FontMetrics.getStringBounds for AttributedString gives wrong result?

Question

As shown in the following picture, an AttributedString is drawn on a JPanel (500X500).

The FontMetrics.getStringBounds() of that AttributedString gives a width of 164.0, as indicated by the trace output.

java.awt.geom.Rectangle2D$Float[x=0.0,y=-12.064453,w=164.0,h=15.09375]

However, the picture suggests the width should be 300-400 (because the width of the panel is 500).

Could you help to comment the reason and the workaround ?

MyJFrame.java

import javax.swing.*;
import java.awt.*;
import java.awt.font.TextAttribute;
import java.text.AttributedString;

class MyJPanel extends JPanel {
    MyJPanel() {
        setPreferredSize(new Dimension(500,500));
    }

    @Override
    public void paintComponent(Graphics gold) {
        super.paintComponent(gold);
        Graphics2D g = (Graphics2D)gold;
        //
        AttributedString text = new AttributedString("Bunny rabits and flying ponies");
        text.addAttribute(TextAttribute.FONT, new Font("Arial", Font.BOLD, 24), 0, "Bunny rabits".length());
        text.addAttribute(TextAttribute.FOREGROUND, Color.RED, 0, "Bunny rabits".length());

        text.addAttribute(TextAttribute.FONT, new Font("Arial", Font.BOLD & Font.ITALIC, 32), 17, 17 + "flying ponies".length());
        text.addAttribute(TextAttribute.FOREGROUND, Color.BLUE, 17, 17 + "flying ponies".length());

        FontMetrics fm = g.getFontMetrics();

        System.out.println(fm.getStringBounds(text.getIterator(), 0, text.getIterator().getEndIndex(), g));
        g.drawString(text.getIterator(), 50, 50);
        //
        g.dispose();
    }
}

public class MyJFrame extends JFrame {

    public static void main(String[] args) {
        MyJFrame frame = new MyJFrame();
        frame.setContentPane(new MyJPanel());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }    
}

Answer 1

FontMetrics fontMetrics = graphics.getFontMetrics() returns a FontMetrics object based on the single font currently set on the graphics object. You are not changing the font used by graphics explicitly so it uses the default font designated for JPanel by the current L&F.

FontMetrics methods related to bounds calculation accept a "simple" CharacterIterator (which does not provide font information) instead of AttributedCharacterIterator (which does). Hence fontMetrics.getStringBounds() simply calculates the text bounds based on the single font of the same size.

You need to use java.awt.font.TextLayout to determine the proper bounds when using AttributedCharacterIterator with different fonts and font sizes:

TextLayout textLayout = new TextLayout( 
        text.getIterator(), 
        g.getFontRenderContext() 
);
Rectangle2D.Float textBounds = ( Rectangle2D.Float ) textLayout.getBounds();

g.drawString( text.getIterator(), 50, 50 );
// lets draw a bounding rect exactly around our text
// to be sure we calculated it properly
g.draw( new Rectangle2D.Float(
        50 + textBounds.x, 50 + textBounds.y,
        textBounds.width, textBounds.height
) );

Answer 2

The FontMetrics only receives a CharacterIterator, and does not take into account that it is actually an AttributedCharacterIterator. You can use a TextMeasurer to compute the actual bounds of your string. For comparison, add this after you called the drawString method:

    // Compensate for the 50,50 of the drawString position
    g.translate(50, 50);

    g.setColor(Color.RED);
    Rectangle2D wrongBounds = fm.getStringBounds(
            text.getIterator(), 0, text.getIterator().getEndIndex(), g);
    g.draw(wrongBounds);
    System.out.println("wrong: "+wrongBounds);

    g.setColor(Color.BLUE);
    AttributedCharacterIterator iterator = text.getIterator();
    TextMeasurer tm = new TextMeasurer(iterator, g.getFontRenderContext());
    Rectangle2D rightBounds = tm.getLayout(0, iterator.getEndIndex()).getBounds();
    g.draw(rightBounds);
    System.out.println("right: "+rightBounds);

(And BTW: Don't call g.dispose() on the Graphics that was handed to you in the paintComponent method)