Trail: 2D Graphics
Lesson: Working with Geometry

绘制几何图元

The Java 2D API provides several classes that define common geometric objects such as points, lines, curves, and rectangles. These geometry classes are part of the java.awt.geom package.
The PathIterator interface defines methods for retrieving elements from a path.
The Shape interface provides a set of methods for describing and inspecting geometric path objects. This interface is implemented by the GeneralPath class and other geometry classes.

All examples represented in this section create geometries by using java.awt.geom package and then render them by using the Graphics2D class. To begin you obtain a Graphics2D object, for example by casting the Graphics parameter of the paint() method.

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

Point

The Point class creates a point representing a location in (x,y) coordinate space. The subclasses Point2D.Float and Point2D.Double provide correspondingly float and double precision for storing the coordinates of the point.

//Create Point2D.Double
Point2D.Double point = new Point2D.Double(x, y);

To create a point with the coordinates 0,0 you use the default constructor, Point2D.Double().
You can use the setLocation method to set the position of the point as follows:

setLocation(double x, double y) – To set the location of the point- defining coordinates as double values.
setLocation(Point2D p) – To set the location of the point using the coordinates of another point.

Also, the Point2D class has methods to calculate the distance between the current point and a point with given coordinates, or the distance between two points.

Line

The Line2D class represents a line segment in (x, y) coordinate space. The Line2D. Float and Line2D.Double subclasses specify lines in float and double precision. 例如：

// draw Line2D.Double
g2.draw(new Line2D.Double(x1, y1, x2, y2));

Line

This class includes several setLine() methods to define the endpoints of the line.
Aternatively, the endpoints of the line could be specified by using the constructor for the Line2D.Float class as follows:

Line2D.Float(float X1, float Y1, float X2, float Y2)
Line2D.Float(Point2D p1, Point2D p2)

Use the Stroke object in the Graphics2D class to define the stroke for the line path.

Curves

The java.awt.geom package enables you to create a quadratic or cubic curve segment.

Quadratic Curve Segment

The QuadCurve2D class implements the Shape interface. This class represents a quadratic parametric curve segment in (x, y) coordinate space. The QuadCurve2D.Float and QuadCurve2D.Double subclasses specify a quadratic curve in float and double precision.

Several setCurve methods are used to specify two endpoints and a control point of the curve, whose coordinates can be defined directly, by the coordinates of other points and by using a given array.
A very useful method, setCurve(QuadCurve2D), sets the quadratic curve with the same endpoints and the control point as a supplied curve. 例如：

// create new QuadCurve2D.Float
QuadCurve2D q = new QuadCurve2D.Float();
// draw QuadCurve2D.Float with set coordinates
q.setCurve(x1, y1, ctrlx, ctrly, x2, y2);
g2.draw(q);

Quadratic parametric curve segment

Cubic Curve Segment

The CubicCurve2D class also implements the Shape interface. This class represents a cubic parametric curve segment in (x, y) coordinate space. CubicCurve2D.Float and CubicCurve2D.Double subclasses specify a cubic curve in float and double precision.

The CubicCurve2D class has similar methods for setting the curve as the QuadraticCurve2Dclass, except with a second control point. 例如：

// create new CubicCurve2D.Double
CubicCurve2D c = new CubicCurve2D.Double();
// draw CubicCurve2D.Double with set coordinates
c.setCurve(x1, y1, ctrlx1,
           ctrly1, ctrlx2, ctrly2, x2, y2);
g2.draw(c);

Rectangle

Classes that specify primitives represented in the following example extend the RectangularShape class, which implements the Shape interface and adds a few methods of its own.

These methods enables you to get information about a shape’s location and size, to examine the center point of a rectangle, and to set the bounds of the shape.

The Rectangle2D class represents a rectangle defined by a location (x, y) and dimension (w x h). The Rectangle2D.Float and Rectangle2D.Double subclasses specify a rectangle in float and double precision. 例如：

// draw Rectangle2D.Double
g2.draw(new Rectangle2D.Double(x, y,
                               rectwidth,
                               rectheight));

Rectangle

The RoundRectangle2D class represents a rectangle with rounded corners defined by a location (x, y), a dimension (w x h), and the width and height of the corner arc. The RoundRectangle2D.Float and RoundRectangle2D.Double subclasses specify a round rectangle in float and double precision.

The rounded rectangle is specified with following parameters:

Location
Width
Height
Width of the corner arc
Height of the corner arc

To set the location, size, and arcs of a RoundRectangle2D object, use the method setRoundRect(double a, double y, double w, double h, double arcWidth, double arcHeight). 例如：

// draw RoundRectangle2D.Double
g2.draw(new RoundRectangle2D.Double(x, y,
                                   rectwidth,
                                   rectheight,
                                   10, 10));

Rounded Rectangle

Ellipse

The Ellipse2D class represents an ellipse defined by a bounding rectangle. The Ellipse2D.Float and Ellipse2D.Double subclasses specify an ellipse in float and double precision.

Ellipse is fully defined by a location, a width and a height. 例如：

// draw Ellipse2D.Double
g2.draw(new Ellipse2D.Double(x, y,
                             rectwidth,
                             rectheight));

Ellipse

Arc

To draw a piece of an ellipse, you use the Arc2D class. This class represents an arc defined by a bounding rectangle, a start angle, an angular extent, and a closure type. The Arc2D.Float and Arc2D.Double subclasses specify an ellipse in float and double precision.

The Arc2D class defines the following three types of arcs, represented by corresponding constants in this class: OPEN, PIE and CHORD.

Several methods set the size and parameters of the arc:

Directly, by coordinates
By supplied Point2D and Dimension2D
By copying an existing Arc2D

Also, you can use the setArcByCenter method to specify an arc from a center point, given by its coordinates and a radius.

// draw Arc2D.Double
g2.draw(new Arc2D.Double(x, y,
                         rectwidth,
                         rectheight,
                         90, 135,
                         Arc2D.OPEN));

Arc

The ShapesDemo2D.java code example contains implementations of all described geometric primitives. For more information about classes and methods represented in this section, see the java.awt.geom specification.