242 ECE Spring 2003
Data Structures in Java
http://rio.ecs.umass.edu/ece242
Stack
Prof. Lixin Gao
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Todays Topics
Abstract Data Types Stack
Implementation of Stack Usage of Stack
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Queue
Queue: First In First Out (FIFO)
Input
D C B A
Output
Toll Station
Car comes, pays, leaves
Check-out in Big Y market
Customer comes, checks out and leaves
More examples: Printer, Office Hours,
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Stack
Stack: Last In First Out (LIFO) Input
C B A
Output
Long and narrow driveway
BMW comes in first, Lexus follows, Benz enters in last. When the cars come out, Benz comes out first, then Lexus follows, and BMW comes out last.
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More Examples of Stack
In our daily life
Elevator Bus Anything else ?
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Data Structures in Java, Prof. Gao
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Abstract Data Types
Stack
Operating on top only Operations: Push(in), Pop(out)
Queue
Operating on both ends Operations: EnQueue(in), DeQueue(out)
List
More general than Stack and Queue, it is a sequence of items. You can insert an item and delete an item anywhere.
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What Is Stack
Stack is an abstract data type Adding an entry on the top (push) Deleting an entry from the top (pop) Simple, but useful in computer engineering
Push Pop
C B A
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Data Structures in Java, Prof. Gao
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Stack
Stack is LIFO (last-in first-out)
A stack is open at one end (the top) only. You can push entry onto the top, or pop the top entry out of the stack. Note that you cannot add/extract entry in the middle of the stack.
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Applications of Stack
Reverse a string Check nesting structure Undo operation in many editor tools. Brackets balancing in compiler Stack frame of procedure call Hardware stack in CPU
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ECE242 Spring 2003
undo operation
In an Excel file, input your data in row
100, 200, 300, 400
Find something wrong, use undo, return to previous state
Operation lists
400 300 200 100
undo
300 200 100
undo
200 100
undo
100
undo
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Last-in First-out (LIFO)
Push A, B, C
C B A
A
B A
The last one pushed in is the first one popped out! (LIFO)
A
B A
Pop C, B, A
ECE242 Spring 2003
When we push entries onto the stack and then pop them out one by one, we will get the entries in reverse order.
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Data Structures in Java, Prof. Gao
Question
Stack is an abstract data structure Item can be Integer, Double, String, also can be any data type, such as Employee, Faculty How to implement a general stack for all those types?
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Abstract Data Type
We use Object data type instead of int or double or String or other data type Use an array in stack, which stores all items popped in.
Object Stack[ ];
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
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Array Implementation of Stack
Use an array to implement stack
n 4 3 2 1 0 Max_Size C B A
Operations: push(item)
pop( ) peek( ) isEmpty( ) isFull( )
Other implementations are possible
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Push & Pop
Push A Push B Push C Pop Pop Pop
A
B A
C B A
B A
A
Top: -1 0 1 2 1 0
15
-1
ECE242 Spring 2003
Data Structures in Java, Prof. Gao
Implementation for Stack
public class ArrayStackPT {
private final static int DEFAULT_CAPACITY = 100; // suppose the default capacity for this stack is 100. private Object stack[ ]; // The array that holds the items
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ArrayStackPT Constructor
// Creates a stack with the default capacity public ArrayStackPT () { this(DEFAULT_CAPACITY); } // Creates a stack with a user-specified capacity public ArrayStackPT (int capacity) { if (capacity < 1) throw new IllegalArgumentException ("Capacity must be > 0"); stack = new Object[capacity]; top = -1; }
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ArrayStackPT---Empty or Full
public int size() { return top + 1; // return the number of elements } // check whether the stack is empty public boolean isEmpty() { return top == -1; } // check whether the stack is full public boolean isFull() { return size() == stack.length; }
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ArrayStackPT Push() Method
public void push(Object item) { if (item == null) throw new IllegalArgumentException ("Item is null"); if (isFull()) throw new IllegalStateException ("Stack is full"); top = top + 1; stack[top] = item; }
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ArrayStackPT Pop() Method
public Object pop( ) { if (isEmpty()) throw new IllegalStateException ("Stack is empty"); Object topItem = stack[top]; stack[top] = null; top = top - 1; return topItem; }
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ArrayStackPT Peek() Method
public Object peek() { if (isEmpty())
throw new IllegalStateException ("Stack is empty");
return stack[top]; }
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Data Structures in Java, Prof. Gao
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Interface
Users dont need to know how Stack is implemented. User only need to know how they can operate the stack. There are many ways to implement Stack class, but all of them have the same interfaces
push, pop, peek, isFull, isEmpty
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Interface and Implementation Class
Interface contains a skeleton for public operations
No real code for each method Like function prototype
A class implements interface
Implements every method
Statement:
public class myclass implements interface { }
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Interface for Stack
public interface StackPT { public boolean isEmpty(); public boolean isFull(); public Object peek(); public Object pop(); public void push(Object item); public int size(); }
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Implementation of ArrayStackPT Class
Example: StackPT.java, ArrayStackPT.java
public class ArrayStackPT implements StackPT { private final static int DEFAULT_CAPACITY = 100; private Object stack[]; // The array holds the stack private int top; // Index of top item on the stack
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Create an object of ArrayStackPT
// create a stack with default capacity StackPT mystack = new ArrayStackPT(); // create a stack with 1024 elements StackPT president = new ArrayStackPT(1024);
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Data Structures in Java, Prof. Gao
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Using ArrayStackPT
push and pop characters in Stack mystack
mystack.push( new Character(a) ) char ch = (Character)mystack.pop()).charValue()
push and pop Employee in Stack president
president.push( new Employee(Bush, 5456) ) Employee emp = (Employee) president.pop();
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Example --- Reverse a String
Input a string Print out the reversed string
Example: Input abcde, then output edcba
Using ArrayStackPT C...