1. CSC 205 – Java Programming II
Lecture 30
April 3, 2002

2. Stack
A stack is an abstract data type (ADT) that contains a sequence of elements and honors the last-in-first-out (LIFO) rule.
Conceptually, the only element that can be removed, accessed, modified is the one which was most recently inserted.
Pop and push are the two basic stack operations

3. Stack
PUSH
top
POP
last-in-first-out

4. Examples
Green
Red
Yellow
Red
Yellow
Green
Red
Yellow
pop
push
green

5. Java Stack Class The Java Stack class represents a stack of objects.
It outdated the Java Collections Framework
Available since JDK1.0
It extends class Vector with 5 additional operations.
With its top at the index size()-1
top

6. Java Stack API boolean empty() Tests if this stack is empty.
Object peek()           Looks at the object at the top of this stack without removing it from the stack.
Object pop()           Removes the object at the top of this stack and returns that object as the value of this function.
Object push(Object item)           Pushes an item onto the top of this stack.
int search(Object o)           Returns the 1-based position where an object is on this stack.

7. Example Checking for balanced braces The braces are balanced if
abc{defg{ijk}{l{mn}}op}qr balanced
abc{def}}ghij{kl}m not balanced
The braces are balanced if
Each time you encounter a }, it matches an already encountered {
When you reach the end of the string, you have matched each {

8. Pseudocode Solution A simplified solution in pseudocode
while (not at the end of the string) {
if (the next character is a ‘{’) {
stack.push(‘{’) }
else if (the character is a ‘}’)
openBrace = stack.pop()

9. Examples {a{b}c} {a{bc} {ab}c} { { { { { { { push “}” pop
Stack empty balanced
{a{b}c} { { {
{a{bc}
push “}”
pop
Stack not empty not balanced {
push “}”
pop
Stack empty balanced
{ab}c}

10. Problems w/ The Stack Class
All the Vector methods can be used
For examples
myBookStack.get(7);
myBookStack.remove(0);
//the element right at the “bottom” can be removed!

11. Convenient or Risky?
myBookStack.get(7);
myBookStack.remove(0);

12. Alternative Designs Implement a RealStack class
Use a LinkedList object as its only field
Provide push and pop methods
public Object push(Object o) {
list.add(o);
return o; }
public Object pop(Object o) {
return list.removeLast(o);

13. Activation Stack
Stacks are used almost exclusively by computer systems
An activation stack is used by a compiler /interpreter to save the method information when a method is called
The stack is part of the main memory.
Other parts include a heap to hold data

14. Activation Records Each activation record includes The return address
The value of each argument
The values of the method’s other local variables

15. Decimal To Binary public void processInput(String s) {
writeBinary(Integer.parseInt(s)); //RA1 }
public void writeBinary(int n) {
if (n<=1)
throw new IllegalArgumentException();
gui.println(n);
else {
writeBinary(n/2); //RA2
gui.println(n%2);

16. Example
RA2 1
RA2
RA2
RA2 3 3 3
RA1
RA1
RA1
RA1
RA1 6 6 6 6 6