Objects and Classes (1)

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1 Objects and Classes (1) Reading: Classes (interface, implementation, garbage collection) Interface Examples Creating and using objects of existing classes via their interfaces. Interface of a Point class Interface of a Rectangle class Object references, equality and identity COMP67021 High-Level Programming 2 1

2 Interface of a Point Class public class Point { /** Create a new point (x,y) */ public Point(int x, int y); /** Get the value of x */ public int getx(); /** Likewise for y */ public int gety(); /** Return a string representation of the point */ public String tostring(); /** Check whether this and another point are equal */ public boolean equals(point other); // End of class Point COMP67021 High-Level Programming 3 Using Points We can make a new Point object, using new: Point p = new Point(42, 99); give it an alternative name: Point q = p; and send it messages: System.out.println(p.getX()); System.out.println(q.toString()); COMP67021 High-Level Programming 4 2

3 Good Design Features Representation is hidden, could be changed without affecting code using Points. Need to be able to get the attributes - x and y -of a Point Fetched using getx() and gety(). Important to use these naming conventions. public String tostring() returns a String representation of a Point. Important to provide such a method and call it tostring(). Cannot change the contents of a Point object once it's been created - Points are immutable. Reflects "the real world" and makes things simpler. COMP67021 High-Level Programming 5 Interface of a Rectangle Class (1) public class Rectangle { /** Create a rectangle with the given width and height, with the bottom left corner at the given position */ public Rectangle(int width, int height, Point position); /** Create a rectangle, bottom left corner at the origin */ public Rectangle(int width, int height); public int getwidth(); public int getheight(); public Point getposition(); COMP67021 High-Level Programming 6 3

4 Interface of a Rectangle Class (2) public void setwidth(int newwidth); public void setheight(int newheight); public void setposition(point newposition); public void setposition(int newx, int newy); /** Return a string giving the position and dimensions */ public String tostring(); /** Check whether this is equal to other */ public boolean equals(rectangle other); /** Display the rectangle on the screen */ public void display(graphics g); // Rectangle COMP67021 High-Level Programming 7 Points to Note Two overloaded constructors - differ in number/types of their parameters. Both get-methods and set-methods (the latter return void). NB. Constructors have no return type, not even void. setposition() shows that methods can also be overloaded - same rule applies. Again, the actual data is hidden - several representations are possible. COMP67021 High-Level Programming 8 4

5 Equality and Identity (1) Point p = new Point(42,99); Rectangle r1 = new Rectangle(27, 32, p); Rectangle r2 = r1; Rectangle r3 = new Rectangle(27,32, p); Questions: are r1 and r2 the same? Or r1 and r3? Or r2 and r3? How does this change if we say: r2.setposition(new Point(33,33)); and does it matter whether we use p as the location of r3 or create a separate new Point(42,99)? COMP67021 High-Level Programming 9 Equality and Identity (2) Need to distinguish: 1. Object identity - are they the very same object (two references to the same set of bits in the store). Tested using ==. 2. Object equality - are they indistinguishable in practice. Tested using.equals() - any class can define this method as appropriate (default is identity). COMP67021 High-Level Programming 10 5

6 Object References Variables such as r1 etc. hold references to objects, which uniquely identify them. After the assignment r2 = r1, both refer to the same object, so r1 == r2 is true. If we change the object with a set-method, they both see the change and remain identical. r1 == r3 and r2 == r3 are both false. For r1.equals(r2) etc. it depends on how the equals() method is defined. COMP67021 High-Level Programming 11 Implementation Examples Implementation of the Point class Implementation of the Rectangle class Garbage collection COMP67021 High-Level Programming 12 6

7 Implementation of Class Point (1) public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; public int getx() { return x; public int gety() { return y; public String tostring() { return "(" + this.getx() + "," + this.gety() + ")"; COMP67021 High-Level Programming 13 Implementation of class Point (2) public boolean equals(point other) { return this.getx() == other.getx() && this.gety() == other.gety(); public static void main (String[] args) { System.out.println(new Point(42,99)); // Point COMP67021 High-Level Programming 14 7

8 Points to Note Instance variables, x and y are private. Instance variables get default values if we don't provide any. Constructor simply stores the values - note naming issue. We try to avoid referring directly to the instance variables even within the class. A main() is provided for unit testing. COMP67021 High-Level Programming 15 Implementation of Class Rectangle (1) import java.awt.graphics; public class Rectangle { private int left, top, width, height; public Rectangle(int width, int height, Point position) { this.width = width; this.height = height; this.left = position.getx(); this.top = position.gety() - height; // NB public Rectangle(int width, int height) { this(width, height, new Point(0,0)); COMP67021 High-Level Programming 16 8

9 Implementation of Class Rectangle (2) public int getwidth() { return width; public int getheight() { return height; public Point getposition() { return new Point(left, top+height); public void setwidth(int newwidth) { width = newwidth; public void setheight(int newheight) { height = newheight; // Spot the bug! COMP67021 High-Level Programming 17 Implementation of Class Rectangle (3) public void setposition(point newposition) { this.setposition(newposition.getx(), newposition.gety()); public void setposition(int newx, int newy) { left = newx; top = newy - height; public String tostring() { return "Rectangle at " + this.getposition() + ", width " + this.getwidth() + " height " + this.getheight(); COMP67021 High-Level Programming 18 9

10 Implementation of Class Rectangle (4) public boolean equals(rectangle other) { return this.getwidth() == other.getwidth() && this.getheight() == other.getheight() && this.getposition().equals(other.getposition()); public void display(graphics g) { g.drawrect(left, top, width, height); public static void main(string[] args) { System.out.println( new Rectangle(33, 44, new Point(72,43))); // End of class Rectangle COMP67021 High-Level Programming 19 Points to Note The representation is non-obvious, optimised for drawing. We define one constructor in terms of the other (using this) to avoid replicating code. Likewise for setposition(). Equality must be defined carefully (see notes) Graphics objects are used to draw things (see later). COMP67021 High-Level Programming 20 10

11 Garbage Collection Need to reclaim the storage used by those objects we no longer need. But who determines whether an object is "no longer needed"? Traditionally, the programmer does - manual storage management. This is a Bad Thing: If an object is deleted while still needed, the program will (eventually) crash. If we fail to reclaim objects we should, long-running programs may run out of memory. Aternative is automatic storage management, GC. The underlying system (in this case the JVM) does it. COMP67021 High-Level Programming 21 GC is a Good Thing Avoids the perils mentioned above, saves the programmer a difficult and time-consuming job. No significant impact on the time it takes a program to run, or to interactive response. In some cases it may actually improve performance. Java programs do tend to use more memory than equivalent programs in other languages - relatively unimportant. COMP67021 High-Level Programming 22 11

Classes as Blueprints: How to Define New Types of Objects

Unit 5, Part 1 Classes as Blueprints: How to Define New Types of Objects Computer Science S-111 Harvard University David G. Sullivan, Ph.D. Types of Decomposition When writing a program, it's important