Stack Program in Java

What is Stack in Java?

Stack is an area of memory that holds all local variables and parameters used by any function and remembers the order in which functions are called so that function returns occur correctly. ‘push‘ operation is used to add an element to stack and ‘pop‘ operation is used to remove an element from stack. ‘peek‘ operation is also implemented returning the value of the top element without removing it. The relation between the push and pop operations is such that the stack is a Last-In-First-Out (LIFO) data structure. The implemented stack has bounded capacity.

Program/Source Code

Here is the source code of the Java program to implement a stack. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

```
/*
```
```
 * Java Program to Implement Stack
```
```
 */
```
```
 
```
```
import java.util.*;
```
```
 
```
```
/*  Class arrayStack  */
```
```
class arrayStack
```
```
{
```
```
    protected int arr[];
```
```
    protected int top, size, len;
```
```
    /*  Constructor for arrayStack */
```
```
    public arrayStack(int n)
```
```
    {
```
```
        size = n;
```
```
        len = 0;
```
```
        arr = new int[size];
```
```
        top = -1;
```
```
    }
```

    /*  Function to check if stack is empty */

```
    public boolean isEmpty()
```
```
    {
```
```
        return top == -1;
```
```
    }
```

    /*  Function to check if stack is full */

```
    public boolean isFull()
```
```
    {
```
```
        return top == size -1 ;        
```
```
    }
```

    /*  Function to get the size of the stack */

```
    public int getSize()
```
```
    {
```
```
        return len ;
```
```
    }
```

    /*  Function to check the top element of the stack */

```
    public int peek()
```
```
    {
```
```
        if( isEmpty() )
```

            throw new NoSuchElementException("Underflow Exception");

```
        return arr[top];
```
```
    }
```

    /*  Function to add an element to the stack */

```
    public void push(int i)
```
```
    {
```
```
        if(top + 1 >= size)
```

            throw new IndexOutOfBoundsException("Overflow Exception");

```
        if(top + 1 < size )
```
```
            arr[++top] = i;
```
```
        len++ ;
```
```
    }
```

    /*  Function to delete an element from the stack */

```
    public int pop()
```
```
    {
```
```
        if( isEmpty() )
```

            throw new NoSuchElementException("Underflow Exception");

```
        len-- ;
```
```
        return arr[top--]; 
```
```
    }    
```

    /*  Function to display the status of the stack */

```
    public void display()
```
```
    {
```
```
        System.out.print("\nStack = ");
```
```
        if (len == 0)
```
```
        {
```

            System.out.print("Empty\n");

```
            return ;
```
```
        }
```
```
        for (int i = top; i >= 0; i--)
```

            System.out.print(arr[i]+" ");

```
        System.out.println();
```
```
    }    
```
```
}
```
```
 
```
```
/*  Class StackImplement  */
```
```
public class StackImplement
```
```
{
```

    public static void main(String[] args)

```
    {
```

        Scanner scan = new Scanner(System.in);

        System.out.println("Stack Test\n");

        System.out.println("Enter Size of Integer Stack ");

```
        int n = scan.nextInt();
```

        /* Creating object of class arrayStack */

        arrayStack stk = new arrayStack(n);

```
        /* Perform Stack Operations */
```
```
        char ch;
```
```
        do{
```

            System.out.println("\nStack Operations");

            System.out.println("1. push");

            System.out.println("2. pop");

            System.out.println("3. peek");

            System.out.println("4. check empty");

            System.out.println("5. check full");

            System.out.println("6. size");

            int choice = scan.nextInt();

```
            switch (choice)
```
```
            {
```
```
            case 1 : 
```

                System.out.println("Enter integer element to push");

```
                try 
```
```
                {
```

                    stk.push( scan.nextInt() );

```
                }
```
```
                catch (Exception e)
```
```
                {
```

                    System.out.println("Error : " + e.getMessage());

                break;

```
            case 2 : 
```
```
                try
```
```
                {
```

                    System.out.println("Popped Element = " + stk.pop());

```
                }
```
```
                catch (Exception e)
```
```
                {
```

                    System.out.println("Error : " + e.getMessage());

```
                }    
```

                break;

```
            case 3 :         
```
```
                try
```
```
                {
```

                    System.out.println("Peek Element = " + stk.peek());

```
                }
```
```
                catch (Exception e)
```
```
                {
```

                    System.out.println("Error : " + e.getMessage());

```
                }
```

                break;

```
            case 4 : 
```

                System.out.println("Empty status = " + stk.isEmpty());

```
                break;                
```
```
            case 5 :
```

                System.out.println("Full status = " + stk.isFull());

```
                break;                 
```
```
            case 6 : 
```

                System.out.println("Size = " + stk.getSize());

                break;

```
            default : 
```

                System.out.println("Wrong Entry \n ");

```
                break;
```
```
            }
```
```
            /* display stack */
```
```
            stk.display();            
```

            System.out.println("\nDo you want to continue (Type y or n) \n");

```
            ch = scan.next().charAt(0);
```
```
 
```

        } while (ch == 'Y'|| ch == 'y');

```
    }
```
```
}
```

Program Explanation

1. The program defines a class called arrayStack for implementing a stack using an array.
2. The arrayStack class initializes its array, size, and other variables in the constructor. It has the following attributes:

arr: The array that stores the elements of the stack.
top: The index of the top element in the stack.
size: The size of the stack.
len: The number of elements in the stack.

3. Stack Operations:

isEmpty() – This method checks if the stack is empty. It returns true if the stack is empty, and false otherwise.
isFull() – This method checks if the stack is full. It returns true if the stack is full, and false otherwise.
getSize() – This method returns the number of elements in the stack.
peek() – This method returns the top element in the stack without removing it.
push() – This method adds an element to the top of the stack.
pop() – This method removes an element from the top of the stack and returns it.
display() – This method prints the contents of the stack to the console.

4. The StackImplement class is a simple driver class that demonstrates how to use the arrayStack class. It creates a stack of integers and allows the user to perform various operations on the stack, such as pushing, popping, peeking, checking if the stack is empty or full, and getting the size of the stack.
Users can push, pop, peek, and check the status of the stack (empty, full, and size).

Time and Space Complexity:

Time Complexity:
The time complexity of the stack operations in the code is as follows:

Push: O(1)
Pop: O(1)
Peek: O(1)
IsEmpty: O(1)
IsFull: O(1)
GetSize: O(1)

This is because all of these operations can be performed in constant time, regardless of the size of the stack.

Space Complexity: O(n)
The space complexity of the stack is O(n), where n is the size of the stack. This is because the stack uses an array to store its elements.

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Program Output:

Stack Test
 
Enter Size of Integer Stack
5
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
4
Empty status = true
 
Stack = Empty
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
24
 
Stack = 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
6
 
Stack = 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
162
 
Stack = 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
19
 
Stack = 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
94
 
Stack = 94 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
5
Full status = true
 
Stack = 94 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
1
Enter integer element to push
32
Error : Overflow Exception
 
Stack = 94 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
3
Peek Element = 94
 
Stack = 94 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Popped Element = 94
 
Stack = 19 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Popped Element = 19
 
Stack = 162 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Popped Element = 162
 
Stack = 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
6
Size = 2
 
Stack = 6 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Popped Element = 6
 
Stack = 24
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Popped Element = 24
 
Stack = Empty
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
2
Error : Underflow Exception
 
Stack = Empty
 
Do you want to continue (Type y or n)
 
y
 
Stack Operations
1. push
2. pop
3. peek
4. check empty
5. check full
6. size
 
4
Empty status = true
 
Stack = Empty
 
Do you want to continue (Type y or n)
 
n

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