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In Java, Stack is a legacy class which extends the Vector legacy class. Stack is used to maintain a last-in, first-out(LIFO) stack i.e. the last element to enter the Stack is the first one to be removed. In other words, Stack can be thought of as a pile of dishes, a new dish is always added to the top of the pile and the first dish to be removed is from the top as well.

Features of Stack

• Stack provides push() method to push/add an object to the top of the Stack and it provides pop() method to pop/remove an object out of stack, from the top.
• Stack legacy class is similar in functioning to the ArrayDeque collection class, although ArrayDeque is an updated version of Stack.







Difference between Stack and ArrayDeque class.



• ArrayDeque is not synchronized and hence it is not thread-safe by default, while on the other hand, Stack class is synchronized and thread-safe.
• ArrayDeque collection class can also be used to implement a LIFO stack and it is faster than Stack legacy class.
• As StackM class is a legacy class, hence most of its methods are legacy methods that are not entirely part of the Collection Framework. While ArrayDeque methods provide a lot more features than Stack.







A simple constructor of Stack class




Stack<Type> st = new Stack<Type>
This constructor creates an empty Stack of a certain Type, where Type could be any class. For example -

Stack <Integer> s = new Stack <Integer>();


The above example of calling the constructor of Stack creates a empty Stack to hold Integer objects.






Some legacy methods in Stack class.


Stack is a subclass of the Vector class, therefore, besides access to the methods defined by the Vector class, Stack class is defined with a few of its own methods as well, which makes it specialized for implementing a LIFO stack.



Methods	Description
int size()	Returns the total number of elements in Stack.
boolean empty()	Returns true if Stack is empty, else false.
E push(E element)	Pushes the element on the top of Stack.
E pop()	Returns and removes the element from the top of Stack
E peek()	Returns but doesn't remove element from the top of Stack.
int search(Object element)	Returns the index of an element(if found), else -1 is returned.



Note: In order to create a Stack, we must first import the java.util.Stack package in our program.




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Let's understand Stack by an example -:


In the upcoming program we have created a Stack to hold <Integer> objects and we will add/remove and search some Integer objects from this Stack by using its own methods. Let's see the code now.



//Java - Example of Stack 


import java.util.*;

class Stack1
{
public static void main(String... ar)
{
	//Calling the constructor of Stack 
	Stack<Integer> st= new Stack<Integer>();
	
	//Calling the push() method to push new objects in Stack
	st.push(10);
	st.push(23);
	st.push(16);
	st.push(5);
	st.push(29);

	//Printing the elements of the Stack 
	System.out.println("Contents of Stack = "+ st);

	//Calling the pop() method to pop the current top element of the Stack
	System.out.println("Popping out the top element = "+ st.pop());
	System.out.println("Popping out the next top element = "+ st.pop());

	//Priting the updated elements of the Stack
	System.out.println("Updated contents of Stack = "+ st);

	//Calling the peek() method of the Stack to print the top element of the Stack without removing it.
	System.out.println("Peeking at the top element = "+ st.peek());

	//Calling the search() method to search an element within the Stack.
	System.out.println("Index of element 5 from the top = "+st.search(5));
}
}





Output is :




Contents of Stack = [10, 23, 16, 5, 29]
Popping out the top element = 29
Popping out the next top element = 5
Updated contents of Stack = [10, 23, 16]
Peeking at the top element = 16
Index of element 5 from the top = 3





Program Analysis



• Method push() is called to add Integer objects to the top of Stack and after populating the Stack with Integer objects, its content are displayed.
• Method pop() is called, which returns and removes the first/top element from the Stack i.e. 29.
• Method pop() is called again, which returns and removes the next first/top element from the Stack i.e. 5. The updated contents of Stack are shown.
• Method peek() is called, which returns(but doesn't remove) the element from the top of Stack i.e. 16.
• Method search() is called, which returns the index of element 5 from the top of Stack i.e. 3.






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