Java Classes and Objects

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2 Table of contents 1 Introduction Case Study - Stack 2 3 Integer String

3 Case Study - Stack Introduction

4 Case Study - Stack Classes Template for creating objects Definition of State (What it knows) Definition of Behaviour (What it can do)

5 Case Study - Stack Object (Instance) A single realization of a Class Has a State Behaviours can be invoked on it

6 Case Study - Stack Analogies from real life People - John and Mary are Instances of a Human Class Human is an abstract definition just like a Class John and Mary are concrete, they exist Houses - Two houses from the same blueprint can be created The blueprints can define a door and how it should open The door can actually be opened on a specific house

7 Case Study - Stack Case Study - Stack

8 Case Study - Stack Case Study - Stack What is the behaviour we want?

9 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object

10 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object We want to request the stack to push an element

11 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object We want to request the stack to push an element We want to query the stack about its top element

12 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object We want to request the stack to push an element We want to query the stack about its top element We want to request the stack to pop an element

13 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object We want to request the stack to push an element We want to query the stack about its top element We want to request the stack to pop an element We want to query the stack about its size

14 Case Study - Stack Case Study - Stack What is the behaviour we want? We want to be able to create a stack object We want to request the stack to push an element We want to query the stack about its top element We want to request the stack to pop an element We want to query the stack about its size Implementation constraints We will build a stack that can hold up to X elements We will build a stack of integers

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16 What should happen when we try to use top/pop on an empty stack?

17 What should happen when we try to use top/pop on an empty stack? What should happen when we try pushing into a full stack?

18 Note What should happen when we try to use top/pop on an empty stack? What should happen when we try pushing into a full stack? We will solve the issues using the tools we know, for now. We will see better ways of dealing with these problems at later stages of the course

19 Data structure What basic data types do we need to represent a stack? Array of elements Integer value to indicate the position of the top in the array Do we need an integer value for the size of the array?

20 Class structure overview 6 class BoundedPositiveIntegerStack { 9 static final int EMPTY_STACK = -1; 14 private int [] elements ; 17 private int numberofelements ; 34 BoundedPositiveIntegerStack () { 44 boolean push ( int element ) { 64 int pop () { 80 int top () { 96 int size () { 109 }

21 Class structure overview - Class definition 6 class BoundedPositiveIntegerStack { 9 static final int EMPTY_STACK = -1; 14 private int [] elements ; 17 private int numberofelements ; 34 BoundedPositiveIntegerStack () { 44 boolean push ( int element ) { 64 int pop () { 80 int top () { 96 int size () { 109 }

22 Class definition 3 /** 4 * A bounded stack of positive integers. 5 */ 6 class BoundedPositiveIntegerStack {... Note This code will be located in a file called BoundedPositiveIntegerStack.java, the name of the file must match the name of the class.

23 Class structure overview - Variables 6 class BoundedPositiveIntegerStack { 9 static final int EMPTY_STACK = -1; 14 private int [] elements ; 17 private int numberofelements ; 34 BoundedPositiveIntegerStack () { 44 boolean push ( int element ) { 64 int pop () { 80 int top () { 96 int size () { 109 }

24 Field / Member variables Just like the definition of a regular variable Only inside the class block instead of a specific method This defines the Possible States of our class 13 // array of elements 14 private int [] elements ; // number of elements in our stack 17 private int numberofelements ;

25 Field variables VS Static variables Member variable Memory allocated per Object Static variable Memory allocated per Class A global variable in a context of a specific class 8 // indication that the stack is empty 9 static final int EMPTY_STACK = -1; Note Static variables should be avoided, mostly used for defining const values in a context of a class

26 Class structure overview - Constructors 6 class BoundedPositiveIntegerStack { 9 static final int EMPTY_STACK = -1; 14 private int [] elements ; 17 private int numberofelements ; 34 BoundedPositiveIntegerStack () { 44 boolean push ( int element ) { 64 int pop () { 80 int top () { 96 int size () { 109 }

