Session 22
Chapter 11: Implications of
Inheritance
Using Images in Java
• Image class in java.awt.Image
• Toolkit (see section 17.6 of text) contains some
useful utilities:
– getImage() takes a string or URL and returns a value of
type Image
– getScreenSize() returns a Dimension object containing
the number of pixels of both the height and width
– getScreenResolution() returns the number of dots per
inch
• Dividing one by the other will yield the physical size of the screen
– getFontList() returns an array of strings that contain the
names of the fonts available on the current system
– etc....
Simple Image Code
import java.awt.*;
public class ImageTestApp extends Frame {
private Image demo;
public static void main(String[] args) {
ImageTestApp app = new ImageTestApp();
}
public ImageTestApp() {
setTitle( "Image Test" );
setSize( 450, 450 );
demo = Toolkit.getDefaultToolkit().getImage("logo.gif" );
show();
}
public void paint( Graphics g ) {
g.drawImage( demo, 200, 200, 50, 50, this );
}
} // end class ImageTestApp
Exercise with Image Code
• Modify the ImageTestApp so the logo
moves to where the mouse is pressed in
the frame.
A Solution
import java.awt.*;
import java.awt.event.*;
public class ImageTestApp extends Frame {
private Image demo;
private int demoX;
private int demoY;
private int demoSize;
public static void main(String[] args) {
ImageTestApp app = new ImageTestApp();
}
public ImageTestApp() {
setTitle( "Image Test" );
setSize( 450, 450 );
demo = Toolkit.getDefaultToolkit().getImage( "logo.gif" );
demoX = 200;
demoY = 200;
demoSize = 50;
addMouseListener( new MouseKeeper() );
show();
}
...
A Solution
import java.awt.*;
import java.awt.event.*;
public class ImageTestApp extends Frame {
...
public void paint( Graphics g ) {
g.drawImage( demo, demoX-demoSize/2, demoY-demoSize/2,
demoSize, demoSize, this );
}
private class MouseKeeper extends MouseAdapter {
public void mousePressed( MouseEvent e ) {
demoX = e.getX();
demoY = e.getY();
repaint();
} // end mousePressed
} // end private class MouseKeeper
} // end class ImageTestApp
Polymorphism
• polymorphism comes from the Greek root for “many
shapes”
• polymorphism is about how we can use different
objects in the same place in our program, i.e.,
polymorphism depends on objects that are
substitutable for one another
• Decorators are great examples of polymorphism:
– Because the decorator class is a subclass, instance of the
decorator can substitute for instances of the superclass
– Because the decorator class holds an instance of the
superclass, it can hold instances of anything substitutable
for the superclass.
Polymorphic Variable
• A polymorphic variable can hold many
different types of values
• Object-oriented languages often restrict
the types of values to being subclasses of
the declared type of the variable.
• Example: “PinBallTarget target” can be
assigned a “Hole”, “ScorePad”, etc.
Implications of
Inheritance/Polymorphism
• At compile-time, the amount of memory for
polymorphic variables cannot be determined, so all
objects reside in the heap
• Because values reside in the heap, reference
semantics is used for assignment and parameter
passing
• Most natural interpretation of equality is identity.
Since programmers often require a different
meaning two operators are needed
• Garbage collection needed since it is hard for a
programmer to know if when an object is no longer
referenced
Typical Memory Layout
Heap
Stack
Global
variables
Program
Stack-based Memory
Main:
ObjA a = new ObjA();
ObjB b = new ObjB();
a.do(5, b)
public class ObjA {
int x = 100;
public void do (int y, ObjB myB) {
int loc = 6;
int t = myB.doMore(loc);
...
}
public class ObjB {
int z = 30;
public int doMore(int i) {
z = z + i;
return z;
}
}
}
• Objects are stored on the heap
• When a method is called, an activation record is allocated
on the stack to hold:
– return address (where to return after execution)
– parameters
– local variables (stuff declared in the method)
• When a method returns, the activation record is popped