1 Todays Objectives Announcements Homework #1 is due next week Return Quiz 1 – answers are posted on the Yahoo discussion page site Basic Java Programming Looping with while Using java.util.Stack Capturing keyboard input with java.util.Scanner Implementing a data structure as a class – a stack example Interfaces – Specifying the methods that a class must implement Generics – Implementing a data structure so it can be used with any data type Exceptions – adding robustness –Using inheritance to create exception classes derived from RuntimeException –Throwing and catching an exception Bonus Lab – creating a generic class Week 4

2 Return Quiz 1 Answers are posted on the Yahoo discussion group site under Files

3 Basic Java Programming while loop, switch, using java.util.Stack, using java.util.Scanner

4 while, switch, java.util.Stack (From last weeks slides)

5 Finishing SimpleSolitaire Capturing keyboard input with java.util.Scanner Implementing the game in SimpleSolitaire

6 Implementing SimpleSolitaire public static final Scanner IN = new Scanner(System.in); public void play() { while (true){ System.out.println("\n" + this); boolean done = true; for (Stack s : stacks){ if (!(s.isEmpty())){ done = false; break; } if (done){ System.out.println("You win!"); return; } System.out.println("pair,suit,deal,or quit?"); String command = IN.nextLine(); if (command.equals("pair")) { removePair(); } else if (command.equals("suit")) { removeLowCard(); } else if (command.equals("deal")) { dealFourCards(); } else return; } Heres the game loop in the play() method. Basic Java Programming (Flanagan; Drake) Print a prompt for the user Capture the user's input as a String Then we test the input and branch to the correct method call We added a Scanner object as an instance variable to help with input SimpleSolitaire.java

7 Implementing SimpleSolitaire public static final Scanner IN = new Scanner(System.in); public void play() { while (true){ System.out.println("\n" + this); boolean done = true; for (Stack s : stacks){ if (!(s.isEmpty())){ done = false; break; } if (done){ System.out.println("You win!"); return; } System.out.println("pair,suit,deal,or quit?"); String command = IN.nextLine(); if (command.equals("pair")) { removePair(); } else if (command.equals("suit")) { removeLowCard(); } else if (command.equals("deal")) { dealFourCards(); } else return; } We still need to implement removePair() and removeLowCard() Basic Java Programming (Flanagan; Drake) SimpleSolitaire.java

8 Implementing SimpleSolitaire public static final Scanner IN = new Scanner(System.in); public void play() { while (true){ System.out.println("\n" + this); boolean done = true; for (Stack s : stacks){ if (!(s.isEmpty())){ done = false; break; } if (done){ System.out.println("You win!"); return; } System.out.println("pair,suit,deal,or quit?"); String command = IN.nextLine(); if (command.equals("pair")) { removePair(); } else if (command.equals("suit")) { removeLowCard(); } else if (command.equals("deal")) { dealFourCards(); } else return; } We still need to implement removePair() and removeLowCard() Basic Java Programming (Flanagan; Drake) public void removePair() { System.out.println("Card location (1-4)"); int i = IN.nextInt(); System.out.println("Card location (1-4)"); int j = IN.nextInt(); IN.nextLine();//clear input stacks[i - 1].pop(); stacks[j - 1].pop(); } The Scanner object can be used in any method SimpleSolitaire.java What happens on these lines? stacks[i - 1].pop(); stacks[j - 1].pop();

9 Implementing SimpleSolitaire public static final Scanner IN = new Scanner(System.in); public void play() { while (true){ System.out.println("\n" + this); boolean done = true; for (Stack s : stacks){ if (!(s.isEmpty())){ done = false; break; } if (done){ System.out.println("You win!"); return; } System.out.println("pair,suit,deal,or quit?"); String command = IN.nextLine(); if (command.equals("pair")) { removePair(); } else if (command.equals("suit")) { removeLowCard(); } else if (command.equals("deal")) { dealFourCards(); } else return; } We still need to implement removePair() and removeLowCard() Basic Java Programming (Flanagan; Drake) public void removePair() { System.out.println("Card location (1-4)"); int i = IN.nextInt(); System.out.println("Card location (1-4)"); int j = IN.nextInt(); IN.nextLine();//clear input stacks[i - 1].pop(); stacks[j - 1].pop(); } The Scanner object can be used in any method public void removeLowCard() { System.out.println("Low card (1-4)"); int i = IN.nextInt(); System.out.println("High card (1-4)"); int j = IN.nextInt(); IN.nextLine();//clear input stacks[i - 1].pop(); } SimpleSolitaire.java

