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2.3 The Java API Documentation
• What follows are the contents of the Java
Application Programming Interface (API)
Documentation for the system supplied class
Point.
• The complete online Java API documentation can
easily be found through a Web search.
• This is a comprehensive reference for questions
about programming in Java.
• An explanation of the excerpt from the
documentation is given following it.
• java.awt
Class Point
• java.lang.Object | +--java.awt.geom.Point2D
|
+--java.awt.Point All Implemented
Interfaces:
• Cloneable, Serializable
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public class Point
extends Point2D
implements Serializable
A point representing a location in (x, y)
coordinate space, specified in integer precision.
Since:
JDK1.0
See Also:
Serialized Form
Inner classes inherited from class java.awt.geom.Point2D
Point2D.Double, Point2D.Float
Field Summary
int
x
The x coordinate.
Inner classes inherited from class java.awt.geom.Point2D
int y
Point2D.Double, Point2D.Float
The y coordinate.
Constructor Summary
Point()
Constructs and initializes a point at the origin (0, 0) of the
coordinate space.
Point(int x, int y)
Constructs and initializes a point at the specified (x, y)
location in the coordinate space.
Point(Point p)
Constructs and initializes a point with the same location as
the specified Point object.
Method Summary
boolean
equals(Object obj)
Determines whether an instance of Point2D is equal to this point.
Point
getLocation()
Returns the location of this point.
double
getX()
Returns the X coordinate of the point in double precision.
double
getY()
Returns the Y coordinate of the point in double precision.
void
move(int x, int y)
Moves this point to the specificed location in the (x, y) coordinate plane.
void setLocation(double x, double y)
Sets the location of this point to the specified float coordinates.
void setLocation(int x, int y)
Changes the point to have the specificed location.
void setLocation(Point p)
Sets the location of the point to the specificed location.
String toString()
Returns a string representation of this point and its location in the (x, y) coordinate space.
void translate(int x, int y)
Translates this point, at location (x, y), by dx along the x axis and dy along the y axis so that it now represents the point
(x + dx, y + dy).
• The information given at the top of the
documentation concerns the naming of the class
and where it is located.
• System supplied classes are arranged in packages.
• If you scan through the information at the top
you will eventually find this:
• java.awt.Point.
• “awt” stands for “abstract windowing toolkit”.
• This is the name of a package in Java which
includes classes related to doing graphical things.
• Point is one of those classes. If a program uses a system
supplied class, a line like this is put at the top of the code:
•
• import java.awt.Point;
•
• The idea is that this will make the class available for use in
the program.
• This will be done in the example programs.
• It is possible to import all classes in a package at once.
• If you chose to do this, you would use the * as a wildcard:
•
• import java.awt.*;
• The next segment of interest in the documentation is
entitled “Field Summary”.
• In the documentation, what are referred to in these
notes as instance variables are referred to as fields.
• In other words, you discover from this documentation
that an object created from the Point class will have
two instance variables, an x coordinate and a y
coordinate.
• These instance variables are given the type “int”, which
signifies that these coordinates can take on integer
values.
• The next segment in the documentation is
entitled “Constructor Summary”.
• Constructors are special pieces of code used
for creating instances of classes.
• Constructors have a form reminiscent of
methods—they have a name followed by a set
of parentheses which may or may not contain
parameters.
• Constructors are not methods.
• Their name is the same as the class they belong to.
• As you can see, a single class may have more than one
constructor.
• The system can tell them apart because they have
different parameter lists.
• In the documentation the types of the parameters are
shown.
• For the time being we will restrict our attention to
examples with parameters of the type “int”.
• The last segment of the documentation is the “Method
Summary”.
• This gives all of the methods by name and all of their
parameters by name, including their types.
• There can be different methods with the same name.
• The system tells them apart by their parameter lists.
• It is only through these methods that a program can
affect the instance variables, the x and y coordinates,
of a Point object that has been created.
• As you can see, int and double are two
different numeric types.
• The meaning of types will be covered in the
next unit.
• For the time being, examples will be restricted
to the int, or integer type.
• This type can hold whole number values.