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This chapter is from the book

This chapter is from the book

10.6 Stroke Styles

In the AWT, the drawXxx methods of Graphics resulted in solid, 1-pixel-wide lines. Furthermore, drawing commands that consisted of multiple-line segments (e.g., drawRect and drawPolygon) had a predefined way of joining the line segments and terminating segments that did not join to others. Java 2D gives you much more flexibility. In addition to setting the pen color or pattern (through setPaint, as discussed in the previous section), with Java 2D you can set the pen thickness and dashing pattern and specify the way in which line segments end and are joined together. To control how lines are drawn, first create a BasicStroke object, then use the setStroke method to tell the Graphics2D object to use the BasicStroke object.

Stroke Attributes

Arguments to setStroke must implement the Stroke interface, and the BasicStroke class is the sole built-in class that implements Stroke. Here are the BasicStroke constructors.

public BasicStroke()

This constructor creates a BasicStroke with a pen width of 1.0, the default cap style of CAP_SQUARE, and the default join style of JOIN_MITER. See the following examples of pen widths and cap/join styles.

public BasicStroke(float penWidth)

This constructor creates a BasicStroke with the specified pen width and the default cap/join styles (CAP_SQUARE and JOIN_MITER).

public BasicStroke(float penWidth, int capStyle, int joinStyle)

This constructor creates a BasicStroke with the specified pen width, cap style, and join style. The cap style can be one of CAP_SQUARE (make a square cap that extends past the end point by half the pen width—the default), CAP_BUTT (cut off segment exactly at end point—use this one for dashed lines), or CAP_ROUND (make a circular cap centered on the end point, with a diameter of the pen width). The join style can be one of JOIN_MITER (extend outside edges of lines until they meet—the default), JOIN_BEVEL (connect outside corners of outlines with straight line), or JOIN_ROUND (round off corner with circle with diameter equal to the pen width).

public BasicStroke(float penWidth, int capStyle, int joinStyle, 
                         float miterLimit)

This constructor is the same as above, but you can limit how far up the line the miter join can proceed (default is 10.0). A miterLimit of 10.0 is a reasonable default, so you rarely need this constructor.

public BasicStroke(float penWidth, int capStyle, int joinStyle, 
               float miterLimit, 
               float[] dashPattern,
                      float dashOffset)

This constructor lets you make dashed lines by specifying an array of opaque (entries at even array indices) and transparent (odd indices) segments. The offset, which is often 0.0, specifies where to start in the dashing pattern.

Two examples of controlling the pen attribute follow. The first example, Listing 10.10, sets the pen width to 8 pixels before drawing an outlined circle, and the second example, Listing 10.11, creates a dashed line. The dash pattern is

float[] dashPattern { 30, 10, 10, 10 };

where the values alternate between the dash length and the gap length. The result is an opaque dash for 30 units, a transparent dash for 10 units, another opaque dash for 10 units, and, finally, a transparent dash for the last 10 units. The pattern is repeated along the line segment. The results for the two examples are shown in Figure 10–6 and Figure 10–7, respectively.

Listing 10.10 StrokeThicknessExample.java

import java.awt.*;

/** An example of controlling the Stroke (pen) widths when
 * drawing.
 */

public class StrokeThicknessExample extends FontExample {
 public void paintComponent(Graphics g) {
  clear(g);
  Graphics2D g2d = (Graphics2D)g;
  drawGradientCircle(g2d);
  drawBigString(g2d);
  drawThickCircleOutline(g2d);
 }

 protected void drawThickCircleOutline(Graphics2D g2d) {
  g2d.setPaint(Color.blue);
  g2d.setStroke(new BasicStroke(8)); // 8-pixel wide pen
  g2d.draw(getCircle());
 }

 public static void main(String[] args) {
  WindowUtilities.openInJFrame(new StrokeThicknessExample(),
                 380, 400);
 }
}

Figure 10–6 The outline of a circle drawn with a pen width of 8 pixels.

