Creating and Using Objects
An Introduction with Examples in Java |
Prof. David Bernstein |
Computer Science Department |
bernstdh@jmu.edu |
boolean
, double
, int
,...
new
operator (which is a unary operator that has a
constructor as its operand))
Color lightRed, darkRed; int blue, green, red, shades; // Initialize the attributes darkRed = new Color(102, 0, 0); // Use a getter/accessor blue = darkRed.getBlue(); // Use another method lightRed = darkRed.brighter(); // Use equals() and a static attribute of the Color class shades = 1; if (!lightRed.equals(Color.RED)) { shades++; }
null
(and is a reference/address that isn't valid)null
null
will result in a run-time error
called a NullPointerException
==
operator.equals()
methodRectangle
and Frame
are
classes of mutable objects)String
and Color
are
both classes of immutable objects)
public
attributes (but they are very dangerous
since there is no control over the changes that can be made)
set
(and, hence, are often
called setters)
increaseBy()
)
Color
class has a
darker()
method that doesn't change the
owning object but returns another Color
object that is "darker")
import java.awt.*; /** * A simple example that uses objects of different kinds * * @author Prof. David Bernstein, James Madison University * @version 1.0 */ public class ObjectExample3 { public static void main(String[] args) { Color purple; Frame window; // Construct a Color object that is JMU purple purple = new Color(69, 0, 132); // Construct a Frame object window = new Frame(); // Set the title of the Frame window.setTitle("CS159"); // Set the size of the Frame window.setSize(200, 200); // Set the background color of the Frame window.setBackground(purple); // Make the Frame visible window.setVisible(true); } }