BM214E Object Oriented Programming Lecture 6. Classes and Objects

Transcription

1 BM214E Object Oriented Programming Lecture 6 Classes and Objects

2 Classes and Objects Class: describes the form of an object, a template or blueprint or mold specifies data representation, behavior, and inheritance (via variables, methods and parents) Object: an instance of a class has unique copy of every non-static variable (i.e., the instance variables but not the class variables). Difference between a class and an object of that class is analogous to the difference between a type and a variable of that type. Key Idea Naming Conventions: Classes: Identifiers begin with cap letters for each word in the Identifier, e.g., class NeuralNetwork Objects: Identifiers begins with lower case letter, then caps for other words in identifier, e.g., thisobjectidentifier

3 class Box { int ilength; int iwidth; int iheight; public void setlength (int inewlength) { ilength = inewlength; } // of setlength public int getlength ( ) { return (ilength); } // of getlength public void setwidth (int inewwidth) { iwidth = inewwidth; } // of setwidth public int getwidth ( ) { return (iwidth); } // of getwidth Specification of a Class public void setheight (int inewheight) { iheight = inewheight; } // of setheight public int getheight ( ) { return (iheight); } // of getheight public int getvolume ( ) { return ( getlength( ) * getwidth( ) * getheight( ) ); } // of getvolume } // of class Box Use accessors and modifiers!

4 Usage tip Box samplebox = new Box(); samplebox.ilength = 10; Legal? Preferred: samplebox.setlength(10); Why?

5 Constructors

6 Constructors Default provided unless... Must have same name as class Must not have return type (if you want a constructor) Multiple constructors typical Each must have unique signature number, type, order of parameters

7 Example class Box { int ilength; int iwidth; int iheight; public Box() { } // Default constructor public Box(int il, int iw, int ih) { ilength = il; iwidth = iw; iheight = ih; } // Box Constructor // class definition continues...

8 Example class Box { int ilength; int iwidth; int iheight; public Box() { this(0, 0, 0); } // I can even do this... public Box(int il, int iw, int ih) { ilength = il; iwidth = iw; iheight = ih; } // Box Constructor // class definition continues...

9 Creating Objects or Instances

10 Involves three things: Creating Instances of Classes 1. Creating the reference: Box thisbox ; 2. Instantiating the object: thisbox = new Box( ); OR do first two steps at once, e.g., Box thisbox = new Box( ); 3. Having constructor(s) set initial values: public Box (int inewlength, int inewwidth, int inewheight) { setlength (inewlength); setwidth (inewwidth); setheight (inewheight); } // of constructor With an appropriate constructor, we can do all three at once: Box thisbox = new Box (10, 5, 25); 10

11 If you just understand one thing......it should be what we mean by a reference. Every object created in Java is created in the dynamic area of memory (heap). The creation operation involves the return (to you) of the location of the object. This is known as a reference. It is the same idea as a pointer. Box somebox; // This creates a reference somebox = new Box(10, 20, 30); // We create the new box and stick its // reference in somebox

12 Objects and References Distinguish between primitives and objects. Assignment with Primitives: Code: int x; x Memory: int y; x y x = 5; x 5 y y = x; x 5 y 5

13 Objects and References Assignment with References to Objects: Code: Memory: Box box1; Box box2; box1 box2 box1 = new Box(8, 5, 7); box1 L=8, W=5, H=7 box2 box2 = box1; // note: two references // but only one object box1 box2 L=8, W=5, H=7 box1 = new Box(3, 9, 2); box1 L=3, W=9, H=2 box2 L=8, W=5, H=7 box1 = box2; // Old reference lost! box1 L=3, W=9, H=2 box2 L=8, W=5, H=7

14 Questions?

15 Parameters

16 Our Next Real Challenge Java only has in parameters public void swap(int a, int b) { int temp; temp = a; a = b; b = temp; } We can return values from functions but this won t be enough. When a parameter is a reference to an object We can access the object s methods We can make persistent changes

17 Pop Quiz! class Pop { public static void swap(int x, int y) { int t; t = x; x = y; y = t; } // swap public static void main(string args[]) { int a = 7; int b = 99; swap(a, b); System.out.println("a = " + a + " b = " + b); } // main } // Pop