10 Implementing SimpleSolitaire public String toString(){ StringBuilder result = new StringBuilder(); for (int i=0; i<4; i++){ if (stacks[i].isEmpty()){ result.append("-- "); } else{ result.append(stacks[i].peek() + " "); } result.append("\n" + deck.size() + " cards left in the deck"); return result.toString(); } The last method to implement is toString() Basic Java Programming (Flanagan; Drake) SimpleSolitaire.java Why use isEmpty? What does peek do? Why use deck.size()?

11 Implementing SimpleSolitaire public String toString(){ StringBuilder result = new StringBuilder(); for (int i=0; i<4; i++){ if (stacks[i].isEmpty()){ result.append("-- "); } else{ result.append(stacks[i].peek() + " "); } result.append("\n" + deck.size() + " cards left in the deck"); return result.toString(); } The last method to implement is toString() Basic Java Programming (Flanagan; Drake) SimpleSolitaire.java Why use isEmpty? What does peek do? Why use deck.size()? Rank SpadesHeartsClubs If the game in progress looks like the picture below, what should toString() print?

12 Implementing Main package edu.uhcl.sce.simplesolitaire; public class Main{ public static void main(String[] args) { System.out.println("Simple Solitaire"); SimpleSolitaire game = new SimpleSolitaire(); game.play(); } The complete code for the console version is posted on the Yahoo site. Basic Java Programming (Flanagan; Drake) Main.java

13 Implementing a Data Structure as a Class A Stack Example

14 What are we making? Start with the Stack ADT Goodrich, p. 189 Implementing a Data Structure as a Class pop() Remove and return the top element on the stack; an error occurs if the stack is empty E top() Return the top element on the stack without removing it; an error occurs if the stack is empty. E push( E element ) Insert element at the top of the stack void isEmpty() Indicate whether the stack is empty boolean size() Return the number of elements in the stack int

15 Java Interface A language element in Java that contains only method declarations Has no data and no method bodies; and it cannot be used to instantiate an object Useful for specifying a list of operations that a class must implement All its methods should be public Use the word interface in its declaration A class can implement an interface by using implements in its declaration and then implementing all methods of the interface Implementing a Data Structure as a Class (Goodrich, 80–83)

16 Using a Java Interface with a Data Structure The ADT specifies what the data structure should do, but not how to do it First create a Java interface that specifies the ADT (The interface should be in a separate file) Then implement the interface with the class that creates the data structure package edu.uhcl.sce.arraystack; public interface Stack{ public void push(Character element); public Character pop(); public Character top(); public int size(); public boolean isEmpty(); } Implementing a Data Structure as a Class (Goodrich, 80–83) package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ //more code here } Stack.java ArrayStack.java

17 Step 1 –Create the interface from the ADT Declare methods in the interface for all the public operations that are called for by the ADT (See Goodrich, p. 189, for the stack ADT.) Implementing a Data Structure as a Class All methods in the interface should be public so that they are accessible in any part of your program where there is an object of this class. package edu.uhcl.sce.arraystack; public interface Stack { public int size(); public boolean isEmpty(); public void push(Character c); public Character top(); public Character pop(); } Stack.java

18 package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ } Step 2 –Start writing the class Open a.java file and name it ArrayStack.java Write the package statement Write the keywords public class followed by the name of the class Implementing a Data Structure as a Class Implement the interface ArrayStack.java

19 package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ public int size(); public boolean isEmpty(); public void push(Character c); public Character top(); public Character pop(); } Step 3 –Declare the public operations Declare member functions in the class for all the public operations that are in the interface Implementing a Data Structure as a Class The declarations of all methods should be identical to the declarations in the interface. ArrayStack.java

20 package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ private int capacity;//Max capacity of the array private Character[] myArray; public int size(); public boolean isEmpty(); public void push(Character c); public Character top(); public Character pop(); } Step 4 –Declare instance variables Next, decide how to hold the data that the data structure will contain. For example, you could put it into either an array or a linked list. Implementing a Data Structure as a Class All instance variables labeled private are only accessible to the methods of this class. ArrayStack.java