Listing 10.11 DashedStrokeExample.java

import java.awt.*;

/** An example of creating a custom dashed line for drawing.
 */

public class DashedStrokeExample extends FontExample {
 public void paintComponent(Graphics g) {
  clear(g);
  Graphics2D g2d = (Graphics2D)g;
  drawGradientCircle(g2d);
  drawBigString(g2d);
  drawDashedCircleOutline(g2d);
 }

 protected void drawDashedCircleOutline(Graphics2D g2d) {
  g2d.setPaint(Color.blue);
  // 30-pixel line, 10-pixel gap, 10-pixel line, 10-pixel gap
  float[] dashPattern = { 30, 10, 10, 10 };
  g2d.setStroke(new BasicStroke(8, BasicStroke.CAP_BUTT,
                 BasicStroke.JOIN_MITER, 10,
                 dashPattern, 0));
  g2d.draw(getCircle());
 }

 public static void main(String[] args) {
  WindowUtilities.openInJFrame(new DashedStrokeExample(),
                 380, 400);
 }
}

As a final example of pen styles, Listing 10.12 demonstrates the effect of different styles for joining line segments, and different styles for creating line end points (cap settings). Figure 10–8 clearly illustrates the differences of the three joining styles (JOIN_MITER, JOIN_BEVEL, and JOIN_ROUND), as well as the differences of the three cap styles (CAP_SQUARE, CAP_BUTT, and CAP_ROUND).

Figure 10–7 The outline of a circle drawn with a dashed line segment.

Listing 10.12 LineStyles.java

import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;

/** A demonstration of different controls when joining two line
 * segments. The style of the line end point is controlled
 * through the capStyle parameter.
 */

public class LineStyles extends JPanel {
 private GeneralPath path;
 private static int x = 30, deltaX = 150, y = 300,
           deltaY = 250, thickness = 40;
 private Circle p1Large, p1Small, p2Large, p2Small,
         p3Large, p3Small;
 private int compositeType = AlphaComposite.SRC_OVER;
 private AlphaComposite transparentComposite =
  AlphaComposite.getInstance(compositeType, 0.4F);
 private int[] caps =
  { BasicStroke.CAP_SQUARE, BasicStroke.CAP_BUTT,
   BasicStroke.CAP_ROUND };
 private String[] capNames =
  { "CAP_SQUARE", "CAP_BUTT", "CAP_ROUND" };
 private int[] joins =
  { BasicStroke.JOIN_MITER, BasicStroke.JOIN_BEVEL,
   BasicStroke.JOIN_ROUND };
 private String[] joinNames =
  { "JOIN_MITER", "JOIN_BEVEL", "JOIN_ROUND" };

 public LineStyles() {
  path = new GeneralPath();
  path.moveTo(x, y);
  p1Large = new Circle(x, y, thickness/2);
  p1Small = new Circle(x, y, 2);
  path.lineTo(x + deltaX, y - deltaY);
  p2Large = new Circle(x + deltaX, y - deltaY, thickness/2);
  p2Small = new Circle(x + deltaX, y - deltaY, 2);
  path.lineTo(x + 2*deltaX, y);
  p3Large = new Circle(x + 2*deltaX, y, thickness/2);
  p3Small = new Circle(x + 2*deltaX, y, 2);
  setFont(new Font("SansSerif", Font.BOLD, 20));
 }

 public void paintComponent(Graphics g) {
  super.paintComponent(g);
  Graphics2D g2d = (Graphics2D)g;
  g2d.setColor(Color.lightGray);
  for(int i=0; i<caps.length; i++) {
   BasicStroke stroke =
    new BasicStroke(thickness, caps[i], joins[i]);
   g2d.setStroke(stroke);
   g2d.draw(path);
   labelEndPoints(g2d, capNames[i], joinNames[i]);
   g2d.translate(3*x + 2*deltaX, 0);
  }
 }

 // Draw translucent circles to illustrate actual end points.
 // Include text labels for cap/join style.
 private void labelEndPoints(Graphics2D g2d, String capLabel,
               String joinLabel) {
  Paint origPaint = g2d.getPaint();
  Composite origComposite = g2d.getComposite();
  g2d.setPaint(Color.black);
  g2d.setComposite(transparentComposite);
  g2d.fill(p1Large);
  g2d.fill(p2Large);
  g2d.fill(p3Large);
  g2d.setPaint(Color.yellow);
  g2d.setComposite(origComposite);
  g2d.fill(p1Small);
  g2d.fill(p2Small);
  g2d.fill(p3Small);
  g2d.setPaint(Color.black);
  g2d.drawString(capLabel, x + thickness - 5, y + 5);
  g2d.drawString(joinLabel, x + deltaX + thickness - 5,
          y - deltaY);
  g2d.setPaint(origPaint);
 }

 public static void main(String[] args) {
  WindowUtilities.openInJFrame(new LineStyles(),
                 9*x + 6*deltaX, y + 60);
 }
}

class Circle extends Ellipse2D.Double {
 public Circle(double centerX, double centerY, double radius) {
  super(centerX - radius, centerY - radius, 2.0*radius,
     2.0*radius);
 }
}

Figure 10–8 A demonstration of the different styles for joining line segments, and styles for ending a line segment.

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