1. Lecture Objectives To understand how Java implements a stack To learn how to implement a stack using an underlying array or linked list Implement a simple calculator CS340 1

2. Implementing a Stack CS340 2

3. A little about information hiding Principle: Hide internal details of a component from other components Why? Prevent damage from wrong external code Make components easier to understand/use Simplify modification and repair Facilitate re-use CS340 3

4. Implementing a Stack with a List Component  ListStack : has a List component  We can use ArrayList, Vector, or the LinkedList classes to implement the List interface.  push method: public E push(E obj) { theData.add(obj); return obj; }  Adapter class: stack in this case is adapter class of List  Method delegation: from stack to list CS340 4

5. Implementing a Stack with a List Component (cont.) public class ListStack implements Stack { private List theData; public ListStack( ) { theData = new ArrayList (); } @Override public E push( E obj ) { theData.add(obj); return obj; } CS340 5

6. 6 Implementing a Stack with a List Component (cont.) @Override public E pop( ) { if( empty( ) ) throw new EmptyStackException( "ListStack pop" ); return theData.remove(theData.size()-1); } @Override public E peek( ) { if( empty( ) ) throw new EmptyStackException( "ListStack top" ); return theData.get(theData.size()-1); }

7. Implementing a Stack with a List Component (cont.) @Override public boolean empty( ) { return(theData.size() == 0); } CS340 7

8. Implementing a Stack Using an Array  If we implement a stack as an array, we would need... public class ArrayStack implements StackInt { private E[] theData; int topOfStack = -1; private static final int INITIAL_CAPACITY = 10; @SupressWarnings("unchecked") public ArrayStack() { theData = (E[])new Object[INITIAL_CAPACITY]; } CS340 8 Allocate storage for an array with a default capacity Keep track of the top of the stack We do not need a size variable or method

9. Implementing a Stack Using an Array (cont.) 9 ArrayStack theData = topOfStack = -1 Object[] [0] = null [1] = null [2] = null [3] = null [4] = null [5] = null [6] = null [7] = null [8] = null [9] = null public E push(E obj) { if (topOfStack == theData.length - 1){ reallocate(); } topOfStack++; theData[topOfStack] = obj; return obj; } 0 Character value = 'J' 1 Character value = 'a' Character value = 'v' 2 Character value = 'a' 3

10. Implementing a Stack Using an Array (cont.) @Override public E pop() { if (empty()) { throw new EmptyStackException(); } return theData[topOfStack--]; } CS340 10  This implementation is O(1)

11. Implementing a Stack as a Linked Data Structure We can also implement a stack using a linked list of nodes 11 It is easiest to insert and delete from the head of a list push inserts a node at the head and pop deletes the node at the head when the list is empty, pop returns null

12. Implementing a Stack as a Linked Data Structure (cont.) public class LinkedStack implements Stack { // Data fields Private Node topOfStackRef = null; // Methods: push, pop, peek, empty } CS340 12

13. Comparison of Stack Implementations  Extending a Vector : poor choice for stack implementation  The easiest implementation uses a List component ( ArrayList is the simplest) for storing data  Array requires reallocation of space when the array becomes full, and  Linked data structure requires allocating storage for links  All insertions and deletions occur at one end: constant time, O(1) CS340 13

14. Additional Stack Applications CS340 14

15. Additional Stack Applications  Postfix and infix notation  Expressions normally are written in infix form, but  it easier to evaluate an expression in postfix form since there is no need to group sub-expressions in parentheses or worry about operator precedence 15

16. Evaluating Postfix Expressions Write a class that evaluates a postfix expression Use the space character as a delimiter between tokens CS340 16

17. Evaluating Postfix Expressions (cont.) CS340 17 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20* 4 44

18. Evaluating Postfix Expressions (cont.) CS340 18 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20* 4 447 7 4

19. Evaluating Postfix Expressions (cont.) CS340 19 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 7 4 4 * 7

20. Evaluating Postfix Expressions (cont.) CS340 20 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 28

21. Evaluating Postfix Expressions (cont.) CS340 21 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 28 20 28

22. Evaluating Postfix Expressions (cont.) CS340 22 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 28 28 - 20

23. Evaluating Postfix Expressions (cont.) CS340 23 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the character on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 8 8

24. Evaluating Postfix Expressions (cont.) CS340 24 1. create an empty stack of integers 2. while there are more tokens 3. get the next token 4. if the first character of the token is a digit 5. push the number on the stack 6. else if the token is an operator 7. pop the right operand off the stack 8. pop the left operand off the stack 9. evaluate the operation 10. push the result onto the stack 11. pop the stack and return the result 7-20*447 8

25. Evaluating Postfix Expressions (cont.) Listing 3.6 ( PostfixEvaluator.java, pages 173 - 175) CS340 25

26. Evaluating Postfix Expressions (cont.)  Testing: write a driver which  creates a PostfixEvaluator object  reads one or more expressions and report the result  catches PostfixEvaluator.SyntaxErrorException  exercises each path by using each operator  exercises each path through the method by trying different orderings and multiple occurrences of operators  tests for syntax errors: an operator without any operands a single operand an extra operand an extra operator a variable name the empty string CS340 26