We want to design a Java class for representing polygons on the plane, where each polygon is constituted by an ordered sequence of colored vertices.

Part 1. (4 points) For each Vertex object, the information of interest is:

• its x and y coordinates (floating point numbers);
• its color (a string).

• creation of a Vertex, given its x and y coordinates and its color;
• getX: that returns the x coordinate of the vertex;
• getY: that returns the y coordinate of the vertex;
• getColor: that returns the color of the vertex.

Solution: class Vertex

Part 2. (16 points) Realize a Java class Polygon, to represent Polygon objects. Each polygon is constituted by an ordered sequence of colored vertices, where each vertex is an instance of the class Vertex and has a position in the sequence. For a polygon with n vertices, the positions go from 0 to n-1. A polygon has also a size, which determines the maximum number of vertices it can have. The class Polygon should export the following functionalities:

• creation of a Polygon, given its size. Initially, the polygon has no vertices;
• getSize: that returns the size of the polygon;
• numVertices: that returns the number of vertices of the polygon;
• getVertex: that, given the position k of a vertex, returns the Vertex object at position k in the polygon, if there are at least k+1 vertices in the polygon, throws an exception otherwise;
• addVertex: that, given a Vertex object v, and a non-negative integer representing a position k, inserts the vertex v before the vertex at position k in the polygon. The positions of vertices from k onwards will increase by 1. If k is not a valid position to insert a new vertex (i.e., there are less than k vertices in the polygon), an exception is thrown. If the vertex cannot be inserted because the number of vertices in the polygon is already equal to its size, an exception is thrown;
• removeVertex: that, given a position k, removes the vertex at position k from the polygon. The positions of vertices from k+1 onwards will decrease by 1. If k is not a valid position for a new vertex (i.e., there are less than k+1 vertices in the polygon), an exception is thrown;
• findVertex: that, given a pair of coordinates x and y, returns the position of the first vertex of the polygon with coordinates x and y, if such a vertex is present in the polygon, -1 otherwise;
• verticesOfColor: that, given a string c representing a color, returns an array of integers containing the positions of all vertices in the polygon that have color c.

Solution: representation of the objects, skeleton of the class, class Polygon, class PolygonException

Part 3. (6 points) Realize a static method coordinatesOfColor, client of the class Polygon (i.e., external to such a class) that, given a Polygon object pol, a string c representing a color, and the name f of a file, writes on the file f the x and y coordinates of all those vertices of pol that have color c, one pair of coordinates per line. The method should handle exceptions due to input/output errors that may occur.

Solution

Additional files to test the solution: class Main, input file vertices.txt

Part 4. (4 points) Discuss the mechanism of dealing with errors in Java through exceptions. Give a complete example, where you show how a class for handling exceptions is defined, how an exception is thrown, and how an exception is handled by catching it.