21 package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ private int capacity;//Max capacity of the array private Character[] myArray; public int size(); public boolean isEmpty(); public void push(Character c); public Character top(); public Character pop(); } Step 4 –Declare instance variables Next, decide how to hold the data that the data structure will contain. For example, you could put it into either an array or a linked list. Implementing a Data Structure as a Class All instance variables labeled private are only accessible to the methods of this class. Information hiding Instance variables are always private so that the objects client cannot change the data by accessing it directly. Data can be accessed only by using the public member functions. ArrayStack.java

22 Step 5 –Define the operations Now, decide how to implement the operations Implementing a Data Structure as a Class package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public int size(); public boolean isEmpty(); public void push(Character c); public Character top(){return myArray[topIndex];} public Character pop(); } ArrayStack.java

23 Step 6 –Add a constructor package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new Character[capacity]; } public int size(); public boolean isEmpty(); public void push(Character c); public Character top(){return myArray[topIndex];} public Character pop(); } Implementing a Data Structure as a Class Public member function with the same name as the class and no return type Called automatically whenever an object is instantiated Initializes the data members ArrayStack.java

24 Step 6 –Add a default constructor package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack{ public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new Character[capacity]; } public int size(); public boolean isEmpty(); public void push(Character c); public Character top(){return myArray[topIndex];} public Character pop(); } Implementing a Data Structure as a Class Constructor with no arguments Calls the overloaded constructor with the default capacity as an argument ArrayStack.java

25 Step 6 –Add exceptions Add robustness – use exceptions Implementing a Data Structure as a Class (Goodrich 76)

26 UML Diagram for ArrayStack – capacity : int – myData : Character[ ] – topIndex : int + ArrayStack(in cap : int ) + ArrayStack() + size() : int + isEmpty() : boolean + push( in c : Character ) + top() : Character + pop() : Character Class name – top section Instance variables – middle section Implementing a Data Structure as a Class (Stevens) Methods – bottom section + size() : int + isEmpty() : boolean + push( in c : Character ) + top() : Character + pop() : Character > Stack ArrayStack

27 Syntax Exercise Handout

28 Using ArrayStack with Any Data So far, our ArrayStack class will do an acceptable job of containing data of the type Character What happens if we want to store something else in our stack? Brute force solution – Write a different stack class for every type of data we might want to store Implementing a Data Structure as a Class Whats the disadvantage of that approach?

29 Generics Better approach Write a generic class A generic class can store any kind of data Instead of making a stack of Character objects, we can make a stack of any kind of objects The behavior of the stack will be the same for any data type that is stored in it and we only need to write the code once The data type is specified by a parameter Implementing a Data Structure as a Class (Goodrich, 89–90)

30 Generics Allow us to use one data structure class for storing objects of any data type Syntax public class ArrayStack implements Stack { private E[] myArray; //more code Implementing a Data Structure as a Class (Goodrich, 89–90)

31 Generics Allow us to use one data structure class for storing objects of any data type Syntax public class ArrayStack implements Stack { private E[] myArray; //more code Implementing a Data Structure as a Class (Goodrich, 89–90) Angle brackets

32 Generics Allow us to use one data structure class for storing objects of any data type Syntax public class ArrayStack implements Stack { private E[] myArray; //more code Implementing a Data Structure as a Class (Goodrich, 89–90) Angle brackets E is a placeholder – the compiler will replace this letter with an actual data type

33 Generics Allow us to use one data structure class for storing objects of any data type Syntax public class ArrayStack implements Stack { private E[] myArray; //more code Implementing a Data Structure as a Class (Goodrich, 89–90) Angle brackets E is a placeholder – the compiler will replace this letter with an actual data type The interface is generic, too

34 Making ArrayStack Into a Generic Class package edu.uhcl.sce.arraystack; public class ArrayStack { public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new Character[capacity]; } public int size(){return topIndex + 1;} public boolean isEmpty(){return topIndex < 0;} public void push(Character c){ myArray[++topIndex] = c; } public Character top(){return myArray[topIndex];} public Character pop(){ return myArray[topIndex--]; } Implementing a Data Structure as a Class 1.Implement the class for a specific data type

