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

Implementing a Stack 2 CS340

Implementing a Stack as an Extension of Vector  Part of the package java.util : public class Stack extends Vector  Vector: a growable array of objects CS340 3

Implementing a Stack as an Extension of Vector (cont.)  We can use Vector 's add method to implement push : public E push(obj E) { add(obj); return obj; }  pop can be coded as public E throws EmptyStackException { try { return remove (size() – 1); } catch (ArrayIndexOutOfBoundsException ex) { throw new EmptyStackException(); } CS340 4

Implementing a Stack as an Extension of Vector (cont.)  All of Vector operations can be applied to a Stack  Such as searches and access by index  This violates the principle of information hiding CS340 5

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 6

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 7

Implementing a Stack with a List Component (cont.) public class ListStack implements Stack { private List theData; public ListStack( ) { theData = new ArrayList (); public E push( E obj ) { theData.add(obj); return obj; } CS340 8

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

Implementing a Stack with a List Component public boolean empty( ) { return(theData.size() == 0); } CS340 10

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 = public ArrayStack() { theData = (E[])new Object[INITIAL_CAPACITY]; } 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 CS340 11

Implementing a Stack Using an Array (cont.) 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 12

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

Implementing a Stack as a Linked Data Structure  We can also implement a stack using a linked list of nodes 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 14

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

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 16

Additional Stack Applications 17 CS340

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 18

Evaluating Postfix Expressions  Write a class that evaluates a postfix expression  Use the space character as a delimiter between tokens CS340 19

Evaluating Postfix Expressions (cont.) 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 CS340 20

Evaluating Postfix Expressions (cont.) 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* CS340 21

Evaluating Postfix Expressions (cont.) 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* * 7 CS340 22

Evaluating Postfix Expressions (cont.) 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* CS340 23

Evaluating Postfix Expressions (cont.) 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* CS340 24

Evaluating Postfix Expressions (cont.) 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* CS340 25

Evaluating Postfix Expressions (cont.) 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* CS340 26

Evaluating Postfix Expressions (cont.) 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 CS340 27

Evaluating Postfix Expressions (cont.) CS  Listing 3.6 ( PostfixEvaluator.java, pages )

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 29