The Stack Class

1
The Stack Class

• Final Review
• Fall 2005
• CS 101
• Aaron Bloomfield

2
Motivation

• Same as for Vectors
• We want an easy way to store elements in a object
  without having to worry about manipulating arrays

3
Properties of our Stack class

• It needs to have an array to hold the values
• That array will be fixed at size 1000
• Well call it array
• Thus, we also need a value to hold the number of
  elements in the stack
• Well call it size
• Our Stack class so far
• public class Stack
• int array new int1000
• int size 0

4
About stacks

• With a stack, you can only add and remove
  elements from ONE end
• Both occur from the same end
• Think of a stack of papers you are always
  adding a new paper to the top or removing it from
  the top
• Adding an element is called pushing an element
• Removing an element is called popping an element

5
Methods in our Stack class

• Create a Stack object
• Insert and remove elements into/from the Stack
• Get the top element
• Find if the stack is empty
• Print it out to the screen
• Search the stack for a value

6
The Stack Constructor

• Ready?
• public Stack ()
• Rather boring
• As we initialized the variables earlier, we dont
  need to do so here
• But we could have done here just as well

7
Pushing an element onto the Stack

• To add an element onto the Stack, we want to do
  two things
• Insert it into the array at the proper position
• Increment the size of the array
• The code
• public void push (int value)
• arraysize value
• We could have done this in two lines as well
• public void push (int value)
• arraysize value
• size size 1

8
Popping an element from the Stack

• To add an element onto the Stack, we want to do
  two things
• Find (and return) the top element in the Stack
• Decrement the size of the array
• The code
• public int pop ()
• return array--size
• We could have done this in two lines as well
• public int pop ()
• size size 1
• return arraysize

9
peek() and empty()

• peek() is a quickie
• public int peek ()
• return arraysize-1
• empty() is also a quickie
• public boolean empty()
• return size 0

10
Searching the Stack

• Note that we want to search up to the value in
  size, not the entire (1,000 element) array
• The method
• public boolean search (int forwhat)
• for ( int i 0 i lt size i )
• if ( arrayi size )
• return true
• return false

11
Printing the Stack

• The method
• public String toString()
• String ret "Stack"
• for ( int i 0 i lt size i )
• ret arrayi
• if ( i ! size-1 )
• ret ", "
• ret ""
• return ret

• This is just so that the method doesnt print a
  comma after the last element
• Our for loop body could also have been
• ret arrayi ", "

12
Using our Stack

• public static void main (String args)
• Stack stack new Stack()
• System.out.println ("pushing elements 5 through
  10 onto the stack")
• for ( int i 5 i lt 10 i )
• stack.push(i)
• System.out.println (stack)
• System.out.println ("peek() returned "
  stack.peek())
• System.out.println ("search(9) returned "
  stack.search(9))
• int ret stack.pop()
• System.out.println ("pop() returned " ret)
• System.out.println (stack)
• System.out.println ("peek() returned "
  stack.peek())

13
Program Demo

• Stack.java