18 Java code for Grades Class public class Grade { String name; double value; public Grade(String newname) { name = newname; } // Constructor public void setvalue(double v) { value = v; } public double getvalue() { return value; } public void setname(string n) { name = n; } public String getname() { return name; } } // Grade

19 Java code for Main Algorithm class Compute_Grade { <other stuff> /** * Calc course grade based on the assignments */ public static void main(string[] args) { double grade_avg; Grade prog = new Grade( Program ); Grade quiz = new Grade( Quiz ); Grade lab = new Grade( Lab ); Grade exam = new Grade( Exam ); get_data(prog, quiz, lab, exam); grade_avg = average( prog, quiz, lab, exam ); output_grade( grade_avg ); } } // end of class Compute_Grade

20 Java code for Get_Data // Still inside the class Compute_Grade /** * Purpose: Prompts user for data & returns them * Pre-condition: none * Post-condition: passes values back to point of call */ public static void get_data(grade g1, Grade g2, Grade g3, Grade g4) { g1.setvalue( IOGadget.readDouble("Enter + g1.getname() ); g2.setvalue( IOGadget.readDouble("Enter + g2.getname() ); g3.setvalue( IOGadget.readDouble("Enter + g3.getname() ); g4.setvalue( IOGadget.readDouble("Enter + g4.getname() ); } //Get_Data

21 Questions?

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23 All Container Classes you build should be Generic Generic classes are easy to build Every class is derived from the class Object Make this the type of the contained data An object of any type can then be put into the container You have to cast the object on output to its specific class Primitive data types [int, char, double] don t work like this each atomic data element has a corresponding wrapper class for this purpose e.g. the Integer class is an Object-type container for an int.

24 The Object Class provides Basic clone() Creates a clone of the object. Used to make a deep copy equals(object) Methods Compares two Objects for equality. getclass() Returns the Class of this Object. hashcode() Returns a hashcode for this Object. Used for indexing this object with others tostring() Why? Returns a String that represents the value of this Object. Used implicitly by print() to stringify the object And some others we ll deal with later, perhaps.

25 Overloading Object s Methods Each of these methods does something generic. However, with the exception of getclass(), they should all be overloaded when you write a specific class consider them as virtual methods should be implemented specifically in derived classes generic behavior is not usually what you want e.g. hashcode() will be unique to every instance of class Object [some representation of its memory address]. However, you probably want objects with identical content to return the same hashcode.

26 Example of a Linked-List Node Pseudocode: LL_Node isoftype record data isoftype <generic type> next isoftype Ptr toa LL_Node endrecord // LL_Node data next

27 public class Node { private Object data; First a Node public Node( Object d ) { setdata( d ); } // Constructor public void setdata( Object d ) { data = d; } public Object getdata() { return data; } public String tostring() { return ( data.tostring() ); } public Object clone() { return new Node( data ); } public int hashcode() { return data.hashcode(); } Ok?

28 Subtle Bug // public class Node (continued) public static void main(string args[]) { Node n = new Node("Hello!"); System.out.println("Should be Hello: " + n); n = new Node(null); System.out.println("Should be null: " + n); } // main }// Node Should be Hello: Hello! Exception in thread "main" java.lang.nullpointerexception: at Node.toString(Node.java:10) at java.lang.string.valueof(string.java:1911) at java.lang.stringbuffer.append(stringbuffer.java:365) at Node.main(Node.java:25)

29 First a Node public class Node { private Object data; public Node( Object d ) { setdata( d ); } // Constructor public void setdata( Object d ) { data = d; } public Object getdata() { return data; } public String tostring() { if(data == null) return "null"; else return data.tostring(); } public Object clone() { return new Node( data ); } public int hashcode() { // need to deal with potential null here too return data.hashcode(); }

30 Now the Linked-List Node class LLNode extends Node { private LLNode next; public LLNode( Object d, LLNode n ) { super( d ); setnext( n ); } public void setnext( LLNode n ) { next = n; }; public LLNode getnext() { return next; }; public String tostring() { return ("{ data = " + super.tostring() + "; next = " + next + " } " ); } } // LLNode

31 Questions?