27 Constructors A special function that will be called, when object is created Has to be the same name as the class We can define what parameters are needed to create an object Initializes State of Objects 19 /** 20 * constructor with configurable max stack size. 21 * 22 maxsize maximum number of elements in stack. 23 */ 24 BoundedPositiveIntegerStack ( int maxsize ) { 25 // create a new array 26 this. elements = new int [ maxsize ]; 27 // initialize number of elements to 0 ( empty stack ) 28 this. numberofelements = 0; 29 } Note Notice that there is no return value in a constructor definition

28 The keyword this A reference to self, for accessing your own field variables / methods 19 /** 20 * constructor with configurable max stack size. 21 * 22 maxsize maximum number of elements in stack. 23 */ 24 BoundedPositiveIntegerStack ( int maxsize ) { 25 // create a new array 26 this. elements = new int [ maxsize ]; 27 // initialize number of elements to 0 ( empty stack ) 28 this. numberofelements = 0; 29 } Note Using this is not required but it s a good practice to distinguish between local variables and field variables

29 the new statement Creates a new object of the requested class (allocates memory on the Heap) Calls the constructor 14 BoundedPositiveIntegerStack stacka = 15 new BoundedPositiveIntegerStack (1000) ;

30 Memory regions Stack (not to be confused with the Stack we implemented) Holds information about what function your are in now and what functions called it Holds all the basic local variables for each executed function Heap Holds all non basic variables Typically much bigger than the Stack

31 Constructors - continued We can define multiple constructors as long as they have different parameters (Constructor overloading) A constructor without any parameters is called a Default Constructor 10 // the default max size of the stack 11 static final int DEFAULT_NEW_STACK_SIZE = 50; /** 35 * Default constructor 36 */ 37 BoundedPositiveIntegerStack () { 38 // create a new array ( default size ) 39 this. elements = new int [ DEFAULT_NEW_STACK_SIZE ]; 40 // initialize number of elements to 0 ( empty stack ) 41 this. numberofelements = 0; 42 }

32 Constructors - continued 2 Why replicate almost the same code twice? We can call other constructors using this() 10 // the default max size of the stack 11 static final int DEFAULT_NEW_STACK_SIZE = 50; BoundedPositiveIntegerStack ( int maxsize ) { 25 // create a new array 26 this. elements = new int [ maxsize ]; 27 // initialize number of elements to 0 ( empty stack ) 28 this. numberofelements = 0; 29 } /** 32 * constructor with default max stack size. 33 */ 34 BoundedPositiveIntegerStack () { 35 this ( DEFAULT_NEW_STACK_SIZE ); 36 }

33 Using overloaded constructors Calls the correct Constructor based on the passed parameters 13 // create a two instances of the stack class 14 BoundedPositiveIntegerStack stacka = 15 new BoundedPositiveIntegerStack (1000) ; BoundedPositiveIntegerStack stackb = 18 new BoundedPositiveIntegerStack (); Note If two constructors had the same parameters, you wouldn t be able to know which one to call either!

34 Class structure overview - Methods 6 class BoundedPositiveIntegerStack { 9 static final int EMPTY_STACK = -1; 14 private int [] elements ; 17 private int numberofelements ; 34 BoundedPositiveIntegerStack () { 44 boolean push ( int element ) { 64 int pop () { 80 int top () { 96 int size () { 109 }

35 Methods Functions belonging to our class Can be invoked on an Instance of a class Define the Behaviour of the class

36 Methods - size Query the size of the stack: 91 /** 92 * The number of elements currently in the stack. 93 * 94 number of elements. 95 */ 96 int size () { 97 return this. numberofelements ; 98 }

