1. Lecture objectives  Collections interface  Learn about stacks and their methods:  push  pop  peek  Empty  Analyze stack applications and why stacks were the best choice in these cases  Use the stack in an actual implementation: “Code stack” CS340 1

2. The Collections Framework Design CS340 2

3. The Collection Interface  Specifies a subset of methods in the List interface, specifically excluding  add(int, E)  get(int)  remove(int)  set(int, E) but including  add(E)  remove(E)  the iterator method CS340 3

4. The Collection Framework 4 CS340

5. Common Features of Collections  Collections  grow as needed  hold references to objects  have at least two constructors: one to create an empty collection one to make a copy of another collection CS340 5

6. Common Features of Collections (cont.)  In a general Collection the order of elements is not specified  For collections implementing the List interface, the order of the elements is determined by the index CS340 6

7. Common Features of Collections (cont.)  In a general Collection, the position where an object is inserted is not specified  In ArrayList and LinkedList, add(E) always inserts at the end and always returns true CS340 7

8. AbstractCollection, AbstractList, and AbstractSequentialList  “Helper" abstract classes: help build implementations of their corresponding interfaces  Extend the AbstractCollection class and implement only the desired methods CS340 8

9. Implementing a Subclass of Collection  Extend AbstractCollection, which implements most operations  You need to implement only:  add(E)  size()  iterator()  An inner class that implements Iterator CS340 9

10. Implementing a Subclass of List  Extend AbstractList  You need to implement only:  add(int, E)  get(int)  remove(int)  set(int, E)  size()  AbstractList implements Iterator using the index CS340 10

11. AbstractCollection, AbstractList, and AbstractSequentialList  Another more complete way to declare CS340ArrayList is: public class CS340ArrayList extends AbstractList implements List  Another more complete, way to declare CS340LinkedList is: public class CS340LinkedList extends AbstractSequentialList implements List CS340 11

12. Stack Abstract Data Type CS340 12

13. Stack Abstract Data Type  A stack is one of the most commonly used data structures in computer science  A stack example: dirty plates  Only the top item can be accessed  You can extract only one item at a time  The stack’s storage policy is Last-In, First- Out, or LIFO CS340 13

14. Specification of the Stack Abstract Data Type  Only the top element of a stack is visible; therefore the number of operations performed by a stack are few  We need the ability to  test for an empty stack (empty)  inspect the top element (peek)  retrieve the top element (pop)  put a new element on the stack (push) CS340 14

15. A Stack of Strings  String last = cities.peek()  String temp = cities.pop()  cities.push(“Tampa”)  cities.push(“Boston”)  String temp = cities.pop() Atlanta Raleigh Norfolk Wichita Raleigh Norfolk Wichita Raleigh Norfolk Wichita Tampa Raleigh Norfolk Wichita Tampa Boston CS340 15

16. A Stack of Strings  String last = cities.peek()  String temp = cities.pop()  cities.push(“Tampa”)  cities.push(“Boston”)  String temp = cities.pop() Raleigh Norfolk Wichita Tampa CS340 16

17. Stack Applications CS340 17

18. Finding Palindromes  Palindrome: a string that reads identically in either direction, letter by letter (ignoring case)  kayak  "I saw I was I"  “Able was I ere I saw Elba”  "Level, madam, level"  Problem: Write a program that reads a string and determines whether it is a palindrome CS340 18

19. Finding Palindromes (cont.) CS340 19

20. Finding Palindromes (cont.) import java.util.*; public class PalindromeFinder { private String inputString; private Stack charStack = new Stack (); public PalindromeFinder(String str) { inputString = str; fillStack(); }... CS340 20

21. Finding Palindromes (cont.)  Solving using a stack:  Push each string character, from left to right, onto a stack ka k y ka ka a ay ayakk k y a k ayakk private void fillStack() { for(int i = 0; i < inputString.length(); i++) { charStack.push(inputString.charAt(i)); } CS340 21

22. kaykaykaa Finding Palindromes (cont.)  Solving using a stack:  Pop each character off the stack, appending each to the StringBuilder result k k a a k y a k y a kayak k private String buildReverse(){ StringBuilder result = new StringBuilder(); while(!charStack.empty()) { result.append(charStack.pop()); } return result.toString(); } CS340 22

23. Finding Palindromes (cont.)... public boolean isPalindrome() { return inputString.equalsIgnoreCase(buildReverse()); } CS340 23

24. Testing  To test this class using the following inputs:  a single character (always a palindrome)  multiple characters in a word  multiple words  different cases  even-length strings  odd-length strings  the empty string (considered a palindrome) CS340 24

25. Balanced Parentheses  When analyzing arithmetic expressions, it is important to determine whether an expression is balanced with respect to parentheses ( a + b * ( c / ( d – e ) ) ) + ( d / e )  The problem is further complicated if braces or brackets are used in conjunction with parentheses  The solution is to use stacks! CS340 25

26. Balanced Parentheses (cont.) CS340 26

27. Balanced Parentheses (cont.) CS340 27

28. Balanced Parentheses (cont.) Expression: balanced : true index : 0 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( CS340 28

29. Balanced Parentheses (cont.) Expression: balanced : true index : 1 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( CS340 29

30. Balanced Parentheses (cont.) Expression: balanced : true index : 2 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( CS340 30

31. Balanced Parentheses (cont.) Expression: balanced : true index : 3 (w * [x + y] / z) 145678910320 wx+y]/z)[*( ( ( [ [( CS340 31

32. Balanced Parentheses (cont.) Expression: balanced : true index : 4 (w * [x + y] / z) 145678910320 wx+y]/z)[*( ( ([ CS340 32

33. Balanced Parentheses (cont.) Expression: balanced : true index : 5 (w * [x + y] / z) 145678910320 wx+y]/z)[*( ( ([ CS340 33

34. Balanced Parentheses (cont.) Expression: balanced : true index : 6 (w * [x + y] / z) 145678910320 wx+y]/z)[*( ( ([ CS340 34

35. Balanced Parentheses (cont.) Expression: balanced : true index : 7 (w * [x + y] / z) 145678910320 wx+y]/z)[*( ( ([( Matches! Balanced still true CS340 35

36. Balanced Parentheses (cont.) Expression: balanced : true index : 8 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( CS340 36

37. Balanced Parentheses (cont.) Expression: balanced : true index : 9 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( CS340 37

38. Balanced Parentheses (cont.) Expression: balanced : true index : 10 (w * [x + y] / z) 145678910320 wx+y]/z)[*((( Matches! Balanced still true CS340 38

39. Testing  Provide a variety of input expressions displaying the result true or false  Try several levels of nested parentheses  Try nested parentheses where corresponding parentheses are not of the same type  Try unbalanced parentheses  PITFALL: attempting to pop an empty stack will throw an EmptyStackException. You can guard against this by either testing for an empty stack or catching the exception CS340 39

40. Code Stack  When analyzing code, we execute methods, code stack changes public static void main(String[] args) { //create a new myLibrary MyLibrary testLibrary = new MyLibrary("Test Drive Library"); …  How do we implement the debugger calls?  The solution is to use stacks! CS340 40