35 Making ArrayStack Into a Generic Class package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack { public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new Character[capacity]; } public int size(){return topIndex + 1;} public boolean isEmpty(){return topIndex < 0;} public void push(Character c){ myArray[++topIndex] = c; } public Character top(){return myArray[topIndex];} public Character pop(){ return myArray[topIndex--]; } Implementing a Data Structure as a Class 2.Add

36 Making ArrayStack Into a Generic Class package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack { public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private Character[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new Character[capacity]; } public int size(){return topIndex + 1;} public boolean isEmpty(){return topIndex < 0;} public void push(Character c){ myArray[++topIndex] = c; } public Character top(){return myArray[topIndex];} public Character pop(){ return myArray[topIndex--]; } Implementing a Data Structure as a Class 3.Find all the locations of the data type of the elements in the class

37 Making ArrayStack Into a Generic Class package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack { public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private E[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = new E[capacity]; } public int size(){return topIndex + 1;} public boolean isEmpty(){return topIndex < 0;} public void push( E c){ myArray[++topIndex] = c; } public E top(){return myArray[topIndex];} public E pop(){ return myArray[topIndex--]; } Implementing a Data Structure as a Class 4.Substitute E for the data type of the elements in the class

38 Making ArrayStack Into a Generic Class package edu.uhcl.sce.arraystack; public class ArrayStack implements Stack { public static final int CAPACITY = 1024; private int topIndex; private int capacity;//Max capacity of the array private E[] myArray; public ArrayStack(){ this(CAPACITY); } public ArrayStack(int cap){ topIndex = -1; capacity = cap; myArray = (E[])new Object[capacity]; } public int size(){return topIndex + 1;} public boolean isEmpty(){return topIndex < 0;} public void push( E c){ myArray[++topIndex] = c; } public E top(){return myArray[topIndex];} public E pop(){ return myArray[topIndex--]; } Implementing a Data Structure as a Class 5.Use this special syntax for the array instance variable

39 Instantiating a Stack with a Generic Class Include it in your package (or import it) package edu.uhcl.sce.arraystack; Instantiate a stack with the default size ArrayStack myStack; myStack = new ArrayStack (); Instantiate a stack with a specific size ArrayStack smallStack; smallStack = new ArrayStack (10); Implementing a Data Structure as a Class Name of the class Type of data in it Use "new" to allocate memory and call the constructor Size of the stack Name of the stack object Parentheses are required

40 Problem with the Implementation ArrayStack is not robust What happens when an operation is used in an unexpected way? package edu.uhcl.sce.arraystack; public class Main{ public static void main(String args[]){ ArrayStack stack; stack = new ArrayStack (); System.out.print(stack.top()); //undefined } Implementing a Data Structure as a Class

41 Exceptions Unexpected problems that occur when the program is running Occur infrequently Affect the operation of the program Examples Trying to access an array outside of its bounds Calling top() on an empty stack Trying to pop() an element from an empty stack Trying to push() an element onto a full stack Implementing a Data Structure as a Class (Goodrich, 92)

42 Java Exception Objects Java exception objects Allow exceptions to be handled so the program doesnt crash and doesnt behave in an undefined way – more robust code The program is written so that the code throws an exception in response to an unexpected event Then the exception is caught and an appropriate action can occur Exception Super class for more specific exception classes RuntimeException The class used for throwing runtime exceptions in your program When theres a potential error, a RuntimeException object is instantiated and used in the exception-handling procedures Implementing a Data Structure as a Class (Goodrich, 76–79)

43 Java Exception Handling Procedure for handling unexpected errors Write methods so that they automatically detect a potential error condition such as popping from an empty stack In the program, we try the method If there is a error condition, the method will throw an exception as a signal to the calling program that something is wrong Then the calling program will catch the exception, and it can take appropriate action Implementing a Data Structure as a Class (Goodrich, 76–79)

44 Try-Catch Block Used for exception handling When a method might throw an exception, its call is placed inside atry block try{ //... } If an exception is thrown by a method call inside the block, then program control goes to the next block after the try block, which must be a catch block catch( RuntimeException e ){ //... } At that point, the program can take appropriate action Implementing a Data Structure as a Class (Goodrich, 92)

45 Try-Catch Example If we write the top() method so that it throws an exception whenever the stack is empty, then we can catch the exception like this: Call top() in a try block. public static void main(String[] args){ ArrayStack myStack = new ArrayStack (); try{ System.out.println(myStack.top()); } catch( RuntimeException e ){ System.out.println(e.getMessage()); } System.out.println("Skipped catch"); } If everything works, the program skips the catch block. Implementing a Data Structure as a Class