37 Methods - push Request the stack to push an element: 38 /** 39 * push an element into the stack. 40 * 41 element the element to push into the stack. 42 true if element was added to the stack, false otherwise. 43 */ 44 boolean push ( int element ) { 45 // check if we have space to add a new element, 46 // and the element is indeed positive 47 if ( element >= 0 && numberofelements < this. elements. length ) { 48 // add the new element 49 this. elements [this. numberofelements ] = element ; 50 this. numberofelements += 1; 51 // return success of adding the element 52 return true ; 53 } else { 54 // return failure since we have no more space left 55 return false ; 56 } 57 }

38 Methods - top Query the stack about its top element: 8 // indication that the stack is empty 9 static final int EMPTY_STACK = -1; /** 76 * The top element in the stack. 77 * 78 top element, or EMPTY_STACK if nothing is the stack. 79 */ 80 int top () { 81 // check if we have any elements in the stack 82 if (this. numberofelements == 0) { 83 // return an indication that the stack is empty 84 return EMPTY_STACK ; 85 } else { 86 // return the correct element 87 return this. elements [this. numberofelements - 1]; 88 } 89 }

39 Methods - pop Request the stack to pop an element: 59 /** 60 * Pop element from the stack. 61 * 62 top element, or EMPTY_STACK if nothing is the stack. 63 */ 64 int pop () { 65 // get the element on top of the stack 66 int top = top (); 67 // decrement the number of elements, unless the stack is already empty 68 if ( top!= EMPTY_STACK ) { 69 this. numberofelements -= 1; 70 } 71 // return the element 72 return top ; 73 }

40 How to arrange all the pieces? Standard order of all components: 1 Statics 2 Field variables 3 Constructors 4 Methods

41 Using methods calling methods on objects using the. (dot) operator 20 // push 1 to 10 into the stack 21 for ( int i = 1; i <= 10; i += 1) { 22 stacka. push (i); 23 } // push all even numbers between 1 and for ( int i = 1; i <= 5; i += 1) { 27 stackb. push (i * 2); 28 }

42 Accessing static variables Using the. (dot) operator on the name of the class (as it s a per class variable) 33 // push elements in A into B 10 to 1 34 while ( stacka.top ()!= BoundedPositiveIntegerStack. EMPTY_STACK ) { 35 stackb.push ( stacka.pop ()); 36 } Note Can be accessed through a variable but you should never use it, it s confusing!

43 Additional Examples Show Towers Of Hanoi code Show compilation of multiple files in command line

44 Integer String

45 Integer String Integer A Simple object wrapper of a single int value 1 Integer number = new Integer (7) Useful static information 1 Integer. MAX_VALUE 2 Integer. MIN_VALUE Can be null 1 Integer number = null ; Note How can we improve our previous?

46 Integer String Integer - continued int will convert to Integer automatically 1 Integer number = 7; Natural assignment 1 int a = 8; 2 Integer b = a; Unnatural assignment, can you see the problem? 1 Integer a = null ; 2 int b = a;

47 Integer String String Every String is an object 1 String str1 = " Hello World "; 2 String str2 = " Goodbye World "; 3 String str3 = " Goodbye World "; Check how long it is 1 str1. length () // 11 Compare two Strings (why == wont work?) 1 str1. equals (str2 ) // returns false 2 str2. equals (str3 ) // returns true

48 Integer String String - continued Get a specific character inside a String 1 String str = " Boring "; 2 char c = str. charat (2) ; // returns r Check prefix / postix 1 String str = " "; 2 str. startswith ("12"); // returns true 3 str. endswith ("45"); // returns true Change case: 1 String str = " abc "; 2 str. touppercase (); // returns new String "ABC "

49 Integer String String - continued 2 Remove prefix and postfix whitespaces 1 String str = " Hello "; 2 str.trim (); // returns new String " Hello " Search for a sub string 1 String str = " "; 2 str. indexof ("34"); // returns 2 3 str. indexof ("78"); // returns -1 to indicate sub string not found Much more in the JavaDoc String.html

50 Integer String Summary 1 Introduction Case Study - Stack 2 3 Integer String