1. COMPUTER 2430 Object Oriented Programming and Data Structures I

3. Quiz 3 1. Evaluate the following expressions a) Postfix Answer: -1
b) Prefix
Answer: 29

4. Quiz 3
3. Convert the following to prefix and postfix a) Prefix + A - * B / C D E b) Postfix A B C D / * E - +

5. Quiz 3: As a User
public Object removeAndReturnBottomElement( Stack s, int size ) { Stack temp = new Stack(size); // Hint: use another stack! if (s.isEmpty()) return null; while (!s.isEmpty()) temp.push( s.pop() ); Object obj = temp.pop(); while (!temp.isEmpty()) s.push) temp.pop() ); return obj; }

6. !s.isEmpty()
public Object removeAndReturnBottomElement( Stack s, int size ) { if (s.isEmpty()) return null; // while (!s.isEmpty()) while (s.isEmpty() == false) temp.push( s.pop() ); Object obj = temp.pop(); while (temp.isEmpty() == false) s.push) temp.pop() ); return obj; }

7. Quiz 3: As a User
public Object removeAndReturnBottomElement( Stack s, int size ) { Stack temp = new Stack(size); // Hint: use another stack! if (s.isEmpty()) return null; while (!s.isEmpty()) temp.push( s.pop() ); Object obj = temp.pop(); s = temp; // will it work? // NO! return obj; }

8. Quiz 3: As an Implementer
public class Stack { private Object[] items; private int top; public Object getBottomElement() if (top == 0) return null; Object obj = items[0]; top --; for (int i = 0; i < top; i ++) items[i] = items[i + 1]; return obj; }

9. Quiz 3: As an Implementer
public class Stack { public Object getBottomElement() if (top == 0) return null; Object obj = items[0]; for (int i = 0; i < top - 1; i ++) items[i] = items[i + 1]; top --; return obj; }

10. public class Stack {
private Object[] items;
private int top = 0;
public Stack( int size )
...
public void push( Object obj )
public Object pop()
public boolean isEmpty()
public boolean isFull() }

11. Example
Stack personList = new Stack(100); Person p = new Person(); personList.push( p ); // Valid? // Yes. // public void push( Object obj ) // Can use child when parent is expected. ... p = personList.pop(); // Valid? // NO! // public Object pop() // Cannot use parent when child is expected. p = (Person) personList.pop(); // Must cast!

12. Stack of Person
public class PersonStack { private Person[] items; private int top = 0; public Stack( int size ) ... public void push( Person obj ) public Person pop() public boolean isEmpty() public boolean isFull() }

13. Stack of Person
public class PersonStack { private Person[] items; ... } PersonStack personList = new PersonStack(100); Person p; p = personList.pop(); // Valid? // Yes! No cast needed. Object obj = new Object(); personList.push( obj ); // Valid? // NO! // Cannot use parent when child is expected.

14. Stack for Each Class
public class PersonStack { private Person[] items; ... } public class DateStack private Date[] items; public class TVectorStack private TVector[] items; The code is almost the same!

15. Generic Stack public class Stack<E> { private E[] items; ... }
E represents a class
The class Stack can be instantiated with a particular class
E is a parameter
E must be a class and cannot be a primitive type

16. public class Stack<E>{
private E[] items;
private int top = 0;
public Stack( int size )
items = (E[])new Object[size];
// Java Warning possible }
public boolean isEmpty()
return top == 0;
public boolean isFull()
return top >= items.length;
...

17. public class Stack<E>{
private E[] items;
private int top = 0;
. . .
public void push ( E x )
items[top ++] = x; }
public E pop()
return items[-- top];

18. Example
Stack<Person> personStack = new Stack<Person>(100); Stack<Date> dateStack = new Stack<Date>(100); Person p; Date d; ... p = personStack.pop(); d = dateStack.pop(); // No cast needed! d = new Date(); dateStack.push( d ); // Valid? // Yes personStack.push( d ); // Valid? // NO!

19. Generic Stack RPN Evaluation
Prog 4
Generic Stack RPN Evaluation

20. Prog 4 Class TVector Java class Vector 4-dimensional
Example: (-7,21,5,-33)
Java class Vector
Generic class
Methods:
size()
get(i)
add(x)

21. Other Generic Classes
Generic Queue
More in CS 2630

22. Prog 3 & Prog 4