46 Try-Catch Example If we write the top() method so that it throws an exception whenever the stack is empty, then we can catch the exception like this: Call top() in a try block. If top() throws an exception, the exception object goes to the catch block. Implementing a Data Structure as a Class public static void main(String[] args){ ArrayStack myStack = new ArrayStack (); try{ System.out.println(myStack.top()); } catch( RuntimeException e ){ System.out.println(e.getMessage()); } System.out.println("Skipped catch"); }

47 Try-Catch Example If we write the top() method so that it throws an exception whenever the stack is empty, then we can catch the exception like this: Call top() in a try block. If top() throws an exception, the exception object goes to the catch block. Implementing a Data Structure as a Class public static void main(String[] args){ ArrayStack myStack = new ArrayStack (); try{ System.out.println(myStack.top()); } catch( RuntimeException e ){ System.out.println(e.getMessage()); } System.out.println("Skipped catch"); } The catch block has a reference to the exception object as an argument.

48 public static void main(String[] args){ ArrayStack myStack = new ArrayStack (); try{ System.out.println(myStack.top()); } catch( RuntimeException e ){ System.out.println(e.getMessage()); } System.out.println("Skipped catch"); } Try-Catch Example If we write the top() method so that it throws an exception whenever the stack is empty, then we can catch the exception like this: Call top() in a try block. If top() throws an exception, the exception object goes to the catch block. The getMessage() method returns a String. Implementing a Data Structure as a Class

49 Throwing an Exception When a method detects a potential error, it can throw an exception 1. Create an exception object and throw it 2. The method stops 3. The exception object is returned to the calling program – control goes to the calling program at the appropriate catch block Example of a method that throws an exception Implementing a Data Structure as a Class public Character top() { if(topIndex < 0) throw new RuntimeException("Empty Stack"); return myArray[topIndex]; } new is required Check for a potential error Use throw Instantiate the exception object

50 Exception Specification When writing a method that throws an exception, you can specify which exceptions the method can throw Called the throw list or exception specification If an exception is listed in this exception specification, the method is not required to catch it – it can be caught in the calling method Example Exception specification public Character top() throws RuntimeException { if(topIndex < 0) throw new RuntimeException("Empty Stack"); return myArray[topIndex]; } Implementing a Data Structure as a Class (Goodrich, 77; Deitel)

51 Using Inheritance to Create Exception Classes Inheritance allows reuse of code that is common between related classes We can use RuntimeException as a base class for specialized exception classes that we use in our data structures Advantage – we can use the built-in capabilities of RuntimeException Advantage – we can make our specialized class send a custom message Create a subclass that extends the super class Implementing a Data Structure as a Class (Goodrich, 76–79)

52 Defining a New Exception /* * FILE: FullStackException.java */ package edu.uhcl.sce.arraystack; public class FullStackException extends RuntimeException{ public FullStackException(){ super("Stack is full"); } Implementing a Data Structure as a Class (Goodrich, 76–79, 190)

53 Using a New Exception public E top() throws FullStackException { if(topIndex < 0) throw new FullStackException(); return myArray[topIndex]; } Implementing a Data Structure as a Class (Goodrich, 76–79, 190)

54 UML Notation for Inheritance Super class Sub classes Implementing a Data Structure as a Class (Deitel, 610–650) A general class that contains code used by all sub classes A specific class that contains specialized code

55 UML Notation for Inheritance from RuntimeException Super class Sub class Implementing a Data Structure as a Class (Deitel, 610–650) RuntimeException FullStackException

56 Bonus Lab Creating a generic class

57 References Deitel, H. M. and P. J. Deitel, Java How to Program. Upper Saddle River, NJ: Prentice Hall. Goodrich, M. T. and R. Tamassia, Data Structures and Algorithms in Java. Hoboken, NJ: John Wiley & Sons, Inc., Flanagan, D., Java in a Nutshell, 5th Edition. Sebastopol, CA:O'Reilly Media Inc., Stevens, P. with R. Pooley, Using UML, Software engineering with objects and components, Updated Edition. Harlow, England:Addison-Wesley., 2000.