrics(f);
int x = fm.stringWidth("Hello Center");
int y = fm.stringWidth(" ");
int z = t.getWidth()/2 - (x/2);
int w = z/y;
String pad ="";
//for (int i=0; i!=w; i++) pad +=" ";
pad = String.format("%"+w+"s", pad);
t.setTitle(pad+"Hello Center");
https://stackoverflow.com/a/9662974/3675925

Thats the only way to do it in Swing. Take note that adding spaces to center the window's title will affect other platforms. For example, for Windows 7 the title is displayed on the top-left, however on Windows 8 the title will be displayed on the top-center. Unless you want to specifically check what OS the client is running, I suggest just leaving the title as it is.

I want to decrease the font size if text doesn't fit in JLabel

There were many posts regarding this problem, but i couldn't understand the answers given by people in there.
Like in this post: "How to change the size of the font of a JLabel to take the maximum size" the answer converts the font size to 14! But that is static and further in other answers; their whole output screen seems to increase.
I display certain numbers in a JLabel named "lnum", it can show numbers upto 3 digits but after that it shows like "4..." I want that if the number is able to fit in the label, it should not change its font size but if like a number is 4 digit, it should decrease the font size in such a way that it fits. NOTE: i do not want that the dimensions of the jLabel change. I just want to change the text in It.
Edit:
Here is what code i tried
String text = lnum.getText();
System.out.println("String Text = "+text);//DEBUG
Font originalFont = (Font)lnum.getClientProperty("originalfont"); // Get the original Font from client properties
if (originalFont == null) { // First time we call it: add it
originalFont = lnum.getFont();
lnum.putClientProperty("originalfont", originalFont);
}
int stringWidth = lnum.getFontMetrics(originalFont).stringWidth(text);
int componentWidth = lnum.getWidth();
stringWidth = stringWidth + 25; //DEBUG TRY
if (stringWidth > componentWidth) { // Resize only if needed
// Find out how much the font can shrink in width.
double widthRatio = (double)componentWidth / (double)stringWidth;
int newFontSize = (int)Math.floor(originalFont.getSize() * widthRatio); // Keep the minimum size
// Set the label's font size to the newly determined size.
lnum.setFont(new Font(originalFont.getName(), originalFont.getStyle(), newFontSize));
}else{
lnum.setFont(originalFont); // Text fits, do not change font size
System.out.println("I didnt change it hahaha");//DEBUG
}
lnum.setText(text);
I have a problem that many a times it doesn't work, like if the text is "-28885" it shows "-28...".
stringWidth = stringWidth + 25; //DEBUG TRY
I had to add this code so that it increases the length that it gets. It was a code i added to just temporarly fix the problem. I want a permanent solution for this.

Adapted from an answer on the question you referred to:
void setTextFit(JLabel label, String text) {
Font originalFont = (Font)label.getClientProperty("originalfont"); // Get the original Font from client properties
if (originalFont == null) { // First time we call it: add it
originalFont = label.getFont();
label.putClientProperty("originalfont", originalFont);
}
int stringWidth = label.getFontMetrics(originalFont).stringWidth(text);
int componentWidth = label.getWidth();
if (stringWidth > componentWidth) { // Resize only if needed
// Find out how much the font can shrink in width.
double widthRatio = (double)componentWidth / (double)stringWidth;
int newFontSize = (int)Math.floor(originalFont.getSize() * widthRatio); // Keep the minimum size
// Set the label's font size to the newly determined size.
label.setFont(new Font(originalFont.getName(), originalFont.getStyle(), newFontSize));
} else
label.setFont(originalFont); // Text fits, do not change font size
label.setText(text);
}
When you'll display a number that would fit, you should reset the Font back to its original (see the else part).
EDIT: If you can't/don't want to keep a reference to the original Font, you can save it as a "client property" (see the first lines).

String length in pixels in Java

Is there a way to calculate the length of a string in pixels, given a certain java.awt.Font object, that does not use any GUI components?

that does not use any GUI components?
It depends on what you mean here. I'm assuming you mean you want to do it without receiving a HeadlessException.
The best way is with a BufferedImage. AFAIK, this won't throw a HeadlessException:
Font font = ... ;
BufferedImage img = new BufferedImage(1, 1, BufferedImage.TYPE_INT_ARGB);
FontMetrics fm = img.getGraphics().getFontMetrics(font);
int width = fm.stringWidth("Your string");
Other than using something like this, I don't think you can. You need a graphics context in order to create a FontMetrics and give you font size information.

You can use the Graphics2D object to get the font bounds (including the width):
Graphics2D g2d = ...
Font font = ...
Rectangle2D f = font.getStringBounds("hello world!", g2d.getFontRenderContext());
But that depends on how you will get the Graphics2D object (for example from an Image).

This gives the output of (137.0, 15.09375) for me. I have no idea what the units are, but it certainly looks proportionally correct and doesn't use Graphics2D directly.
Font f = new Font("Ariel", Font.PLAIN, 12);
Rectangle2D r = f.getStringBounds("Hello World! Hello World!", new FontRenderContext(null, RenderingHints.VALUE_TEXT_ANTIALIAS_DEFAULT, RenderingHints.VALUE_FRACTIONALMETRICS_DEFAULT));
System.out.println("(" + r.getWidth() + ", " + r.getHeight() + ")");

I needed to get length and width of a string before paintComponent was called so I could size the enclosing panel to the text dimensions. None of these techniques provided a sufficiently width and I did not have a Graphics object available. I resolved the issue by setting my font to "Monospaced".

Getting the visible text in a JEditorPane

I have a JeditorPane in a JScrollPane. At certain points in the application, I would like to retrieve the text that is visible in the scrollPane (the text that is currently showing) and only this text. Is there a way to do this?
Thank you,
Elliott

You can use the viewport to get the view position and size.
JViewport viewport = scrollPane.getViewport();
Point startPoint = viewport.getViewPosition();
Dimension size = viewport.getExtentSize();
Point endPoint = new Point(startPoint.x + size.width, startPoint.y + size.height);
Once you know the start/end points of the viewport you can use:
int start = editorPane.viewToModel( startPoint );
int end = editorPane.viewToModel( endPoint );
Once you know the offsets of the text you want you can get the text from the component:
String text = editorPane.getText(start, end - start);
None of the code is tested.

Java: Fonts and Pixels

I'm making a game and in the menu I want to display the text in the center of the screen.
Is there a way in Java to get/calculate the width of a piece of text in a specified font with specified size and style.
Martijn

The FontMetrics.stringWidth method does just that -- it will return the width in pixels for a given String.
One can obtain the FontMetrics from a Graphics object by the getFontMetrics method.
For example:
g.setFont(new Font("Serif", Font.BOLD, 24));
int width = g.getFontMetrics().stringWidth("Hello World!");
System.out.println(width);
The result was:
135

In the class Font you have methods such like getLineMetrics or getStringBounds that may help you.

Just use a JLabel that is center aligned and the proper layout manager and you don't have to worry about this.

JLabel label = new JLabel("Text");
frame.add(label , SwingConstants.CENTER);