[Chapter 2] 2.6 Polygon
Author: John Zukowski
Date: Thu Jul 31 14:25:05 1997

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| Chapter 2 Simple Graphics |

## 2.6 Polygon

A `Polygon` is a collection of points used to create a series of line segments. Its primary purpose is to draw arbitrary shapes like triangles or pentagons. If the points are sufficiently close, you can create a curve. To display the `Polygon`, call `drawPolygon()` or `fillPolygon()`.

### Polygon Methods

Variables

The collection of points maintained by `Polygon` are stored in three variables:

public int npoints

  The `npoints` variable stores the number of points.

public int xpoints[]

  The `xpoints` array holds the x component of each point.

public int ypoints[]

  The `ypoints` array holds the y component of each point.

You might expect the `Polygon` class to use an array of points, rather than separate arrays of integers. More important, you might expect the instance variables to be private or protected, which would prevent them from being modified directly. Since the three instance variables are public, there is no guarantee that the array sizes are in sync with each other or with `npoints`. To avoid trouble, always use `addPoints()` to modify your polygons, and avoid modifying the instance variables directly. Constructors

public Polygon ()

  This constructor creates an empty `Polygon`.

public Polygon (int xPoints[], int yPoints[], int numPoints)

  This constructor creates a `Polygon` that consists of `numPoints` points. Those points are formed from the first `numPoints` elements of the `xPoints` and `yPoints` arrays. If the `xPoints` or `yPoints` arrays are larger than `numPoints`, the additional entries are ignored. If the `xPoints` or `yPoints` arrays do not contain at least `numPoints` elements, the constructor throws the run-time exception `ArrayIndexOutOfBoundsException`.

Miscellaneous methods

public void addPoint (int x, int y)

  The `addPoint()` method adds the point (`x`, `y`) to the `Polygon` as its last point. If you alter the `xpoints`, `ypoints`, and `npoints` instance variables directly, `addPoint()` could add the new point at a place other than the end, or it could throw the run-time exception `ArrayIndexOutOfBoundsException` with a message showing the position at which it tried to add the point. Again, for safety, don't modify a `Polygon`'s instance variables yourself; always use `addPoint()`.

public Rectangle getBounds ()
public Rectangle getBoundingBox ()

  The `getBounds()` method returns the `Polygon`'s bounding `Rectangle` (i.e., the smallest rectangle that contains all the points within the polygon). Once you have the bounding box, it's easy to use methods like `copyArea()` to copy the `Polygon`.

  `getBoundingBox()` is the Java 1.0 name for this method.

public boolean contains (int x, int y)
public boolean inside (int x, int y)

  The `contains()` method checks to see if the (`x`, `y`) point is within an area that would be filled if the `Polygon` was drawn with `Graphics.fillPolygon()`. A point may be within the bounding rectangle of the polygon, but `contains()` can still return `false` if not within a closed part of the polygon.

  `inside()` is the Java 1.0 name for this method.

public boolean contains (Point p)

  The `contains()` method checks to see if the point `p` is within an area that would be filled if the `Polygon` were drawn with `Graphics.fillPolygon()`.

public void translate (int x, int y)

  The `translate()` method moves all the `Polygon`'s points by the amount (`x`, `y`). This allows you to alter the location of the `Polygon` by shifting the points.

Rectangle |  | Image