Trail: 2D Graphics
Lesson: Working with Geometry

绘制任意形状

You have already learned how to draw most of shapes represented in the java.awt.geom package. To create more complicated geometry, such as polygons, polylines, or stars you use another class from this package, GeneralPath.

This class implements the Shape interface and represents a geometric path constructed from lines, and quadratic and cubic curves. The three constructors in this class can create the GeneralPath object with the default winding rule (WIND_NON_ZERO), the given winding rule (WIND_NON_ZERO or WIND_EVEN_ODD), or the specified initial coordinate capacity. The winding rule specifies how the interior of a path is determined.

public void paint (Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    ...
}

To create an empty GeneralPath instance call new GeneralPath() and then add segments to the shape by using the following methods:

moveTo(float x, float y) – Moves the current point of the path to the given point
lineTo(float x, float y) – Adds a line segment to the current path
quadTo(float ctrlx, float ctrly, float x2, floaty2) – Adds a quadratic curve segment to the current path
curveTo(float ctrlx1, float ctrly1, float ctrlx2, float ctrly2, float x3, floaty3) – Adds a cubic curve segment to the current path
closePath() – Closes the current path

The following example illustrates how to draw a polyline by using GeneralPath:

// draw GeneralPath (polyline)
int x2Points[] = {0, 100, 0, 100};
int y2Points[] = {0, 50, 50, 0};
GeneralPath polyline = 
        new GeneralPath(GeneralPath.WIND_EVEN_ODD, x2Points.length);

polyline.moveTo (x2Points[0], y2Points[0]);

for (int index = 1; index < x2Points.length; index++) {
         polyline.lineTo(x2Points[index], y2Points[index]);
};

g2.draw(polyline);

This example illustrates how to draw a polygon by using GeneralPath:

// draw GeneralPath (polygon)
int x1Points[] = {0, 100, 0, 100};
int y1Points[] = {0, 50, 50, 0};
GeneralPath polygon = 
        new GeneralPath(GeneralPath.WIND_EVEN_ODD,
                        x1Points.length);
polygon.moveTo(x1Points[0], y1Points[0]);

for (int index = 1; index < x1Points.length; index++) {
        polygon.lineTo(x1Points[index], y1Points[index]);
};

polygon.closePath();
g2.draw(polygon);

Note that the only difference between two last code examples is the closePath() method. This method makes a polygon from a polyline by drawing a straight line back to the coordinates of the last moveTo.

To add a specific path to the end of your GeneralPath object you use one of the append() methods. The ShapesDemo2D.java code example contains additional implementations of arbitrary shapes.