1 Todays Objectives Announcements Homework #1 is due next week Return Quiz 1 – answers are posted...

1

Today’s 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

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Return Quiz 1

Answers are posted on the Yahoo discussion group site under “Files”

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Basic Java Programming

while loop, switch, using java.util.Stack,

using java.util.Scanner

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while, switch, java.util.Stack

(From last week’s slides)

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Finishing SimpleSolitaire

Capturing keyboard input with java.util.Scanner

Implementing the game in SimpleSolitaire

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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<Card> 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;}

Here’s 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

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We still need to implement removePair() and removeLowCard()



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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


What happens on these lines? stacks[i - 1].pop(); stacks[j - 1].pop();

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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();}


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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()



Why use “isEmpty”?

What does “peek” do?

Why use “deck.size()”?

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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()



Why use “isEmpty”?

What does “peek” do?

Why use “deck.size()”?

Rank

Spades Hearts Clubs

If the game in progress looks like the picture below, what should toString() print?

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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.


Main.java

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Implementing a Data Structureas a Class

A Stack Example

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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

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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)

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Using a Java Interfacewith 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();}



public class ArrayStack implements Stack{ //more code here}

Stack.javaArrayStack.java

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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.)


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.


public interface Stack { public int size(); public boolean isEmpty(); public void push(Character c); public Character top(); public Character pop();}

Stack.java

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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


Implement the interface

ArrayStack.java

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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


The declarations of all methods should be identical to the declarations in the interface.

ArrayStack.java

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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.


All instance variables labeled “private” are only accessible to the methods of this class.

ArrayStack.java

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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.


All instance variables labeled “private” are only accessible to the methods of this class.

Information hiding

Instance variables are always “private” so that the object’s client cannot change the data by accessing it directly.

Data can be accessed only by using the “public” member functions.

ArrayStack.java

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Step 5 – Define the operations

Now, decide how to implement the operations


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

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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();

}


• 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

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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();

}


• Constructor with no arguments

• Calls the overloaded constructor with the default capacity as an argument

ArrayStack.java

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Step 6 – Add exceptions

Add robustness – use exceptions

Implementing a Data Structure as a Class (Goodrich 76)

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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

<<interface>>Stack

ArrayStack

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Syntax Exercise

Handout

Handout

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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


What’s the disadvantage of that approach?

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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


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Generics

Allow us to use one data structure class for storing objects of any data type

Syntax

public class ArrayStack<E>implements Stack<E>{

private E[] myArray;

//more code


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Generics


Syntax



//more code


Angle brackets

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Generics


Syntax



//more code


Angle brackets

“E” is a placeholder – the compiler will replace this letter

with an actual data type

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Generics


Syntax



//more code


Angle brackets

“E” is a placeholder – the compiler will replace this letter

with an actual data type

The interface is generic, too

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Making ArrayStack Intoa 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--]; }}


1. Implement the class for a specific data type

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package edu.uhcl.sce.arraystack;public class ArrayStack<E> implements Stack<E>{ 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--]; }}


2. Add <E>

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package edu.uhcl.sce.arraystack;public class ArrayStack<E> implements Stack<E>{ 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--]; }}


3. Find all the locations of the data type of the elements in the class

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package edu.uhcl.sce.arraystack;public class ArrayStack<E> implements Stack<E>{ 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--]; }}


4. Substitute “E” for the data type of the elements in the class

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package edu.uhcl.sce.arraystack;public class ArrayStack<E> implements Stack<E>{ 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--]; }}


5. Use this special syntax for the array instance variable

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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 sizeArrayStack<Character> myStack;

myStack = new ArrayStack<Character>();

Instantiate a stack with a specific sizeArrayStack<Character> smallStack;smallStack = new ArrayStack<Character>(10);


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

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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<Character> stack; stack = new ArrayStack<Character>(); System.out.print(stack.top()); //undefined }}


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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)

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Java Exception Objects

Java exception objects• Allow exceptions to be handled so the program doesn’t crash and

doesn’t 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 there’s a potential error, a RuntimeException object is

instantiated and used in the exception-handling procedures


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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


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Try-Catch Block

Used for exception handling

When a method might throw an exception, its call is placed inside a “try 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)

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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<Character> myStack = new ArrayStack<Character>(); 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.


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Try-Catch Example



If top() throws an exception, the exception object goes to the catch block.



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Try-Catch Example






The catch block has a reference to the exception object as an argument.

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Try-Catch Example




The getMessage() method returns a String.


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Throwing an Exception

When a method detects a potential error, it can “throw an exception”1. Create an exception object and throw it2. The method stops3. 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


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

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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");


}

Implementing a Data Structure as a Class (Goodrich, 77; Deitel)

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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


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Defining a New Exception

/*

* FILE: FullStackException.java

*/


public class FullStackException extends RuntimeException{

public FullStackException(){

super("Stack is full");

}

}

Implementing a Data Structure as a Class (Goodrich, 76–79, 190)

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Using a New Exception

public E top() throws FullStackException {

if(topIndex < 0) throw new FullStackException();


}

Implementing a Data Structure as a Class (Goodrich, 76–79, 190)

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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

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UML Notation for Inheritance from RuntimeException

Super class

Sub class

Implementing a Data Structure as a Class (Deitel, 610–650)

RuntimeException

FullStackException

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Bonus Lab

Creating a generic class

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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., 2006.

Flanagan, D., Java in a Nutshell, 5th Edition. Sebastopol, CA:O'Reilly Media Inc., 2005.

Stevens, P. with R. Pooley, Using UML, Software engineering with objects and components, Updated Edition. Harlow, England:Addison-Wesley., 2000.

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